2. B-ISDN REFERENCE MODEL

2. B-ISDN REFERENCE MODEL
and PROTOCOL LAYERS

B-ISDN Protocol Reference Model
Management Plane AAL ATM Layer Physical Layer Signaling Protocol Applications TCP/IP Native User Plane Control Plane SNMP: Simple Network Management Protocol CMIP: Common Information Control Plane Supports Signaling Call Setup, Call Control, Connection Control User Plane Data Transfer, Flow Control, Error Recovery Management Plane Operation, Administration, & Maintenance

(Provides Control of ATM Switch)
Management Plane (Provides Control of ATM Switch) Layer Management (Layered) Plane Management (No Layered) Use to manage each of the ATM layers with entity corresponding to each ATM layer OAM issues Concerned with management of all the planes All management functions (Fault, Performance, Configuration, Operation, & Security) which relates to the whole system are located in the Plane Management Provides coordination between all planes

Broadband Networking with SONET and ATM
USER ATM SW Video Image Data etc… UNI NNI Higher Layers Convergence Sublayers (CS) Segmentation Reassembly Sublayer (SAR) ATM Layer Physical Layer Higher Layers Convergence Sublayers (CS) Segmentation Reassembly Sublayer (SAR) ATM Layer Physical Layer Flow Control Error Handling Message Segmentation Adaptation Layer Segmentation Type Message Number Message ID 5 Byte Header 48 Byte Payload Handles cont. and bursty traffic ATM Layer Physical Layer ATM Layer Physical Layer SONET USER USER

Protocol Reference Model in the User Plane
Upper Layers Abbreviations class A class B class C class D AAL = ATM Adaptation Layer SAR = Segmentation and Reassembly CS = Convergence Sublayer PL = Physical Layer TC = Transmission Convergence PM = Physical Medium 1 2 3 4 Handling lost / misdelivered cells Timing recovery Interleaving CS Cell Information Field AAL Split frames / bit stream info cells Re-assemble frames / bit stream SAR Service Classes for AAL Class Type Cell routing Multiplexing / demultiplexing Generic flow control Cell Header A B C D Constant Bit Rate Variable Bit Rate Connection Oriented Data Connectionless Data ATM Cell header verification and cell delineation Rate decoupling (insert idle cells) Transmission frame adaptation TC PL SEAL = Simple and Efficient Adaptation Layer Type 5 AAL Acknowledged info transfer Bit timing Physical medium PM Remark: See next page

Remarks: PMD  Physical Medium Dependent
TC  Transmission Convergence Sublayer It separates transmission from the physical interface and allows ATM interfaces to be built on a large variety of physical interfaces

Physical Layer Functions
a) Physical Medium (PM) PM sublayer provides the bit transmission capability including bit alignment Line coding and, if necessary, electrical/optical conversion is performed in this sublayer Optical fiber is used for the physical medium. Other media, coax cables are also possible Bit rates  155 Mbps or Mbps.

PHYSICAL LAYER FUNCTIONS
b) Bit Timing Generation and reception of waveforms which are suitable for the medium, the insertion, and extraction of bit timing information and the line coding if required CMI (Code Mark Inversion) (CCITT G.703) proposed for Mbps interface. NRZ “Nonreturn to Zero” code proposed for optical interface.

LINE CODING Electrical Interface: Coded Mark Inversion (CMI)
For binary 0  always a positive transition at the midpoint of the binary unit time interval. For binary 1  always a constant signal level for the duration of the bit time. This level alternates between high and low for successive binary 1s. 1 1 1 1 1 Level A2 Level A1

LINE CODING Optical Interface: Nonreturn to Zero (NRZ)
For binary 0  Emission of light For binary 1  No emission of light Transition: 0  1 or 1  0 Otherwise no transition 1 1 1 1 1 Level A2 Level A1

ATM INTERFACES ATM INTERFACES
SONET/SDH : 155 Mbps and 622 Mbps over OC-3 (single mode fiber) Cell Based PDH Based (ATM cells mapped into PDH signals) (59 columns and 9 rows frame). Frame at Mbps. FDDI based or 100 Mbps (same as in FDDI PMD uses multimode fiber and line coding of 4B/5B). (called TAXI interface). Early private UNI interfaces were based on TAXI interfaces. DS-3 (45 Mbps) Transfer of ATM cells on T3 (DS-3) public carrier interface. It is cheaper than SONET links. STS-3 (155 Mbps) over Multimode fiber uses line coding of 8B/10B. STS-3 (155 Mbps) over Twisted Pair (using Taxi interface) uses line coding of 8B/10B. D1-T1 carriers (1.5 Mbps) ATM INTERFACES

CELL BASED INTERFACE Physical layer OAM cell
This interface consists of a continuous stream of cells where each cell contains 53 octets. 26 1 26 1 Physical layer OAM cell Synchronization achieved through HEC basis. Maximum spacing between successive physical layer cells is 26 ATM layer cells. After 26 consecutive ATM layer cells, a physical layer cell (idle cells or OAM cells) is enforced to adapt transfer capability to the interface rate.

Transmission Convergence Sublayer (TC)
A. Transmission Frame Adaptation Adapts the cell flow according to the used payload structure of the transmission system in the sending direction. In the opposite direction, it extracts the cell flow out of the transmission frame.

B. Header Error Control (HEC)
Multiple-bit errror (Cell discarded) After initialization receiver is in the “Correction Mode” Single bit error detected  corrected Multiple bit error detected  cell discarded Receiver switches to “Detection Mode” In “Detection Mode”, each cell with a detected single-bit error is discarded. If a correct header is found, receiver switches to “Correction Mode” NOTE: A noise burst of errors or other events that might cause a sequence of errors!! Error detected Cell discarded No Error Correction Mode No error Detection Mode Correction Single-bit error

Example: p Probability that a bit is in error
(1-p) Probability that a bit is NOT in error p40 Probability that 40 bits are in error (1-p)40 Probability that 40 bits are correct

Correction Mode With what probability a cell is rejected when
the HEC state machine is in the "Correction Mode"? Correction Mode Probability of a cell being rejected Different Perspective: When is a cell accepted? * Probability of having no errors in cell header OR * Probability of having a single bit error in cell header

HEC will only accept ERROR-FREE cells.
With what probability a cell is rejected when the HEC state machine is in the "Detection Mode"? Detection Mode HEC will only accept ERROR-FREE cells. Different Perspective: What is the probability that a cell header is correct?

Assume that the HEC state machine is in the “Correction Mode
Assume that the HEC state machine is in the “Correction Mode.” What is the probability that n successive cells will be rejected, where n >= 1 ? Correction Mode Probability of n successive cells being accepted (n>1) n=1: Probability that 1 cell is accepted, i.e., the entire header is error-free. What is that probability? OR There is at most one bit error in the header.

Probability that the cell header (2) is correct AND
1 n=2: Probability that the cell header (2) is correct AND Previous case for cell 1 OR Probability that the cell header (2) has at most 1 bit error AND Probability that the cell header (1) is correct (error free)

Probability that the cell header (3) is correct AND
2 1 n=3: Probability that the cell header (3) is correct AND Previous case for cell n=1 OR Probability that the cell header (3) has at most 1 bit error AND Probability that the cell header (2) is correct AND The case for n=1

First cell is rejected:
Assume that the HEC state machine is in the “Correction Mode.” What is the probability p(n) that n successive cells will be accepted, where n >= 1 ? First cell is rejected: What is the probability that a cell is rejected?  Case a) Different Perspective:  Probability that all header bits of a cell are correct  Probability that one single bit error in a cell header

Now, HEC is in Detection Mode
Remaining n-1 successive cells: Now, HEC is in Detection Mode What is the probability that (n-1) successive cells are rejected, i.e., there will be errors in the headers for the remaining (n-1) cells

EFFECT OF ERROR IN CELL HEADER
Incoming Cell Error in Header? No Valid cell (intended service) Yes Error detected Apparently valid cell With errored header (unintended service) No Yes Current mode? Detection Discarded Cell Correction Error incorrectable? Yes No Correction attempt Unsuccessful Successful

