ECS5365 Lecture 5 ATM Protocols and Networks

1

Copyright © Monash University

ECS5365 Lecture 5ATM Protocols and Networks

Philip BranchCentre for Telecommunications and Information

Engineering (CTIE)

Monash University

2


ATM Protocols and Networks Overview

• ATM

• Virtual Circuits

• ATM Cell

• Services

• ATM Adaptation Layers

• Examples

3


Circuit Switching vs Packet Switching

• Circuit switching– fixed delay

– low latency

– bandwidth allocation difficult

– signalling slow

– excellent for voice

– not adaptable for bursty traffic

• Packet switching– delay is variable

– latency is high

– bandwidth allocation flexible

– signalling simple

– difficult for voice

– very flexible for bursty traffic

4


Asynchronous and Synchronous Time Division Multiplexing

• Synchronous Time Division Multiplexing– eg narrow band ISDN– fixed assignment of slots to channels in frames– position specifies channel

• Asynchronous Time Division Multiplexing– ATM– no fixed assignment of slots to channels in frames– tag needed to specify channel

5


ATM

• Asynchronous Transfer Mode– Also: Asynchronous Time Division Multiplexing

• Connection Oriented Fast Packet Switching• Small header

– Mostly next hop information

• Fixed size information field• Bandwidth flexibility, rather then efficiency

6


Virtual Circuits in ATM

• Non Broadcast Multiple Access (NBMA)

• Virtual circuits across same medium

• Order preserving– simplifies switch design

7


Types of Virtual Circuits

• Permanent Virtual Circuits– Established once manually

• Semi-permanent Virtual Circuits– Time based

• Switched Virtual Circuits– On demand

8


The ATM connection hierarchy

• Virtual Channel

• Virtual Path

• Physical Medium

9


Switching on Virtual Paths and Channels

• Multiple Virtual Channels can be bundled into a single Virtual Path

• Switching can be on path or circuit

• Useful for interconnection via second party

10


Tunnelling SVCs through PVPs

• Carriers may offer only Permanent Virtual Paths

• Can ‘tunnel’ Switched Virtual Channels through them

11


ATM Layer Functions

• Cell multiplexing and demultiplexing

• Virtual Path and Virtual Circuit Identifier translation

• Cell header generation and extraction

12


ATM Cells

• 5 Byte header– mostly next hop information

• 48 Byte Payload– Compromise between 32 and 64 bytes

• Different format for UNI and for NNI– UNI has GFC field

13


Fields in an ATM Cell

• GFC - Generic Flow Control

• VPI - Virtual Path Indicator

• VCI - Virtual Connection Indicator

• PTI - Payload Type Indicator

• CLP- Cell Loss Priority

• HEC- Header Error Check

• Payload

14


Generic Flow Control

• Present in UNI but not in NNI

• Use not fully specified

• Intended for priority scheme

• Rarely used

15


VPI / VCI

• Virtual Path Identifier / Virtual Channel Identifier

• Local only to the switch– Will change as cell passes through switch

• Index into lookup tables setup at connection time

16


Payload Type

• 3 bits

• bit 1 – 0 = user cell– 1 = management cell

• bit 3 in user cells– signalling bit– used to signal end of datagram in AAL5

17


Cell Loss Priority

• 1 bit

• Switch must drop CLP=1 cells before CLP=0 cells

• Can be set by network– non-conforming cells

• Can be set by application– lower priority cells

18


Header CRC

• Cyclic Redundancy Check

• Calculated over 5 byte cell header

• Can correct single bit and detect large class of multiple bit errors

• Recalculated at each hop in the ATM network

19


Problems with ATM UNI Header

• Generic Flow Control– Better done at higher layer

• GFC limits number of VCI values

• User-network interface and network-network interface distinction artificial

20


Quality of Service

– Traffic parameters• Peak Cell Rate

• Sustainable Cell Rate

• Maximum Burst Size

• Minimum Cell Rate (ABR only)

