University of Arizona ECE 478/ Packet Relay u Relaying: Switching packets asynchronously u Types of packet relay: 1. Cell relay: s Fixed-size packets s Used in ATM and SMDS 2. Frame relay: s Variable-length packets

University of Arizona ECE 478/ Advantages of Fixed-Size Packets u Simple switch-hardware design n Hardware store-and-forward is easier n Dynamic storage allocation is easier (no memory fragmentation) u More deterministic scheduling (for performance guarantees) u High degree of parallelism in large switches n Synchronized multiprocessors n Multiple levels of buffering can be easily clocked

University of Arizona ECE 478/ Disadvantages of Fixed-Size Packets u Segmentation and reassembly (SAR) u Overhead in the case of small-size cells

University of Arizona ECE 478/ Integrated Services Digital Network (ISDN) u Evolutionary technology from digital telephony u Intended as a digital interface for voice and data u More popular in Europe u ISDN terminology: n Functional grouping: A set of capabilities in an ISDN user interface (similar to layer functions) n Reference points: Logical interfaces between functional groupings (similar to SAPs)

University of Arizona ECE 478/ ISDN Specification u Types of functional groupings: n Terminal Type 1 and 2 (TE1 and TE2) n Network Termination 1 and 2 (NT1 and NT2) u Four reference points: R, S, T, and U

University of Arizona ECE 478/ Typical ISDN Topology TE 1NT 2NT 1LT/ET S TU Network TANT 2NT 1LT/ET S TU TE 2 R

University of Arizona ECE 478/ Typical ISDN Topology (Cont.) u TE1 = end-user ISDN terminal u TE2 = non-ISDN terminal u TA = terminal adaptor u NT1 = device that connects 4-wire subscriber wiring to 2-wire local loop. Responsible for physical layer functions u NT2 = more complex than NT1. Contains layer 2 and 3 functions. Performs concentration

University of Arizona ECE 478/ ISDN Access Rates u Basic Rate Interface (BRI) n Two 64-kbps B channels for data n One 16-kbps D channel for control (out of band) n Designated as 2B+D n Up to eight TE1s can be multiplexed onto a BRI u Primary Rate Interface (PRI) n kbps B channels for data (total of Mbps) n One 64-kbps D channel for control n Designated as 23B+D n In Europe the PRI consists of 31B+D (E1 line)

University of Arizona ECE 478/ Broadband ISDN (B-ISDN) u Set of protocols that is standardized by ITU-T u Started as an extension of ISDN. However, ISDN and ISDN interfaces are NOT compatible u ATM is the transport protocol for B-ISDN

University of Arizona ECE 478/ Driving Forces Behind B-ISDN u Emergence of bandwidth-intensive applications u Desire to integrate data, voice, and video over a single channel (why?) u Need to provide performance guarantees for real- time applications

University of Arizona ECE 478/ Synchronous Transfer Mode (STM) u Based on time-division multiplexing (TDM) u Each connection is reserved a time slot u Bandwidth is wasted if user is idle Time Division Multiplexer Stream #1 frame wasted bandwidth

University of Arizona ECE 478/ Asynchronous Transfer Mode (ATM) u Based on statistical (i.e., asynchronous) multiplexing u Bandwidth is allocated on demand u Each packet (cell) carries its connection ID Statistical Multiplexer Stream #

University of Arizona ECE 478/ So What is ATM? u Transport technology for B-ISDN u Based on fixed-length packets (cells) u A cell consists of 53 bytes: n User payload: 48 bytes n Cell header: 5 bytes u Hardwired store-and-forward architecture u Connection-oriented fast packet switching!

University of Arizona ECE 478/ What Does ATM Provide? u Efficient bandwidth utilization (via statistical multiplexing) u Quality of service (QoS) n Maximum cell transfer delay n Cell delay variation (jitter) n Cell loss rate u Cell sequencing (important for real-time apps) u Unified transport solution for diverse traffic types u Scalability

University of Arizona ECE 478/ Cell Size Considerations u Transmission efficiency n PL  no. of payload bytes n HD  no. of header bytes u Impact of cell loss on voice quality n Loss of 32-byte cell  4 ms interruption n > 32 ms interruption is quite disruptive u Echo cancellation

University of Arizona ECE 478/ Physical Layer ATM Layer ATM Adaptation Layer Higher Layers Control Plane User Plane Layer Management Plane Management B-ISDN Protocol Stack u Three “planes”: n User plane n Control plane n Management plane s Plane management s Layer management

University of Arizona ECE 478/ B-ISDN Physical Layer u Consists of two sublayers: n Transmission Convergence (TC) sublayer n Physical Medium (PM) sublayer u Functions of the TC sublayer: n Generation/recovery of transmission frames n Transmission frame adaptation n Cell delineation n Cell header processing (HEC generation) n Cell rate decoupling

University of Arizona ECE 478/ B-ISDN Physical Layer (Cont.) u Functions of the PM sublayer: n Bit timing n Line encoding n Other medium-dependent functions

University of Arizona ECE 478/ Common Physical Layer Interfaces u Multimode Fiber: n 155 Mbps SONET STS-3c (SDH) n 100 Mbps 4B/5B coding u Single-Mode Fiber at 100 Mbps 4B/5B coding u Coax cable at 45 Mbps DS3 rate u Subrates (for ATM over unshielded twisted pair) n Mbps n Mbps n Mbps

University of Arizona ECE 478/ SONET Hierarchy

University of Arizona ECE 478/ SONET STC-3c Physical Layer Transmission Convergence Sublayer Physical Media Dependent Sublayer - HEC generation/verification - Cell scrambling/descrambling - Cell delineation - Path signal identification - Frequency justification/Pointer processing - Multiplexing - Scrambling/descrambling - Transmission frame generation/recovery - Bit timing, Line coding - Physical medium B-ISDN Specific Functions B-ISDN Independent Functions