HEC Generation Algorithm (I.432)
Every ATM cell transmitter calculates the HEC value across the first 4 octets of the cell header and inserts the result in the fifth octet (HEC field) of the cell header. The HEC value is defined as “the remainder of the division (modulo 2) by the generator polynomial x8+x2+x+1 of the product x8 multiplied by the content of the header excluding the HEC field to which the fixed pattern will be added modulo 2.” The receiver must subtract first the coset value of the 8 HEC bits before calculating the syndrome of the header. Device always preset to 0s. [Key Word: CRC (Cyclic Redundancy Check Algorithm)]

ATM CELL STRUCTURE HEADER (5 octets) PAYLOAD (48 octets) GFC PT HEC
1 2 3 4 5 : 53 Octet HEADER (5 octets) PAYLOAD (48 octets) 1 2 3 4 5 : 53 GFC VPI VPI VCI VCI VCI PT PR HEC PAYLOAD (48 octets) 11

HEC GENERATION ALGORITHM
The HEC field contains the 8-bit FCS (Frame Check Sequence) obtained by dividing the first 4 octets (32 bits) of the cell header multiplied by 2^8 by the CRC code (generator polynomial) (x8+x2+x+1)

HEC Generation Algorithm (I.432)
This HEC code can Correct single bit errors Detect multiple bit errors REMARK: If a code corrects “t” errors, it can detect (2t + 1) errors!!!!! i.e., Here  (up to 3 bits) Purpose: Protects the header control information Helps to find a valid cell (cell delineation and boundaries)

CELL DELINEATION (This process allows identification of cell boundaries) Correct HEC Bit-by-Bit Cell-by-Cell HUNT PRESYNC Incorrect HEC  consecutive incorrect HEC  consecutive correct HEC SYNCH

Cell Delineation (cont.)
In Hunt State  a cell delineation algorithm is performed bit-by-bit to determine if the HEC coding law is observed (i.e., match between received HEC and calculated HEC). Once a match is achieved, it is assumed that one header has been found and the method enters the PRESYNCH state. The HEC algorithm is performed cell-by-cell. If  consecutive correct HECs are found, SYNCH state is entered; if not the system goes back to HUNT state. SYNCH is only left (to HUNT) state if  consecutive incorrect HECs are identified.

 and  are design parameters that influence the performance of cell delineation process. (=7 and =6). Greater values of  result in longer delays in recognizing a misalignment but in a greater robustness against false alignment. Greater values of  result in longer delays in establishing synchronization but in greater robustness against false delineation.

Remarks: A Mbps ATM system will be in SYNCH state for more than 5349 years even when the bit error probability is BER=10-4. This method may fail if the header HEC occurs in the info field (maliciously or accidentally)  Cell Payload Scrambling. To overcome  the info field contents scrambled using a self-synchronizing scrambler with polynomial X Header itself is not scrambled.

The probability of 7 consecutive incorrect HEC with BER= A= The probability that 7 consecutive cells are in error. [1- (1-10-4)40 ]7 = * = A /A  The number of cells sent in order to have a 7 consecutive error cells; (Unit Cells); How often does event A occur in terms of ATM cells.

{53 * 8} / { Mbps} = C (53*8) = # of bits/cell ; Link Speed = # of bits/sec C is how long does it take to send one ATM cell through the 155 Mbps link. k = [1 / A] * C = {6.187*106} * {53 * 8 / Mbps} = *1011 k  in terms of seconds k / (365*24*60*60)  approx years..

Cell Rate Decoupling (Speed Matching)
Adapts cell stream into Transmission Bit Rate (Insertion / Discarding idle cells; in particular for SONET Interface). SONET uses synchronous cell time slots! Note: Cell Based Interface  No need for this function.

Cell Rate Decoupling (cont.) (Speed Matching)
ATM Transmitter ATM Receiver B u f e r + VPI/VCI - Insert Idle or Unassigned cells Remove the Idle or Unassigned cells Transmitter multiplexes multiple streams; queueing them if an ATM cell is not immediately available. If the queue is empty, when the time arrives to fill the next synchronous cell time slot, then the Transmission Convergence Sublayer inserts an Idle cell (or the ATM layer inserts an Unassigned cell.)

ATM Layer Functions Cell Multiplexing/Demultiplexing
Cell VPI/VCI Translation Cell Header Generation/Extraction GFC Function

ATM Layer Functions i) CELL MULTIPLEXING/DEMULTIPLEXING
In the transmit direction, cells from individual VPs and VCs are multiplexed into one resulting stream. At the receiving side  the cell demultiplexing function splits the arriving cell stream into the individual cell flows appropriate to the VP or VC.

ATM Layer Functions ii) CELL VPI/VCI TRANSLATION
- At ATM switching nodes, the VPI and VCI translation must be performed. - Within VP switch, the value of the VPI field of each incoming cell is translated into a new VPI value for the outgoing cell. - At a VC switch, the values of the VPI as well as the VCI are translated into new values.

ATM Layer Functions iii) CELL HEADER GENERATION/EXTRACTION
- This function is applied at the termination points of the ATM layer. - Transmit Side: After receiving the cell information from the AAL, the cell header generation adds the appropriate ATM cell header except for the HEC values. HEC is done at Physical Layer. VPI/VCI values could be obtained by a translation from the SAP identifier. - Receive Side: The cell header extraction function removes the cell header. Only the cell information is passed to the AAL. - This function could also translate a VPI/VCI value into a SAP identifier.

ATM Layer Functions iv) GFC FUNCTIONS
- Supports the control of the ATM traffic flow in a UNI. It can be used to alleviate short overload conditions. - Control of cell flows toward the network but not flow control from the network. - No effect within the network.

Virtual Path and Virtual Circuit Concept
ATM cells flow along entities known as VIRTUAL CHANNELS. A VC is identified by its virtual circuit identifier (VCI). VC  set up between 2 end-users (like VC in X.25 => Indiv. Log connection). VP  Bundle of VCs  having the same end points (Group logical connection; reserved trunk of connections). All cells in a given VC follow the same route across the network and are delivered in the order they were transmitted. VCs are transported within Virtual Paths (VPs). A VP is identified by its virtual path identifier (VPI). VPs are used for aggregating VCs together or for providing an unstructured data pipe.

Virtual Path and Virtual Circuit Concept
Optical links will be capable of transporting hundreds of Mbps where VCs fill kbps. Thus, a large number of simultaneous channels have to be supported in a transmission link. Typically 10K simultaneous channels are considered (thus, VCI field up to 16bits). Since ATM is connection oriented, each connection is characterized by a VCI which is assigned at Call-Set-Up. When connection is released, VCI values on the involved links will be released or can be reused by other components.

VIRTUAL PATH / VIRTUAL CIRCUIT
CONCEPT VP TRANSMISSION PATH VC Virtual Path Text VCI =1 (text) Voice VCI =2 (voice) Video VCI =3 (video) ATM Network Interface

VIRTUAL PATH/VIRTUAL CIRCUIT CONCEPT
Each VP has a different VPI value and each VC within a VP has a different value. Two VCs belonging to different VPs at the same interface may have identical VCI values. VPI is changed at points where a VP link is terminated. VCI is changed at points where a VC link is terminated.

