William Stallings Data and Computer Communications Chapter 11

William Stallings Data and Computer Communications Chapter 11 Asynchronous Transfer Mode and Frame Relay

Protocol Architecture z Similarities between ATM and packet switching y. Transfer of data in discrete chunks y. Multiple logical connections over single physical interface z In ATM flow on each logical connection is in fixed sized packets called cells z Minimal error and flow control y. Reduced overhead z Data rates (physical layer) 25. 6 Mbps to 622. 08 Mbps

Protocol Architecture (diag)

Reference Model Planes z User plane y. Provides for user information transfer z Control plane y. Call and connection control z Management plane y. Plane management xwhole system functions y. Layer management x. Resources and parameters in protocol entities

ATM Logical Connections z Virtual channel connections (VCC) z Analogous to virtual circuit in X. 25 z Basic unit of switching z Between two end users z Full duplex z Fixed size cells z Data, user-network exchange (control) and network-network exchange (network management and routing) z Virtual path connection (VPC) y. Bundle of VCC with same end points

ATM Connection Relationships

Advantages of Virtual Paths z Simplified network architecture z Increased network performance and reliability z Reduced processing z Short connection setup time z Enhanced network services

Call Establishment Using VPs

Virtual Channel Connection Uses z Between end users y. End to end user data y. Control signals y. VPC provides overall capacity x. VCC organization done by users z Between end user and network y. Control signaling z Between network entities y. Network traffic management y. Routing

VP/VC Characteristics z Quality of service z Switched and semi-permanent channel connections z Call sequence integrity z Traffic parameter negotiation and usage monitoring z VPC only y. Virtual channel identifier restriction within VPC

Control Signaling - VCC z Done on separate connection z Semi-permanent VCC z Meta-signaling channel y. Used as permanent control signal channel z User to network signaling virtual channel y. For control signaling y. Used to set up VCCs to carry user data z User to user signaling virtual channel y. Within pre-established VPC y. Used by two end users without network intervention to establish and release user to user VCC

Control Signaling - VPC z Semi-permanent z Customer controlled z Network controlled

ATM Cells z Fixed size z 5 octet header z 48 octet information field z Small cells reduce queuing delay for high priority cells z Small cells can be switched more efficiently z Easier to implement switching of small cells in hardware

ATM Cell Format

Header Format z Generic flow control y. Only at user to network interface y. Controls flow only at this point z Virtual path identifier z Virtual channel identifier z Payload type ye. g. user info or network management z Cell loss priority z Header error control

Generic Flow Control (GFC) z Control traffic flow at user to network interface (UNI) to alleviate short term overload z Two sets of procedures y. Uncontrolled transmission y. Controlled transmission z Every connection either subject to flow control or not z Subject to flow control y. May be one group (A) default y. May be two groups (A and B) z Flow control is from subscriber to network y. Controlled by network side

Single Group of Connections (1) z Terminal equipment (TE) initializes two variables y. TRANSMIT flag to 1 y. GO_CNTR (credit counter) to 0 z If TRANSMIT=1 cells on uncontrolled connection may be sent any time z If TRANSMIT=0 no cells may be sent (on controlled or uncontrolled connections) z If HALT received, TRANSMIT set to 0 and remains until NO_HALT

Single Group of Connections (2) z If TRANSMIT=1 and no cell to transmit on any uncontrolled connection: y. If GO_CNTR>0, TE may send cell on controlled connection x. Cell marked as being on controlled connection x. GO_CNTR decremented y. If GO_CNTR=0, TE may not send on controlled connection z TE sets GO_CNTR to GO_VALUE upon receiving SET signal y. Null signal has no effect

Use of HALT z To limit effective data rate on ATM z Should be cyclic z To reduce data rate by half, HALT issued to be in effect 50% of time z Done on regular pattern over lifetime of connection

Two Queue Model z Two counters y. GO_CNTR_A, GO_VALUE_A, GO_CNTR_B, GO_VALUE_B

Header Error Control z 8 bit error control field z Calculated on remaining 32 bits of header z Allows some error correction

HEC Operation at Receiver

Effect of Error in Cell Header

Impact of Random Bit Errors

Transmission of ATM Cells z 622. 08 Mbps z 155. 52 Mbps z 51. 84 Mbps z 25. 6 Mbps z Cell Based physical layer z SDH based physical layer

Cell Based Physical Layer z No framing imposed z Continuous stream of 53 octet cells z Cell delineation based on header error control field

Cell Delineation State Diagram

Impact of Random Bit Errors on Cell Delineation Performance

Acquisition Time v Bit Error Rate

SDH Based Physical Layer z Imposes structure on ATM stream z e. g. for 155. 52 Mbps z Use STM-1 (STS-3) frame z Can carry ATM and STM payloads z Specific connections can be circuit switched using SDH channel z SDH multiplexing techniques can combine several ATM streams