University of Arizona ECE 478/ ATM Layer Functions u Cell multiplexing and demultiplexing u VPI/VCI translation u Traffic management (e.g., shaping, policing) u Cell header processing (except for the HEC field) u Cell rate decoupling (for SONET and DS3) u OAM functions Switch

University of Arizona ECE 478/ ATM Cell Format OCTET GFC VPI VCI PTCLP HEC Cell Payload (48 octets) BIT

University of Arizona ECE 478/ Remarks u The previous cell format is for the User-to- Network Interface (UNI) n Between an end-system and an ATM switch n An end system could be, an IP router with an ATM interface, a PC/workstation, or a LAN switch u In the Network-to-Network Interface (NNI), the GFC field is used as part of the VPI field n NNI is typically between two ATM switches n Two flavors of NNI are used (Private and Public)

University of Arizona ECE 478/ ATM Switching UNI = User Network Interface PNNI = Private Network Node Interface AAL ATM Physical AAL ATM Physical ATM Network UNI PNNI User AUser B

University of Arizona ECE 478/ Generic Flow Control (GFC) u Four bits in the cell header u Only in cells at UNI (intermediate switches overwrite it) u Intended for link-by-link flow control u Typically, GFC is not used GFC VPI VCI PT CLP HEC Cell Payload (48 octets)

University of Arizona ECE 478/ Connection Identifiers u ATM uses a 2-level connection hierarchy: n Virtual channel connection (VCC or VC) n Virtual path connection (VPC or VP) u A VP is a bundle of VCs u Each connection has a VP identifier (VPI) and a VC identifier (VCI) u Cell switching is performed based on: n VPI alone (VP switching), or n Both VCI and VPI (VP/VC switching) GFC VPI VCI PT CLP HEC Cell Payload (48 octets)

University of Arizona ECE 478/ Connection Identifiers (Cont.) u Some VPI and VCI values are reserved for signaling and control functions: n Connection requests: VPI=0, VCI=5 n PNNI topology state packets: VPI=0, VCI=18 n Resource Management (RM) cells: VCI=6 n VCI values < 32 are reserved for control functions u VCIs and VPIs have local scope

University of Arizona ECE 478/ VP and VP/VC Switching VCI 1 VCI 2 VPI 1 VPI 3 VCI 1 VCI 2 VP Switching VCI 1 VCI 2 VPI 1 VPI 3 VCI 5 VCI 3 VP/VC Switching VPI 4

University of Arizona ECE 478/ Payload TypeMeaning Payload Type (PT) Field 000user cell, no congestion, cell type 0 001user cell, no congestion, cell type 1 010user cell, congestion indication, cell type 0 011user cell, congestion indication, cell type 1 100OAM cell (link-by-link) 101OAM cell (end-to-end) 110RM cell (used in ABR service) 111reserved for future use GFC VPI VCI PT CLP HEC Cell Payload (48 octets)

University of Arizona ECE 478/ Cell Loss Priority (CLP) u CLP = 1 for low priority u CLP = 0 for high priority u CLP is used in selective cell discarding to: n penalize greedy users (traffic policing) n request differential QoS (e.g., coded video) u CLP is a key parameter in traffic management GFC VPI VCI PT CLP HEC Cell Payload (48 octets)

University of Arizona ECE 478/ Header Error Control (HEC) u Checksum over cell header u Performed by the physical layer u Corrects all single-bit errors u Detects about 84% of multiple-bit errors n Cells with multiple errors are discarded GFC VPI VCI PT CLP HEC Cell Payload (48 octets)

University of Arizona ECE 478/ ATM Layer in the OSI Model u Different opinions: n Network layer (since it performs routing) n Data-link layer (in IP over ATM and in MPOA) n Physical layer (in LAN emulation) u Conclusion: n There is no 1-to-1 correspondence between B-ISDN and OSI layered models

University of Arizona ECE 478/ ATM Adaptation Layer (AAL) u Purpose: Adapt upper “applications” to ATM layer u Different applications have different needs  Four AALs are used u AALs were originally classified according to: n Real-time versus non-real-time n Connection oriented versus connectionless n Constant bit rate (CBR) versus variable bit rate (VBR)

University of Arizona ECE 478/ AAL Functions u Segmentation and reassembly of upper-layer PDUs u Delay variation recovery u Cell losses recovery u Circuit emulation (e.g., voice over ATM) u Connectionless service over ATM u Clock synchronization u And others...

University of Arizona ECE 478/ AAL Structure Convergence Sublayer (service specific part) Convergence Sublayer (common part) Segmentation & Reassembly Sublayer Note: In some AALs, the convergence sublayer consists of one part only

University of Arizona ECE 478/ Types of AAL 1. AAL1 n Intended for TDM-like circuit emulation n Supports clock synchronization and timing recovery n Provides sequence numbers 2. AAL2 n Optimized for the transport of VBR video traffic n Provides timing information and sequence numbers n Not quite popular

University of Arizona ECE 478/ Types of AAL (Cont.) Types of AAL (Cont.) 3. AAL3/4 n Provides both connectionless and connection-oriented services over ATM n Supports the multiplexing of messages from multiple users over the same VC n Not popular either 4. AAL5 n Intended for data applications (e.g., TCP over AAL5) n Provides minimal functionality n Most popular AAL

University of Arizona ECE 478/ Barriers to the Deployment of ATM u Lack of “killer applications” u Cost of new infrastructure u Other competitive technologies for LANs u Uncertainty about the new technology u Incomplete standards ATM is mainly being deployed in the Internet backbone and within specialized networks