Goal  Multimedia Communication
Video & Voice  Time Sensitive (Delay bounds) Data  Loss Sensitive (Loss bounds) Allows the network to add or remove components during the connection e.g. Video Telephony  Start with voice (only single VC)  Add video later (on another VC)  Add data (on another VC)  Signaling (on another VC)

EXAMPLE Three VP connections exist from A to B. They are seen by A as corresponding to the values p, q, r of the VPI field, and by B as corresponding to the values p2, q2, r2. Whenever A wants to send some information to B on the VP connection seen as p, it writes the value p in the VPI field of the cell. The VP switches T1, T2 and T3 swap the VPI labels according to the lookup tables. The VCI field is not changed by the VP switches, so it can be used by A to multiplex several VC connections on any one of the three VP connections. Therefore, at the VC level, A has at its disposal three direct links to B. A B A VC Level B VP Level p p2 p p2 p1 T1 T2 q q2 q q2 T3 r r2 r r2

SWITCHING OF VCs and VPs
Routing functions for VPs are performed at a VP switch. This routing involves translation of the VPI values of the incoming VP links to the VPI values of the outgoing VP links. VCI values remain unchanged. VC switches terminate both VC links and necessarily VP links. VPI and VCI translation is performed. VP Switch/Cross Connect VPI1 VPI2 VPI3 VPI4 VPI5 VPI6 VCI 21 VCI 22 VCI 23 VCI 24 VCI 25 VP Switching

VC Switch/Cross Connect
VP and VC SWITCHING VPI 2 VCI 25 VCI 21 VPI 4 VPI 5 VCI 23 VCI 24 VC Switch/Cross Connect VCI 23 VCI 24

Virtual Channel Connection
MORE ABOUT VCs and VPs A VP Connection: Contains multiple VC connections. VC connections may be built up of multiple VP connections. Use of VPI simplifies routing table lookup. Virtual Channel Connection A B T D1 D2 D3 D4 Virtual Path Connection x Virtual Path Connection y VCI = a1 VCI = a2 VPI=x1 VPI=x3 VPI=x2 VPI=y3 VPI=y2 VPI=y1 Virtual Channel View A T B VCI=a1 VCI=a2 Other VCI

VCs and VPs (Cont.) The inter-networking of the VP and VC switches is illustrated in Figure. There exist VP connections (x and y) between A and T; T and B. Assume now that A wants to setup a VC connection to B using those two VP connections. The network has to provide a VCI value, say a1, for the A to T link, and a VCI value, say a2, for the T to B link. The VC connection from A to B is thus made of two VC links only. At switching points D1 through D4, only the VPI field is swapped. At the switching point T, both VPI and VCI fields are swapped. The situation is thus similar to that where A and B would be access nodes in a circuit switched network, T would be a transit node, and D1 through D4 would be cross-connects.

Example for VCIs and VPIs
A VP is established between Subscriber A and Subscriber C transporting 2 individual connections, each with a separate VCI. Remark: The VCI values used (1,2,3 and 3,4 in the example) are NOT translated in the switches, which are only switching on the VPI field. ATM Node 1 Node 2 Node 3 A B C VPI=4, VCI=1,2,3 VPI=6 VCI=3,4 VPI=2 VCI=1,2,3 VPI=8 2 6 VPIOUT VPIIN 8 4

Namings VC VC Link VC Identifier VCC (Virtual Channel Connection)
Virtual Channel  Virtual Circuit VC Link A point where a VCI value is assigned to another where that value is translated or terminated. VC Identifier A value which identifies a particular VC link for a given VP Connection. VCC (Virtual Channel Connection) A concatenation of VC links that extends between 2 points. (cell sequence integrity preserved)

VP VP Link VP Identifier VPC (Connection) Bundle of VCs.
A group of VC links, identified by a common value of VPI, between a point where a VPI value is assigned and the point where that value is translated as terminated. VP Identifier Identifies a particular VP Link. VPC (Connection) A concatenation of VP Links.

PVC and SVC Permanent Virtual Circuits (PVC)
Established by a network operator in which appropriate VPI/VCI values are programmed for a given source and destination (for long time). VPs  0, …, 256 (manually configured) PVCs are established by provisioning & usually last a long time (months/years). Switched Virtual Circuits (SVC) Established automatically through a signalling protocol (Q.2931B) and lasts for short time (minutes/hours). VCs  0, …, (automatically configured)

SOFT PVC Part of the connection is permanent and part of it is switched. Hybrid of PVC and SVC!!!

VCC  0, 5  Call set up (Signalling) 0, 16  Network Management (Integrated Local Management Interface ILMI)  User Data 0, 17  For LAN Emulation Configuration Server (LECS) 0, 18  For Private NNI (PNNI) 0, 19 or 0, 20  Reserved for future use.

Advantages of VP/VC Concept
Simplified Network Architecture: Network transport functions can be separated into those related to an individual logical connection (VC) and those related to a group of logical connections (VP). Increased Network Performance and Reliability: The network deals with fewer, aggregated entities. Reduced Processing and Short Connection Setup Time: Much of the work is done when the VP is set up. The addition of new VCs to an existing VP involves minimal processing. Enhanced Network Services: The VP is used internal to the network but is also visible to the end user. Thus, the user may define closed user groups or closed networks of VC bundles.

ATM Adaptation Layer (AAL)
AAL is responsible for adaptation of information of higher layers to the ATM cells (in the transmission direction) or adaptation of ATM cells into the information of the higher layer (receiver direction). AAL is subdivided into two sublayers: - SAR (Segmentation and Reassembly) - CS (Convergence Sublayer): Multiplexing, loss detection, timing recovery, message identification

ATM Adaptation Layer (AAL)
AAL provides a variety of services: Class 1: Circuit Emulation with Constant Bit Rates (CBR). Voice of 64 kbps Fixed Bit Rate (Voice,Video) Class 2: Connection-oriented service with Variable Bit Rates (VBR) and timing between source and destination. VBR Video & Audio Class 3: Connection-Oriented Service. Data Transfer and Signaling ABR Traffic with no timing Class 4: Connectionless Data Service SMDS, Ethernet, Internet, Data Traffic, No constraints.

Traffic Classes A D C B 1 2 ¾ or 5 Yes No Constant Variable
Connectionless Connection Oriented 1 2 ¾ or 5 Timing Between Source and Destination Bit Rate Connection Mode AAL Example DS1, E1 N64 Kbps Emulation Packet Video, Audio (Real Time) Frame Relay IP, Ethernet

General Structure of AAL
AAL-SAP ATM-SAP CONVERGENCE SUBLAYER(CS) SAR SUBLAYER Primitives AAL Convergence Sublayer (CS) Segmentatioin & Reassembly (SAR) Service Access Point • Service Data Unit (SDU) crosses the SAP • PDU is data unit between peer layers

General Data Unit Naming Convention
AAL-SAP AAL-SDU CS-PDU Payload CS-PDU Header Trailer SAR-PDU Payload SAR-PDU ATM-SAP ATM-SDU Cell information Field (Cell Payload) Cell PL-SAP No SAP is defined between CS and SAR ATM Cell ATM Layer Physical Layer Segmentation And Reassembly (SAR) Sublayer Convergence Sublayer(CS) AAL Interfaces AAL of CS-PDU

Structure of AAL with SSCS and CPCS
AAL-SAP AAL-PDU Primitives Service Specific Convergence Sublayer (SSCS) SSCS SSCS-PDU CS AAL Common Part (CP) Primitives AAL Common Part Convergence Sublayer (CPCS) CPCS CPCS-PDU Primitives Segmentation And Reassembly (SAR) SAR SAR-PDU Primitives ATM-SAP

AAL Type 1 AAL 1 provides the foll. services to the AAL users:
Transfer of service date unit with a constant source bit-rate and their delivery with the same bit rate - Voice traffic 64kbps: as in N-ISDN to be transported over an ATM network. This service is called circuit emulation. In other words, how TDM type circuits can be emulated over ATM. CBR-Voice; CBR-Video (fixed (constant) bit rate video)

AAL Type 1 Transfer of timing information between source and destination. Transfer of structure information between source and destination; some users may require to transfer of structured data, e.g., 8 kHz structured data for circuit mode device for 64 kbps (N-ISDN). Indication of lost or errored information which is not covered by AAL1, if needed.