STM-1 Payload for SDH-Based ATM Cell Transmission

ATM Service Categories z Real time y. Constant bit rate (CBR) y. Real time variable bit rate (rt-VBR) z Non-real time y. Non-real time variable bit rate (nrt-VBR) y. Available bit rate (ABR) y. Unspecified bit rate (UBR)

Real Time Services z Amount of delay z Variation of delay (jitter)

CBR z Fixed data rate continuously available z Tight upper bound on delay z Uncompressed audio and video y. Video conferencing y. Interactive audio y. A/V distribution and retrieval

rt-VBR z Time sensitive application y. Tightly constrained delay and delay variation z rt-VBR applications transmit at a rate that varies with time z e. g. compressed video y. Produces varying sized image frames y. Original (uncompressed) frame rate constant y. So compressed data rate varies z Can statistically multiplex connections

nrt-VBR z May be able to characterize expected traffic flow z Improve Qo. S in loss and delay z End system specifies: y. Peak cell rate y. Sustainable or average rate y. Measure of how bursty traffic is z e. g. Airline reservations, banking transactions

UBR z May be additional capacity over and above that used by CBR and VBR traffic y. Not all resources dedicated y. Bursty nature of VBR z For application that can tolerate some cell loss or variable delays ye. g. TCP based traffic z Cells forwarded on FIFO basis z Best efforts service

ABR z Application specifies peak cell rate (PCR) and minimum cell rate (MCR) z Resources allocated to give at least MCR z Spare capacity shared among all ARB sources z e. g. LAN interconnection

ATM Adaptation Layer z Support for information transfer protocol not based on ATM z PCM (voice) y. Assemble bits into cells y. Re-assemble into constant flow z IP y. Map IP packets onto ATM cells y. Fragment IP packets y. Use LAPF over ATM to retain all IP infrastructure

ATM Bit Rate Services

Adaptation Layer Services z Handle transmission errors z Segmentation and re-assembly z Handle lost and misinserted cells z Flow control and timing

Supported Application types z Circuit emulation z VBR voice and video z General data service z IP over ATM z Multiprotocol encapsulation over ATM (MPOA) y. IPX, Apple. Talk, DECNET) z LAN emulation

AAL Protocols z Convergence sublayer (CS) y. Support for specific applications y. AAL user attaches at SAP z Segmentation and re-assembly sublayer (SAR) y. Packages and unpacks info received from CS into cells z Four types y. Type 1 2 3/4 5

AAL Protocols

Segmentation and Reassembly PDU

AAL Type 1 z CBR source z SAR packs and unpacks bits z Block accompanied by sequence number

AAL Type 2 z VBR z Analog applications

AAL Type 3/4 z Connectionless or connected z Message mode or stream mode

AAL Type 5 z Streamlined transport for connection oriented higher layer protocols

CPCS PDUs

Example AAL 5 Transmission

Frame Relay z Designed to be more efficient than X. 25 z Developed before ATM z Larger installed base than ATM z ATM now of more interest on high speed networks

Frame Relay Background - X. 25 z Call control packets, in band signaling z Multiplexing of virtual circuits at layer 3 z Layer 2 and 3 include flow and error control z Considerable overhead z Not appropriate for modern digital systems with high reliability

Frame Relay - Differences z Call control carried in separate logical connection z Multiplexing and switching at layer 2 y. Eliminates one layer of processing z No hop by hop error or flow control z End to end flow and error control (if used) are done by higher layer z Single user data frame sent from source to destination and ACK (from higher layer) sent back

Advantages and Disadvantages z Lost link by link error and flow control y. Increased reliability makes this less of a problem z Streamlined communications process y. Lower delay y. Higher throughput z ITU-T recommend frame relay above 2 Mbps

Protocol Architecture

Control Plane z Between subscriber and network z Separate logical channel used y. Similar to common channel signaling for circuit switching services z Data link layer y. LAPD (Q. 921) y. Reliable data link control y. Error and flow control y. Between user (TE) and network (NT) y. Used for exchange of Q. 933 control signal messages

User Plane z End to end functionality z Transfer of info between ends z LAPF (Link Access Procedure for Frame Mode Bearer Services) Q. 922 y. Frame delimiting, alignment and transparency y. Frame mux and demux using addressing field y. Ensure frame is integral number of octets (zero bit insertion/extraction) y. Ensure frame is neither too long nor short y. Detection of transmission errors y. Congestion control functions

LAPF Core Formats

User Data Transfer z One frame type y. User data y. No control frame z No inband signaling z No sequence numbers y. No flow nor error control

Required Reading z Stallings Chapter 11 z ATM Forum Web site z Frame Relay forum