• Cell Delay Variation Tolerance

– Negotiated Quality of Service Parameters• Cell Loss Ratio

• Cell Delay

• cell errors, cell misinsertions, block errors

21


Classes of Service Defined in UNI 3.1

• Class A - constant bit rate

• Class B - variable bit rate, real time

• Class C - connection oriented data

• Class D - connectionless data

22


ATM Services

• Constant Bit Rate (CBR)

• Variable Bit Rate (VBR)– real time – non-realtime

• Available Bit Rate (ABR)

• Unspecified Bit Rate (UBR)

23


Constant Bit Rate

• Circuit emulation– voice, H.320 Videoconferencing

• Parameters– Peak Cell Rate– Cell Delay Variation Tolerance– Quality of service parameters

24


Variable Bit Rate

• Variable encoded video and voice (rt)

• Bursty data (nrt)

• Parameters– Peak Cell Rate– Cell Delay Variation Tolerance– Sustained Cell Rate (rt only)– Maximum Burst Size (rt only)– Quality of service parameters

25


Available Bit Rate

• Uses feedback for congestion control– Resource management cells

• Used mostly for TCP/IP data

• Parameters– Peak Cell Rate– Minimum Cell Rate– Cell Loss Ratio

26


Unspecified Bit Rate

• No parameters specified

• No QoS guarantees

• ATM Forum only (ITU-T not defined)– VBR with SCR 0 and CLP 1

27


ATM Adaptation Layer

• Adapts service to ATM cell transport

• Maps AAL Service Data Units to Cells

• Originally one AAL per class of service

• Now AAL independent of class of service

• One AAL can support more than one class of service

28


ATM Adaptation Layer Functions (AAL)

• Two sublayers– Convergence sublayer (CS)– Segmentation and reassembly sublayer (SAR)

• CS handles flow of data to and from SAR– deals with cell delay variation– not really necessary for a separate layer

• SAR breaks data into cells at sender and reassembles them at receiver

29


AALs

• AAL1 - constant bit rate service

• AAL3/4 - connectionless data based on DQDB protocol

• AAL5 - simple adaptation for connection oriented traffic

• AAL1 and AAL5 widely used

30


AAL1

• Constant bit rate services

• Uses 1 byte per cell from the payload for AAL Service Data Unit information– convergence sublayer indicator– sequence count– CRC– parity

31


AAL3/4

• Variable bit rate services

• 2 bytes per cell header– type– sequence number– Multiplexing ID

• 2 bytes per cell trailer– Length– CRC

32


Problems with AAL3/4

• Wasteful– 44 bytes data / 53 byte cell (17% overhead)

• Process data a cell at a time– examine type to identify end of packet

• Complex to implement

33


AAL5

• Most commonly used AAL– video– data

• Uses full 48 bytes per cell for data– efficient use of cell space

• End of PDU indicated in cell header– PTI indicator

34


Early Packet Discard

• Available in AAL5

• Uses PTI indicator

• If switch drops part of packet– overflow, error

• Then switch drops rest of packet

• Prevents transmission of cells that will be retransmitted.

35


Information Transmission in an ATM Network

• Connection set up (Signalling)– routing done at connection setup time– resources allocated within switches– VPI/VCI translation tables set up

• Information flow– adaptation of higher layer to cells– switching of cells based on VPI/VCI

36


Example: Circuit Emulation

• Traffic class A

• AAL1

• Constant bit rate connection

• CS layer packs frames into SAR-PDU

• SAR layer prefixes header– sequence number and check sum

• ATM layer generates cells from SAR-PDU

37


Example: IP Packets over ATM using AAL5

• Traffic class D

• UBR or ABR

• CS layer segments IP packet into 48 byte SAR-PDU payloads

• SAR layer presents SAR-PDU payloads to ATM layer

• ATM layer generates cells

• Sets PTI indicator for end of PDU

38


Summary

• ATM

• Virtual Circuits

• ATM Cell

• Services

• ATM Adaptation Layers

• Examples

39


Review Questions

– ATM has been attacked as inefficient, since it has a large cell header. What is the transmission efficiency of ATM?

– Write pseudo-code describing an algorithm to implement early packet discard.

– Why does the header CRC need to be recalculated at each hop in the ATM network?

Documents

ECS5365 Lecture 5 ATM Protocols and Networks