AAL Type 1 (Cont.) The functions listed below may be performed in the AAL in order to enhance the layer service provided by the ATM layer: Segmentation and reassembly of user information Handling of cell delay variation  to achieve constant rate delivery (playout buffer) Handling of cell payload assembly delay Handling of lost and misinserted cells (SN processing)  Discarded

AAL Type 1 (Cont.) - 4 bit RTS is transferred by CSI
Source clock frequency recovery at the receiver - 4 bit RTS is transferred by CSI - handling of timing relation for Asynchronous transfer (SRTS Synchronous Residual Time Stamp) Monitoring of AAL-PCI (Protocol Control Information) for bit errors Handling of AAL-PCI bit errors SAR-PDU Header CS-PDU Header CS-PDU Trailer PCI

AAL Type 1 (Cont.) Monitoring of the user information field for bit
errors and possible corrective action - FEC maybe performed for high quality video or audio (124,128 Reed Solomon code)

AAL Type 1 (cont.) Receiver’s Responsibilities are as follows.
Examine the CRC and parity bit for error detection. Correct single bit errors in SN field. If multiple bit errors in SN field, then declare invalid. Reassemble the CS-PDU in correct sequence using SN-numbers. Discard misinserted CS-PDUs and generate dummy information for missing CS-PDU.

AAL Type 1 (Cont.) Buffer the received CS-PDUs to compensate for cell
delay variation (jitter) to achieve constant rate delivery. (PLAYOUT Buffer) Clock frequency recovery (Handling of timing relationship for asynchronous circuit transport) Monitoring and handling AAL-PCI (Protocol Control Information) SAR-PDU Header, SAR-PDU Trailer, CS-PDU Trailer are collectively called AAL-PCI.

AAL 1 STACK Forward Error Correction No Retransmission ATM
Convergence Sublayer - accepts 124-byte blocks from user - appends 4-byte FEC - writes to matrix “row” - forwards CS-PDU to SAR when 47 blocks (rows) have been written Forward Error Correction No Retransmission Segmentation/Re-assembly Sublayer - reads matrix “columns”(47bytes) - effect: interleaving * (124,128) Reed-Solomon Code * Polynomial undefined * Corrects 2 errored bytes per row * Corrects 4 “erasure” bytes (knows position) * Uses interleaving ATM

FEC in AAL1 Reading R-S Code with 4 byte FEC
Cell 1 Byte 1 Cell 2 Byte 1 Cell 124 Byte 1 Cell 1 Byte 2 Cell 2 Byte 2 Cell 124 Byte 2 Cell 1 Byte 47 Cell 2 Byte 47 Cell 124 Byte 47 Reed-Solomon Code recovers up to 4 lost cells in a block of 128.

AAL 1 Higher Layers CS AAL SAR ATM Layer … … User Data Bit Stream
AAL-SAP CS CPCS-PDU Payload AAL SAR-PDU Payload SAR-PDU Payload H … SAR-PDU Payload H H SAR 1B 47B 48 Bytes ATM-SAP ATM Layer … H Cell Payload H Cell Payload H Cell Payload 5B 53 Bytes

SAR-PDU of AAL 1 47 Octets SAR-PDU Payload 1 Octet Cell Header SN SNP
4 bits 4 bits SAR-PDU Header SAR-PDU (48 Octets) SN (Sequence Number) for numbering of the SAR-PDUs SNP (Sequence Number Protection) to protect the SN field To detect lost or mis-inserted cells (Error Detection & Correction)

SAR-PDU Header of AAL 1 SN Field SNP Field Sequence Count Even Parity
1 bit 3 bits 3 bits 1 bit Sequence Count Even Parity CSI CRC SN Field SNP Field

CSI Field Sequence Count 0, .., 7
CSI bit used to transfer TIMING or DATA STRUCTURE information. CSI values in cells 1,3,5,7 are inserted as a 4-bit timing value. In even numbered cells 0,2,4,6, CSI used to support blocking of info. from a higher layer. If CSI bit is set to 1 in a cell 0,2,4,6, then the first octet of SAR-PDU payload is a pointer that indicates the start of the next structured block within the payload of this cell and the next cell, i.e., 2 cells (0-1, 2-3, 4-5, 6-7) are created as containing a 1-octet pointer and a 93-octet payload and pointer indicates where in that 93 octet payload is the first octet of the next block of data.

P & Non-P Formats AAL-1 CS uses a pointer to delineate the structure boundaries. Supported by 2 types of CS_PDUs called  Non-P & P SN SNP SAR-PDU Payload (User Data) Non P-format 1 Octet 47 Octets SAR-PDU Header SAR-PDU (48 Octets) SAR-PDU Payload 46 Octets (CSI = 1) Reserved for Pointer P-format Can be used only in SAR PDUs with even SN values (because SRT scheme uses the CSI bits in SAR PDUs with odd SN values)

User Data P-Format Sequence Counter 0,2,4,6 Offset Field 7 Bits
SAR-PDU Header Structure Pointer Field User Data P-Format Sequence Counter 0,2,4,6 Reserved Bit Offset Field 7 Bits 7 Bits are the offset measured in Bytes between the end of the pointer field & start of the structured block in 93 bytes consisting of remaining 46 bytes in this CS-PDU & 47 Bytes of the next CS-PDU. This offset may range from 0-92. SN even  uses 1 Octet Pointer field to indicate the offset into the current payload of the first octet of a n*DSO payload. Value of n may be as large as 92 in the P-format since pointer is repeated every other cell when supporting AAL 1.

(supports an octet structured (4-bit RTS included in CSI Bit !!)
AAL1 STD Mode (Structured Data Transfer) Unstructured Data Transfer n x DSO (64kbps) Service (supports an octet structured n – DSO Service) DS1/E1 (1.544Mbps) DS3/E3 (45Mbps) including timing SRTS Method CSI bit (in even SN values) for SDT to convey information about internal byte alignment structure of the user data bit stream. (4-bit RTS included in CSI Bit !!) One sent in (1,3,5,7)

Structured Data Transfer
Kind of fractional DS1/E1 service where the user only requires an n*64kbps (DS0) connection where n can be small as 1 and as high as 24 for DS1 (T1) and 30 for E1. An n*64 kbps service generates blocks of n bytes which are carried in P and non-P format CS-PDUs. The beginning of a block is pointed to by the pointer in the 1-byte header of the CS-PDU-- > P format.

EXAMPLE: STRUCTURED DATA TRANSFER
1 192 1 192 1 192 1 192 1 192 1 192 1 192 1 192 DS1 Signal 46=368 47=376 46=368 47=376 CS-PDUs p SN=0 CSI=1 P-Format SN=1 CSI=0 Non-P-Format SN=2 CSI=1 P-Format SN=3 CSI=0 Non-P-Format 0-1 = 93 Octets 2-3=93 Octets Pointer indicates where in that 93 octet payload is the first octet of the next block of data. No structured boundary, then use dummy offset value of 127.

Unstructured Data Transfer
The entire DS-1/E1 signal is carried over an ATM network. The DS-1 signal is received from user A which is packed bit-by-bit into the 47-byte non-P format CS-PDU which then becomes the payload of a SAR-PDU.

DS1 CIRCUIT EMULATION USING AAL 1
EXAMPLE: UNSTRUCTURED DATA TRANSFER DS1 CIRCUIT EMULATION USING AAL 1 ATM Cells DS1 Signal SRTS CS SAR-PDUs octets bits Header SAR- PDU octets 1 47 5 48 RTS 1 192 1 1 192 Header SAR- PDU octets 1 47 5 48 Time 192 1 192 Header SAR- PDU octets 1 47 5 48 1 192 1 192 Transmitter uses AAL 1 operating in SRTS mode to emulate a DS 1 digital bit stream created by a video codec. DS1 frame has 193 bit frames that repeat 8000 times per second (192 user data bit + 1 framing bit). CS computes the RTS every 8 cell times and provides this to the SAR sublayer for insertion in the SAR header. 193 bit frames are packed into 47 octet SAR-PDUs by SAR layer. SAR then adds the SN, inserts the data from CS, computes CRC and parity over SAR header and passes 48-octet SAR-PDU to ATM layer.

Handling of Lost and Misinserted Cells in AAL1
At the transmitter, CS provides SAR with a Sequence Count Value and a CSI associated with each SAR-PDU payload. Sequence Count Value starts with 0, and incremented sequentially and is numbered modulo 8. At the receiver, CS receives Sequence Count, CS indication from SAR, and check status of Sequence Count and CS indication. CS identifies SAR-PDU payload sequence SAR-PDU loss, and SAR-PDU misinsertion. CSI is used to transfer timing information and default value of CSI is “0”. 4 bit RTS is sent in odd-sequence-numbered PDUs (1,3,5,7) in SRTS approach.

