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 ADCB YesNo ConstantVariable Connectionless Connection Oriented 12¾ 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) Sublayer 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 CS-PDU Trailer SAR-PDU Payload SAR-PDU Header SAR-PDU Trailer ATM-SAP ATM-SDU Cell information Field (Cell Payload) Cell Header PL-SAP CS-PDU No SAP is defined between CS and SAR SAR-PDU ATM Cell ATM Layer Physical Layer Segmentation And Reassembly (SAR) Sublayer Convergence Sublayer(CS) AAL Interfaces AAL Segmentation of CS-PDU

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

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

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 AAL Type 1 (Cont.) PCI SAR-PDU Header CS-PDU Header CS-PDU Trailer

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.)

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.

STACK ATM Convergence Sublayer- accepts 124-byte blocks from user- appends 4-byte FEC- writes to matrix “row”- forwards CS-PDU to SARwhen 47 blocks (rows) havebeen written Segmentation/Re-assembly Sublayer- reads matrix “columns”(47bytes) - effect: interleaving Adaptation Layer Type 1 (AAL1) Forward Error Correction No Retransmission * (124,128) Reed-Solomon Code * Polynomial undefined * Corrects 2 errored bytes per row * Corrects 4 “erasure” bytes (knows position) * Uses interleaving

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

AAL 1 User Data Bit Stream CPCS-PDU Payload AAL-SAP … SAR-PDU Payload H SAR-PDU Payload H SAR-PDU Payload H ATM-SAP Cell Payload 1B1B47B 48 Bytes H Cell Payload H H … 53 Bytes 5B5B Higher Layers CS SAR ATM Layer AAL

SAR-PDU of AAL 1 Cell Header SNSNPSAR-PDU Payload 4 bits SAR-PDU Header 1 Octet47 Octets 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 misinserted cells (Error Detection & Correction)

SAR-PDU Header of AAL 1 CSI Sequence Count CRC Even Parity 1 bit3 bits 1 bit SN FieldSNP Field

CSI Field Sequence Count 0,.., 7 CSI bit used to transfer timing info. or info. about data structure. 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 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) SNSNPSAR-PDU Payload (User Data) Non P-format 1 Octet47 Octets SAR-PDU Header SAR-PDU (48 Octets) SAR-PDU PayloadSNSNP 1 Octet 46 Octets SAR-PDU Header (CSI = 1) Reserved for Pointer P-format

SAR-PDU Header Structure Pointer Field User Data P-Format Sequence Counter 0,2,4,6 Reserved BitOffset Field 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 Bits 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.

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

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.

p 47=376 SN=0 CSI=1 P-Format 46=368 SN=1 CSI=0 Non-P-Format 46=368 SN=2 CSI=1 P-Format 47=376 SN=3 CSI=0 Non-P-Format 0-1 = 93 Octets2-3=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. EXAMPLE: STRUCTURED DATA TRANSFER DS1 Signal CS-PDUs

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 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. octets Header SAR- PDU SAR- PDU octets bits Time DS1 SignalSRTS CSSAR-PDUs ATM Cells Header SAR- PDU SAR- PDU octets Header SAR- PDU SAR- PDU octets RTS EXAMPLE: UNSTRUCTURED DATA TRANSFER

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 (Cont.) 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.

TIMING (CLOCK) RECOVERY TECHNIQUES IN AAL 1 1.Adaptive Clocking in AAL 1 (No Network clock is available). 2.Synchronous Residual Time Stamp Approach (SRTS) (Global Network Clock is available)

Common network reference clock is not available!!! 1. Adaptive Clocking (Receiver) Cells PLAYOUT BUFFER Used for Transfer Delay Variable Receiver reads info. with a local clock. Receiver writes received info field in this buffer. PLL (Phase Lock Loop) Provides local clock. (Content) Filling level of the buffer is used to control the frequency of the 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. 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 Network Data FIFO Data Terminal Filling Level Local Clock Jitter Filter PLL. Adaptive Clocking in AAL 1

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! Sender NETWORK Common Network Clock Receiver Difference between the local and network clocks. Local Clock TIMESTAMP CSI field Sequence # field Odd # of segments Difference between 2 clocks Transport this info. in odd numbered Cells (CSI Field) to destination Local Clock

Source Transmitter (Assumed) Common Network clock is available Source (local) clock is not synchronized with it. 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 M nom (fixed and known for the service) and the actual value of M may vary anywhere within a certain range (M min & M max ) around this nominal value M nom. The actual value of M will be the sum of M nom and a residual part. By transmitting the residual part, the receiver has enough info to construct the source clock.

t Tolerance Source Frequency (f s ) Source clock N cycles T seconds t Derived Network Frequency (f nx ) MMM minnommax yy 2 4 Residual value M M nom

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

Source clock fs is divided by N to sample the 4-bit counter C t driven by the network clock f nx once every N = 3008 = 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 even SAR-PDUs have CSI=0. The CSI bits in SAR-PDUs with even SN values are used in SDT. 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!!!!

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 AAL 2

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 AAL SAP Service Specific Convergence Sublayer (SSCS) Common Part Sublayer (CPS) ATM SAP ATM Layer 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 AAL-SDU SSCS-PDU Trailer User Packet SSCS-PDU Header SSCS-PDU AALAAL ATMATM CPS-SDU CPS Packet CPS-Packet Header CS-Packet Payload CPS-PDU Header CPS-Packet CPS- Packet PAD CPS-PDU (48 octets) ATM Header ATM Cell Payload ATM Cell Start Field

CPS-PACKET FORMAT CPS-Packet (45 octets default 64 octets optimal) CPS-INFOCID| PPT|LIUUIHEC CPS-Packet Header (3 octets) CPS-Packet Payload (Variable length) CID: Channel Identifier (8 bits): 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 ATM PHY AAL2 ATM PHY ATM Network ABCD A’B’C’D’ AAL2 can multiplex several data streams

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

CPS-PACKET FORMAT (Contd) 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.

CPS-PACKET FORMAT (Contd) * 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 = L P => LI = L P -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.

CPS-PACKET FORMAT (Contd) * 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 Future use Layer management

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

CPS-PDU FORMAT SNPCPS-Packet PADOSF CPS-PDU Payload (47 octets) Start Field (STF) indicates the position of the first packet CPS-PDU (48 octets) CPS-PDU Header 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)

ATM Header Packets are streamed into successive payloads Cell Period Padding: All 0’s First Packet CPS Packet ATM Cell Pointer in OSF points to find start of a CPS packet in 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.

CPS Packets ATM Layer User 1User 2User 3User 4User 5 48 bytes ATM Header ATM Header EXAMPLE 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. Rt VBR Sources CPS PDUs SAR STF … … … …

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

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!!