Handling of Lost and Misinserted Cells in AAL1
Remark: For each SAR-PDU, SAR receives a sequence number (SN) value from CS. At the receiver, SAR passes the SN to CS. The CS may use these SNs to detect lost or misinserted SAR-PDU payloads. SAR protects the SN value and CSI against bit errors. It informs the CS when SN value and the CSI are in error and cannot be corrected. Transmitter computes the CRC value across the 4 bits of SAR-PDU header and inserts into CRC field. CRC contains the remainder of the division (mod 2) by polynomial of the product multiplied by the contents of SN field. After completing the above operations, transmitter inserts the even parity bit.  7 bit code word protected.

Adaptive Clocking in AAL 1 (No Network clock is available).
TIMING (CLOCK) RECOVERY TECHNIQUES IN AAL 1 Adaptive Clocking in AAL 1 (No Network clock is available). Synchronous Residual Time Stamp Approach (SRTS) (Global Network Clock is available)

Adaptive Clocking in AAL 1
Common network reference clock is not available!!! Network Data FIFO Terminal Filling Level Local Clock Jitter Filter PLL . 1. Adaptive Clocking (Receiver) Used for Transfer Delay Variable Cells PLAYOUT BUFFER Receiver writes received info field in this buffer. Receiver reads info. with a local clock. CONTROL is performed by continuously measuring the fill level around its median position & by using this measure to drive the PLL providing the local clock. (Content) Filling level of the buffer is used to control the frequency of the local clock. The content level of the buffer may be maintained within an upper limit and lower limit to present buffer overflow and underflow. Underflow => PLL slowed down Overflow=> PLL speeded up PLL (Phase Lock Loop) Provides local clock.

Synchronous Residual Time Stamp (SRTS) Approach
BASIC IDEA: Convey a measure of the frequency difference between the reference clock and source clock. Network reference clock is available, source clock is not syncronized! NETWORK Sender Receiver Common Network Clock Local Clock Local Clock CSI field Sequence # field Odd # of segments TIMESTAMP Difference between the local and network clocks. Transport this info. in odd numbered Cells (CSI Field) to destination Difference between 2 clocks

Source Transmitter (Assumed)
Common Network clock is available Source (local) clock is not synchronized with it. Source Transmitter SRTS method conveys a measure of the frequency difference between the derived network reference clock and the source (local) clock. The derived network reference clock is determined from the frequency of the network clock divided by some integer. Within a time interval of N “source clock cycles” suppose there are M cycles of the derived “network reference clock”. There is a nominal value Mnom (fixed and known for the service) and the actual value of M may vary anywhere within a certain range (Mmin & Mmax) around this nominal value Mnom. The actual value of M will be the sum of Mnom and a residual part. By transmitting the residual part, the receiver has enough info to construct the source clock.

Tolerance Source Frequency (fs) Derived Network Frequency (fnx)
Source clock N cycles T seconds Source Frequency (fs) t M nom M M M min nom max Derived Network Frequency (fnx) t y y Residual value M 4 2

Sample & Hold 4 Bit Counter
1 C fs t N 4 Bit SRTS encoded in CSI bit for SAR-PDUs with Sequence Numbers 1,3,5,7 1 fnx 4 Bit Counter fn X Network Reference clock frequency fn is divided by x such that 1 < fnx/fs < 2

Source clock fs is divided by N to sample the 4-bit counter Ct driven by
the network clock fnx once every N = = 47 x 8 x 8 bits generated by the source. This sampled counter output 4 bits (residual part) is transmitted as the SRTS in SAR-PDU. It is sent in the CSI bits of SAR-PDUs which have odd SN values. The method can accept a frequency tolerance for source frequency of 200 parts per million (ppm). Ct, X, Mnom, N, fn are available at the destination and the clock value can be recovered accordingly!!!! ATM INTERFACES

AAL 2 For low bit rate communications, e.g., for compressed voice traffic. Main Idea: multiplex many users within a single ATM VCC, where each user’s information (SDT) is carried in variable length packets with a header (3 octets) identifying the user channel with control information. (kind of variable ATM cell) In the minicell header, the field for user identification has 8 bits limiting the number of AAL 2 users sharing a VCC to 256. Short and variable length payload. User packet multiplexing Minicell Header 3 octets Payload (1-64) octets SDU

WHY AAL 2? AAL 1 needs not be filled with full 47 bytes. e.g., to transmit digitized voice at a rate of 1 byte every 125 sec, filling a cell with 47 bytes means collecting samples for msec. If this delay before transmission is unacceptable, we send partially filled cells  waste of bandwidth!!!

STRUCTURE OF AAL TYPE 2 A L A T M Service Specific Convergence
AAL SAP Service Specific Convergence Sublayer (SSCS) AAL-SDU SSCS-PDU Header User Packet SSCS-PDU Trailer A L SSCS-PDU Common Part Sublayer (CPS) CPS-SDU CPS-Packet Header CS-Packet Payload Start Field CPS Packet CPS-PDU Header CPS-Packet CPS- Packet PAD CPS-PDU (48 octets) ATM SAP A T M ATM Layer ATM Header ATM Cell Payload ATM Cell PHY SAP Transfer of Service Data Unit with a Variable Bit Rate Transfer of timing information between source and destination Indication of lost or errored information which is not covered by AAL 2

CPS-PACKET FORMAT CID| PPT| LI UUI HEC CPS-INFO
CPS-Packet Header (3 octets) CPS-Packet Payload (Variable length) CPS-Packet (48 octets default 64 octets optimal) CID: Channel Identifier (8 bits): Values: 0: Not used 1: Reserved for Layer Management (AAL2 ANP packets) 2-7: Reserved 8-255: ID of SSCS entity (valid CID values to identify channels)

CPS-PACKET FORMAT (Contd)
CID helps to multiplex multiple AAL2 users/streams (channels) onto a single VCC (ATM connection). Each channel is identified by the CID. A channel is bidirectional and has the same CID value. * CID field supports up to 248 individual users per VCC.

AAL2 can multiplex several data streams
B C D A’ B’ C’ D’ AAL2 AAL2 ATM ATM ATM Network PHY PHY AAL2 can multiplex several data streams

Functional model of AAL2 (sender side)
AAL-SAP SSCS SSCS CID=Z SSCS CID=Y CID=X CSP ATM-SAP Functional model of AAL2 (sender side)

Packet Payload Type (2 bits): serves 2 functions: * When PPT =/ 3, the CPS packet is serving a specific application, such as carrying voice data, or carrying an ANP packet. * When PPT=3, the CPS packet is serving an AAL network management function associated with the management of the channel identified in the CID field.

* LI: Length Indicator (6 bits) * LI specifies the number of octets (minus 1) in the variable length user payload. * LI Coding: One less than CPS-Packet payload length CPS-Packet payload length = LP => LI = LP -1 * CPS-INFO: Information (variable size: (min. 1- max. 45 or 64 octets))  45 means that exactly one CPS packet fits inside the 48 octet ATM cell payload.

* UUI: User-to-User Information (5 bits): Allows the functions of an SSCS to be specific according to a purpose. UUI serves two purposes: To convey specific info transparently between CPS users, SSCS entities or layer management. To distinguish between SSCS entities and layer management users. Codepoints: 0-27 SSCS entities 28-29 Future use 30-34 Layer management

CPS-PACKET FORMAT (Ctd)
HEC: Header Error Control (5 bits) 5 bit CRC : Generator Polynomial x5+x2+1 (excluding CPS packet payload and error correction). Detectable 1 and 2 bit errors.

CPS-PDU Payload (47 octets)
CPS-PDU FORMAT Start Field (STF) indicates the position of the first packet OSF SN P CPS-Packet CPS-Packet PAD CPS-PDU Header CPS-PDU Payload (47 octets) CPS-PDU (48 octets) OSF: Offset Field (6 bits) 6 bit pointer => Position Indication of first CPS-packet (starting point of the next CPS packet header within the cell) Values: 0-40: First CPS packet boundary (0=Next to OSF) 47-63: No CPS packet boundary SN: Sequence Number (1 bit): mod 2 (value 1 or 0) P: Parity (1 bit) : Odd parity for STF PAD: Padding (0-47 octets)

OSF identifies the starting point of the next CPS packet header
Packets are streamed into successive payloads CPS Packet Cell Period Pointer in OSF points to find start of a CPS packet in cell ATM Header First Packet Padding: All 0’s ATM Cell OSF identifies the starting point of the next CPS packet header within the cell. If more than one CPS packet is present in a cell, then AAL2 uses the LI in the CPS packet header to compute the boundary of the next packet.

EXAMPLE User 1 User 2 User 3 User 4 User 5 Rt VBR Sources … 16 16 16 16 16 … CPS Packets 3 16 3 16 3 16 3 16 3 16 CPS PDUs SAR … 1 19 19 9 1 10 19 18 STF STF ATM Layer … ATM Header 48 bytes ATM Header 48 bytes PURPOSE: Accommodation of low bit rate (below 64 kbps) and delay sensitive applications into ATM networks, e.g., cellular systems. Requirements: Short Cell Assembly Time and High Efficiency.

PACKET CAN STRADDLE CELLS!!
ATM Cell 1 ATM Cell 2 H H Packet 1 Packet 2 Pac- ket 3 Packet 4 Packe-

AAL Negotiation Procedures (ANP)
This is the function that provides the dynamic allocation of AAL2 channels on demand. This function is carried out by an AAL2 layer management entity at each side of an AAL 2 link. This layer management entity uses the services provided by AAL2 through a SAP for the purpose of transmitting and receiving ANP messages. These messages are carried on a dedicated AAL2 channel with CID=1, and they control the assignment, removal and status of an AAL2 channel. The following types of messages have been defined: Assignment request, assignment confirm, assignment denied, removal request, removal confirm, status poll, and status response.

Open Questions: Timing mechanisms??? Error correction schemes? FEC but with QoS considerations!!

AAL 3/4 AAL-SDU SSCS SSCS-PDU Payload SSCS-PDU Trailer Header SSCS-PDU
AAL-SAP AAL-SDU SSCS SSCS-PDU Payload SSCS-PDU Trailer Header SSCS-PDU CPCS-SDU CPCS CPCS-PDU Payload CPCS-PDU Trailer Header CPCS-PDU SAR SAR-PDU Payload SAR-PDU Trailer Header SAR-PDU ATM-SAP

Small AAL-PDUs allowed
Non-Assured Mode (Unreliable) Assured Mode (ARQ Protocols) - Go_Back_N - Selective Repeat Request Message Mode Entire AAL-PDU needed Stream Mode Small AAL-PDUs allowed

a) MESSAGE MODE AAL-SDU is passed across the AAL interface in exactly one AAL-SDU. This service provides transport of fixed size of variable length AAL-SDUs. 1:1 mapping, i.e., one SSCS-PDU consists of one AAL-SDU. SSCS accepts a block of information from a user and creates a SSCS-PDU. This includes a Header & Trailer with protocol information and padding to make the PDU an integral multiple of 32 bits. SAR accepts the SSCS-PDU from SSCS and segments it into N 44-octet SAR-PDUs (this last segment may contain some unused portion).

Message Mode . . . AAL-SDU SSCS-PDU AAL Interface SAR-PDUs Data
SSCS-PDU Header (4 octets) SSCS-PDU Trailer (4 octets) Padding octets ( 0-3 octets ) H SAR-PDU Header SAR-PDU Trailer Unused AAL-SDU AAL Interface SSCS-PDU SAR-PDUs . . . H H H

Message mode is used for “framed data transfer”, e. g
Message mode is used for “framed data transfer”, e.g., high level protocols and applications would fit into this category, e.g., LAPD or Frame Relay would be in message mode. Advantage: Detects errored SSCS-PDUs and discards them. Disadvantage: Requires large buffer capacity.

Streaming Mode The AAL service data unit is passed across the AAL interface in one or more AAL interface data units (AAL IDUs). The transfer of these AAL-IDUs across the AAL interface may occur separately in time and this service provides the transport of the variable length AAL-SDUs. It provides transport of variable length AAL-SDU. The AAL-SDU may be small as 1 octet and is always delivered as 1 unit because only this unit will be recognized by the application.

Streaming mode AAL SDUs Data AAL Interface SSCS-PDU SAR-PDU Trailer
Header (4 octets) AAL SDUs SSCS-PDU Trailer (4 octets) AAL Interface Padding octets(0-3) SSCS-PDU Unused SAR-PDU Header H H H H SAR-PDU Trailer SAR-PDUs

Streaming mode is used for low speed continuous data with low delay requirements which may be as small as 1 octet. 1 block is transferred per cell. Data are presented to AAL in fixed size slots. • Advantage: Transfer delay of a message is low. A single SDU is passed to the AAL layer and transmitted in multiple SSCS-PDUs (pipelined or streamed mode).

AAL 3/4 Details Cell Payload 53 octets Higher layer LI LI
CPI: Common Part Indicator (1 Octet) Btag: Beginning Tag (1 octet) BA Size: Buffer Size Allocation (2 octets) Length: Length of CPCS-PDU Payload (2 octets) AL: Alignment (1 octet) Etag: End Tag (1 octet) PAD: Padding (0-3 octets) ST: Segment Type (2 bits) SN: Sequence Number (4 bits) MID: Multiplexing Identification (10 bits) LI: Length Indicator (6 bits) CRC: Cyclic Redundancy Check Code (10 bits) Higher layer Bytes AAL-SAP H T CPCS-PDU Payload Etag CPI Btag BASize PAD AL Length Length Bytes CPCS SAR T H T H 44 44 SAR-PDU Payload LI CRC SAR-PDU Payload LI CRC ST SN MID ST SN MID … 48 octets ATM-SAP ATM Layer Cell Header Cell Payload ……. 53 octets

The SAR sublayer is depicted in the Figure
The SAR sublayer is depicted in the Figure. The SAR sublayer accepts variable length CS-PDUs from the convergence sublayer and generates SAR-PDUs with a payload of 44 octets, each containing a segment of the CS-PDU. ST (Segment Type) The ST identifies a SAR-PDU as containing a beginning of message (BOM), a continuation of message (COM), an end of message (EOM), or a single segment message (SSM). All BOMs and COMs contain exactly 44 octets where EOM and SSM may have variable lengths. ST ST Field BOM COM EOM SSM 10 00 01 11 Segment Type Value

AAL 3/4 Segmentation User Data CPCS CPCS-H CPCS-PDU Payload CPCS-T PDU
SAR PDU SAR-H SAR-PDU Payload SAR-T BOM SAR PDU COM SAR-H SAR-PDU Payload SAR-T SAR-H SAR-PDU Payload SAR-T SAR PDU EOM ATM Cell ATM-H ATM Cell Payload

SN (Sequence Number) The SN allows the sequence of SAR-PDUs to be numbered modulo 16. SN is incremented by 1 relative to the SN of the previous SAR-PDU belonging to the same AAL connection (numbering modulo 16). These two fields enable the segments of the CS-PDU to be reassembled in the correct sequence and minimize the effect of errors on the reassembly process (counts for lost or misinserted cells, buffer overflows, and underflows & bit errors).

MID (Multiplexing Identification)
* The MID is used to identify a CPCS connection on a single ATM-layer connection. This allows for more than one CPCS connection for a single ATM-layer connection. The SAR sublayer, therefore, provides the means for the transfer of multiple, variable-length CS-PDUs concurrently, over a single ATM layer connection between AAL entities. Different AAL connections on a single ATM layer connection where AAL connections must have identical QoS requirements.

Multiplexing/Demultiplexing is performed on an end-to-end basis.
AAL 3/4 multiplex different streams of AAL/SDUs across a single Virtual Connection. For CO, each logical connection between AAL users is assigned a unique MID value. Thus, the cell traffic from up to 210 different AAL connections can be multiplexed and interleaved over a single ATM connection. For CL service, MID field can be used to communicate a unique identifier associated with each CL user and again traffic from multiple AAL users can be multiplexed.

VC1 VP 3 sessions VC2 Multiplexed onto VC2
From a single host to forward along the same VC and be separated at the destination. All sessions having the same QoS  MID finds which cell belongs to which session. MID  desirable  Carriers charge for each connection set up and for each second for an open connection. If a pair of hosts have several sessions open simultaneously giving each one its own VC  expensive. If 1 VC can handle the job (enough BW use)

AAL 3/4 Multiplexing Example
A data communication terminal has 2 inputs with a 98-octet packets arriving simultaneously destined for a single ATM output port using the AAL 3/4 protocol. Two parallel instances of the CPCS sublayer encapsulate the packets the packets with a header and trailer. These are passed to 2 parallel SAR processes that request the CPCS-PDU or two different MIDs resulting in a BOM, COM, and EOM segment for each input packet. Since all these occurs in parallel, the ATM cells are interleaved on output.

Time SAR-PDUs ATM Cells Input Packets CPCS-PDUs CPCS Payload Payload
Header PAD Trailer CPCS Payload Time SAR-SDU Payload 5 48 octets 98 2 44 4 26 CPCS-PDUs SAR-PDUs ATM Cells Input Packets

LI (Length Indicator) The LI contains the number of octets (binary coded) from the CS-PDU which are included in the SAR-PDU payload. Maximum value is 44. It aids in the detection of reassembly errors such as loss or gain of cells.

CRC ( Cyclic Redundancy Check )
The CRC is a 10-bit sequence used to detect bit errors across the whole SAR-PDU. This includes the CS-PDU segment and hence the user data. Remainder of the division (modulo 2) by the generator polynomial of the product of x10 and the content of the SAR-PDU, including the SAR-PDU header, SAR-PDU payload and LI field of SAR-PDU. The polynomial is G(x) = x10+x9+x5+x4+x+1. Result of CRC calculation is placed with the LSB right justified in the CRC-field (CRC-10 to detect errors).

CPI ( Common Part Indicator )
* The CPI is used to interpret subsequent fields for the CPCS functions in the Header/Trailer. * CPI of 0 indicates that the BAsize field contains an estimate of incoming CPCS-PDU and LI exact size.

BTag ( Beginning Tag ) Sender inserts same value in BTag and ETag for a given CPCS-PDU and changes the value for each successive CPCS-PDU. Receiver checks the values for each successive CPCS-PDU. It also checks the value of BTag in the CPCS-Header with the value of ETag in trailer. BTag and ETag are set to the same value to help error detection.

BASize ( Buffer Allocation Size )
The BAsize indicates the receiver the maximum buffering requirements to receive the CPCS-PDU. BAsize is binary encoded as number of counting units. Size of counting units is identified by the CPI field. BAsize field estimates the incoming CPCS-PDU size in bytes. Length field contains the exact size of CPCS-PDU in bytes. In Message Mode, BAsize value is encoded equal to the CPCS-PDU payload length. In Streaming Mode, BAsize value is encoded equal to or greater than the CPCS-PDU length.

PAD Between end of CPCS-PDU payload and 32-bit
aligned CPCS-PDU trailer, there will be 0-3 unused octets for padding makes the CPCS-PDU an integral multiple of 32 bits to make end system processing more effificient. These are used as filler octets and do not convey any information. It may be set to zero and its value is ignored at the receiving end.

AL (Alignment ) The AL is used to achieve 32-bit alignment in the CPCS-PDU trailer. AL field complements the CPCS-PDU trailer to 32 bits. This unused octet is strictly used as a filler octet and does not convey any information, i.e., it simply makes the trailer a full of 32 bits to simplify the receiver design. AL field should be set to 0. ETag ( End Tag ) The ETag is used to associate the CPCS-PDU trailer with the CPCS-PDU header the transmitter will insert the same value into the BTag and ETag fields.

Reassembly Process * Normally, BOM-COMs-EOM..etc. * The first BOM causes the AAL to note the MID and SN fields, and then look for following COMs which contain the same MID and have correctly incremented the SN fields. * Payload is extracted from each SAR-PDU to from the CPCS-PDU. * Finally, when EOM arrives, in sequence and matching MID value, then the CPCS-PDU is complete. * Final error checking  Matching ETag & BTag and ensuring the Length field matches that the received data.

Remark: being reassembled, COM & EOM SAR-PDU’s arriving
If a BOM occurs with the MID of a current CPCS-PDU being reassembled, COM & EOM SAR-PDU’s arriving with a MID value not corresponding to a current CPCS-PDU are ignored. Those arriving with an out-of-sequence, SN field indicates an error occurred so reassembly aborted.

Example 1 Suppose a sequence of SAR-PDU is transmitted through AAL 3/4. 1. Suppose BOM SAR-PDU is lost on the way. What happens at the receiving end? CS-PDU will be discarded. BOM COM EOM Discard Detect Btag missing Length will also can do it !

2. One of the COM SAR-PDU is lost. What happens at the receiving end?
CS-PDU will be discarded. (same as above: violation of sequence number) Note: SAR layer cannot detect the problem with CS. Since it has LI field (that complete data is not received), ETAG and BTAG fields. BOM COM EOM Discard SN  Violated Buf  CS  SAR will not detect the problem SN  will be missing Length  will also detect.

Btag & Etag will be different!  Discard !!
3. Special Case: Suppose COM & successive EOM & BOM are lost assuming SN is matched. What happens at the receiving end? 2 PDUs get concatenated into the same CS-PDU On the CPCS layer, Btag and Etag will be different for 2 PDUs (Error occurred). Hence, everything will be discarded. EOM BOM COM Concatenated Btag & Etag will be different!  Discard !!

4. 16 consecutive COM SAR-PDUs are lost
4. 16 consecutive COM SAR-PDUs are lost. What happens at the receiving end? When EOM SAR-PDU is received, the CS-PDU will be discarded because it is shorter than BAsize indication (Buffer Allocation size) field. SAR does not recognize that SAR_PDUs were lost because it uses mod 16 SN, and hence after 16 data units, the SN is repeated. However, when EOM is delivered to the CPCS, the CPCS will check the length field in the trailer of CPCS-PDU that it has assembled and will detect the assembled data is shorter than the length field. CPCS will discard it. BOM COM EOM 16 SN modulo 16 BA size will detect it!  SAR will not compare it!

5. Multiple 16 consecutive COM SAR-PDUs are lost
5. Multiple 16 consecutive COM SAR-PDUs are lost. What happens at the receiving end? Any sequence of lost COM SAR-PDU that is multiple of 16 result same as before because mod 16 SN.

Example 2 1. EOM-SAR-PDU of the first block sequence is lost. 
The partial CS-PDU of the first block will be discarded when another BOM SAR-PDU is received. (SAR will send an ABORT signal to CPCS to terminate the Re-assembly) So that the CPCS can release the re-assembly buffer.

2. EOM-SAR-PDU of the first block & BOM-SAR-PDU of the second block are both lost 
i) Sequence numbers or ii) E-Tag of the CPCS trailer of the second block or iii) Length field of the second message will catch the errors. SN of 2 subsequent messages are randomly related (AAL is free to pick any # between 0 and 15 range for initial SN of the first SAR-PDU of a message). Suppose first message ends with a sequence like …, 6, 7 and the next message starts with 6, 7 …, if EOM (SN=6) of first message is lost BOM (SN=7) of second message is lost, sequence will appear correctly, …,6 ,7… So in this case if SN does not help in the SAR, the E-Tag will help. If they agree, then the length field in the CPCS-PDU of second message will catch it.

AAL 5 The new AAL was introduced in the study process of CCITT at the end of 1991. Its description was published in the 1994 CCITT recommendations. Designed for the same class of service as AAL 3/4, it has the advantage of being simpler and requiring less overhead. Unlike AAL 3/4, it allows all 48 octets of the cell information field to be used for the transport of CS-PDU segments, the only SAR protocol information being provided by a bit in the ATM cell header, as explained below.

AAL 5 This means that there is neither multiplexing nor error control at the SAR sublayer. However, there is a CRC field (CRC-32) in the CS sublayer. There are also similarities with AAL 3/4. The two modes of service defined, message and streaming mode are the same as in AAL3/4.

The Convergence Sublayer of AAL 5 has been subdivided into a CPCS part and a SSCS part.
--supports streaming mode and message mode SSCS: uses the same SSCS as AAL 3/4 and provides assured or non-assured data delivery.

The protocol control information field of the SAR sublayer uses the ATM-layer-user-to-ATM-layer-user parameter (AAU) contained in the ATM header to indicate that a SAR-PDU contains the end of a CS-PDU. * When the bit is set to 1, it indicates the end of the CPCS-PDU; when the bit is set to 0 it indicates the continuation or the beginning of a CS-PDU. * This is necessary to enable the SAR to copy with reassembly of the CS-PDU in the presence of errors. * If no indication of the end of the CS-PDU was provided, the loss of a cell, and hence the loss of a segment of the CS-PDU, would mean that all subsequent reassembly operation would be incorrect. * By indicating the end of the CS_PDU, the loss of a single cell would limit the error to one CS-PDU, unless the lost cell contained the end indication in which case the error would be restricted to 2 CS-PDUs.

User Network Interface (UNI) Network Network Interface (NNI)
ATM CELL STRUCTURE 1 2 3 4 5 : 53 Octet HEADER (5 octets) Octets are sent in increasing order  1,2,3 … Within an octet the bits are sent in decreasing order  8,7,6,5,4 ... PAYLOAD (48 octets) User Network Interface (UNI) Cell Structure Network Network Interface (NNI) Cell Structure GFC : Generic Flow Control VPI : Virtual Path Identifier VCI : Virtual Channel Identifier PT : Payload Type PR : Priority HEC : Header Error Control 1 2 3 4 5 : 53 GFC VPI 1 2 3 4 5 : 53 VPI VPI VCI VPI VCI VCI VCI VCI PT PR VCI PT PR HEC HEC PAYLOAD (48 octets) PAYLOAD (48 octets) 11

First Bit  1  Network Management or Maintenance Function
PAYLOAD TYPE (PT) First Bit   User Information First Bit   Network Management or Maintenance Function Second Bit  Whether CONGESTION has been experienced or not. Third Bit  known as AAU (ATM-User-to-ATM-User) used in AAL5 to convey information between end users.

Remark: Lack of LI field  No way for SAR to distinguish between CPCS-PDU octets and filler in the lost SAR-PDU. There exists no way for SAR entity to find the CPCS-PDU trailer in the last SAR-PDU. To avoid these situations - CPCS-PDU payload be added out so that the last bit of the CPCS trailer occurs as the last bit of the final SAR-PDU. No Sequence Number  Receiver must assure that all SAR-PDUs arrive in proper order for reassembly. CRC should guarantee that.

G(x)=x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1
Lack of MID: It is not possible to interleave cells from different CPCS-PDUs. (Each successive SAR-PDU carries a portion of the current CPCS-PDU or the first block of the next CPCS-PDU). 32-bit CRC for AAL 5  G(x)=x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1 Probability of undetected cell misordering is 2-32.

AAL 5 CPCS-PDU Payload PAD CPCS-UU CPI Length CRC 8 Octets
Trailer CPCS-PDU Payload CPCS-UU CPI Length CRC 8 Octets PAD: Padding (0 to 47 octets); Length: Length of CPCS-SDU (2 octets) CPCS-UU: CPCS user-to-user indication (1 octet) CRC: Cycle Redundancy Check (4 octets) CPI: Common Part Indicator (1 octet)

SAR PDU Format for AAL5 SAR-PDU Payload Cell Header PT
SAR-PDU (48 Octets) PT (Payload Type): The PT belongs to the ATM header and it conveys the value of the ATM-layer-user-to-ATM-layer-user indication.

AAL 5 Example * Two 98-byte packets arrive simultaneously.
* Two parallel instances of this CPCS sublayer. * Add a trailer to each packet. * Note that the entire packet does not have to be received before it can begin the SAR function as in AAL 3/4 to insert the correct buffer allocation size. * The packets are segmented by 2 parallel SAR processes. * Here these cells are destined for the same VPI/VCI and hence only one can be sent at a time

Input Packets CPCS-PDUs SAR-PDUs ATM Cells CPCS Payload Time octets 98
48 5 Time octets PAD SAR-SDU Header Trailer 38 8 98

Example 1. Single bit error in 1 of the SAR-PDUs occurs.
CS-PDU will be discarded when lost SAR-PDU is received due to CRC failing. In trailer  (CRC - checking CS layer  AAU=1 after getting EOM-SAR 2. Suppose one of the cells with AAU=0 is lost.  Find SAR-PDU (AAU=1) will cause the CPCS check the LENGTH FIELD of the CPCS-PDU trailer. Trailer is always in the last cell (AAU=1), and CRC will ALSO be checked. 3. One of the cells AAU=1 is lost?  Error either by CRC or by mismatch of the length field in CPCS-PDU trailer OF THE NEXT ARRIVING CELL with AAU=1.

When trailer for next message (the one that is lost with AAU=1 cell) is received. This will result in a loss of both corrupted and the next message. REMARK: The LEN field of CS-PDU is limited to 2 bytes. So a max. of 64K bytes can be sent before the end of message error can be triggered off. If the total size of both CS-PDUs is greater than 64K, receiver will detect the error.

Effect of Cell Loss on Reassembly Cell Loss
Sources of Cell Losses a) Errors on the transmission media b) Discarding cells for congestion control c) Processing errors in switching nodes and end-points Effect of Cell Loss on Reassembly Cell Loss AAL 3/4 may pass partially reassembled CPCS-PDUs to the user along with an error indicator. AAL 5 Can only pass up an error indicator

AAL 3/4 Reassembly Receiver SAR/CPCS rejects all COM & EOM cells passed to it. BOM is required. If BOM lost, the entire CPCS-PDU is discarded. Incorrect SN progression between SAR-PDU reveals the loss of a COM. If a multiple of 16 consecutive cells is lost, then the SN wraps around, but the loss of data is detected by the CPCS-PDU being undersized. To detect EOM loss, two methods exist:

METHOD 1: If the BOM of the next CPCS-PDU on the same MID arrives before the EOM for the current CPCS-PDU, then the partially reassembled CPCS- PDU must be released by the SAR/CPCS. Entire partially reassembled CPCS-PDU received to that point is considered valid & passed to the AAL user along with an error indication. This is only the case when EOM is lost or where a cell burst knocks out some COMs followed by the EOM. A cell loss burst that knocks out the EOM & the following BOM & slips past the SN checks, will be detected when B-tag & E-tag fields fail to match.

REMARK: The length indicator may fail to pick up this error, if the cell burst loses as many cells as are added by concatenating the 2 CPCS-PDU fragments. In this case only the first 44 bytes of the first CPCS-PDU may be legitimately retrieved. For the second one ---> bad luck ....

METHOD 2: AAL 5 Reassembly Errors
Attach a timer to each CPCS-PDU under reconstruction & signal an error when it is not reassembled within a certain time frame. AAL 5 Encapsulation & Seq-Checking DO NOT EXIST as in AAL 3/4. Reassembly Errors are detected only when CPCS-PDU trailer arrives. Impossible to know how much has been received already is correct. Single-bit errors in SAR- PDU are not picked up until the CPCS-PDU CRC is calculated ---> if incorrect the entire CPCS-PDU is discarded.

Lost of cells with AAU=0, detected by an incorrect CRC when the trailer arrives. If CRC fails to flag the error, the length field mismatch ensures the CPCS-PDU is discarded. Loss of Cells with AAU=1 detected in 3 ways SAR-PDU of the following CPCS-PDU may be appended to the first, resulting in a CRC error (or length mismatch). AAL may enforce second CPCS-PDU can flag an error and cause the assembled data to be discarded. A timer attached to CPCS-PDU reassembly. If it expires, assembled is discarded.

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