1. Asynchronous Transfer Mode (ATM) Architecture and Operation
IP over ATM

2. Background / Overview
1990’s/00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architecture
Goal: integrated, end-end transport of voice, video, data
Thus meeting timing / QoS requirements of voice, video
(in contrast to the Internet best-effort model)
“next generation” telephony: technical roots in telephone world
packet-switching.
Based on cell switching (fixed length packets, called “cells”) using virtual circuits.
Designed primarily as WAN / Backbone / Trunk technology.
3 layer architecture.
ATM protocol stack
ATM Physical Layer
ATM Adaptation Layer
(AAL)
ATM Layer 2
Richard Anthony, University of Greenwich

3. Background / Overview - Switched end-end ‘connections’
UNI=User Network Interface
NNI = Network-Network Interface 3
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4. Background / Overview - ATM Protocol Layers
ATM has three protocol layers:
AAL = ATM Adaptation Layer - actually a group of protocols,
- selection is based on QoS requirements.
ATM = ATM Layer - the cell-switching layer.
Physical = ATM Physical layer 4
Richard Anthony, University of Greenwich

5. ATM Protocol AAL sub-layers
The ATM Adaptation Layer comprises 2 sublayers:
Convergence sublayer (CS): adds a header and trailer onto user-data PDUs.
The information in the header and trailer depends on the type of information
to be transported but usually includes error handling and data priority
preservation information.
Segmentation & Reassembly sublayer (SAR): recieves the CS PDU and divides it up into ATM cell-sized pieces. Each SAR PDU includes a header containing information used to reassemble the pieces at the destination.
Convergence sublayer (CS)
Segmentation & Reassembly sublayer (SAR) 5
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6. Background / Overview – Layered End-End view
Switch
Endpoint
interface
AAL = ATM Adaptation Layer
ATM = ATM Layer
Physical = ATM Physical layer 6
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7. End System ATM Switch ATM Switch End System
Background / Overview – End-End data path
Application data
encapsulation
decapsulation
Switching
decision
ATM Physical Layer
ATM Layer
ATM Adaptation Layer
(AAL)
End System ATM Switch ATM Switch End System
Adaptation layer: only at edge of ATM network
data segmentation / reassembly
partially analogous to Internet transport layer
ATM layer: partially analogous to Internet network layer
cell switching 7
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8. Cell Switching - Overview
Fixed size cells - more efficient switching,
- predictable delays.
Asynchronous TDM - fixed slots (empty cells sent when no data). 8
Richard Anthony, University of Greenwich

9. Cell Switching – Benefits of fixed cell size
Fixed cell size facilitates hardware switching (due to fixed offset in buffer)
- can be 1000 x faster than software switching
Payload
Head
Head of buffer
(position 0)
Cell flow through buffer
Buffer
Offset of start of cells at positions 0, 53, 106, 159 etc.
(add 53 every time)
Fixed cell size also avoids the need for a length field in the header:
Improves payload capacity / efficiency
Saves some computation when creating / processing headers
Richard Anthony, University of Greenwich

10. Cell Switching – Cell Structure 1
GFC is only used at the endpoints
These 4 bits form part of the VPI during transit across NNI interfaces 10
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11. Cell Switching – Cell Structure 2
5-byte ATM cell header
48-byte payload, Why only 48 bytes?
Small payload -> short cell-creation delay for digitized voice
- Larger payload better to encapsulate higher-layer protocols
- Compromise between 32 (telephony choice) and 64 (Data network choice)
Cell header 11
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12. Cell format and encapsulation
Cell Switching – Cell Structure 3
Cell format and encapsulation
Segmentation & Reassembly (SAR) PDU
Cell payload (48 bytes)
Cell header
Convergence Sublayer (CS) PDU
CS to SAR mapping depends on AAL type
AAL
ATM 12
Richard Anthony, University of Greenwich

13. Cell Switching – Cell Header
VCI: 28 bits - virtual channel ID
Will change from link to link through network
PT: 3 bits - Payload type (e.g. RM cell versus data cell)
CLP: Cell Loss Priority bit
CLP = 1 implies low priority cell, can be discarded if congestion
HEC: 8 bits - Header Error Checksum
Cyclic Redundancy Check (CRC) 13
Richard Anthony, University of Greenwich

14. ATM Adaptation Layer (AAL) - Overview
AAL “Adapts” upper layers (IP packets, streaming multimedia) to ATM layer below.
- Encapsulation and treatment is based on QoS requirements,
- Thus several variants of the AAL.
AAL is present only in end systems, not in switches.
AAL segment (header/trailer fields, data) fragmented over multiple ATM cells.
- Uses Convergence Sublayer (CS) and Segmentation & Reassembly (SAR). 14
Richard Anthony, University of Greenwich

15. ATM Adaptation Layer (AAL) – AAL Classes
AAL1: for CBR (Constant Bit Rate) services, e.g. circuit emulation.
AAL2: originally for VBR (Variable Bit Rate) services, e.g., MPEG video;
Now used for low bit rate or short frame traffic in general.
AAL3/4: Originally Connection Oriented & Connectionless data services,
Now merged into one.
AAL5: Data (e.g. IP datagrams) (Available BR) ABR / (Unassigned BR) UBR.
User data
AAL
PDU
ATM cell 15
Note. CPCS = Common Part Convergence Sublayer (see later)
Richard Anthony, University of Greenwich

16. ATM Adaptation Layer (AAL) – Service Classes and QoS 1
Real time
Constant bit rate (CBR) (Highest priority)
Real time variable bit rate (rt-VBR) (High priority)
Non-real time
Guaranteed frame rate (GFR) (High priority)
Non-real time variable bit rate (nrt-VBR)
Available bit rate (ABR)
Unspecified bit rate (UBR) (Lowest priority) 16
Richard Anthony, University of Greenwich

17. ATM Adaptation Layer (AAL) – Service Classes and QoS 2
CBR
Fixed data rate continuously available
Tight upper bound on delay
Uncompressed audio and video
- Video conferencing
- Interactive audio
- Audio / Visual distribution and retrieval
Rt-VBR
Time sensitive application
Tightly constrained delay and delay variation
rt-VBR applications transmit at a rate that varies with time
- e.g. compressed video produces varying sized image frames
- Original (uncompressed) frame rate constant, but compressed
data rate varies.
Can statistically multiplex connections 17
Richard Anthony, University of Greenwich

18. ATM Adaptation Layer (AAL) – Service Classes and QoS 3
GFR
Examples of services carried: IP and Ethernet
Optimize traffic from LAN through IP router to ATM BackBone
Used by enterprise, carrier and ISP networks
Consolidation and extension of IP over WAN
ABR difficult between routers over ATM network (unpredictable bandwidth)
GFR better alternative for traffic originating on Ethernet
ATM network is aware of frame / packet boundaries
When congested, all cells from frame discarded (pointless to treat independently)
Guaranteed minimum capacity 18
Richard Anthony, University of Greenwich

19. ATM Adaptation Layer (AAL) – Service Classes and QoS 4
Nrt-VBR
For some VBR applications, may be able to characterize expected traffic flow. End system (application) specifies:
- Peak cell rate
- Sustainable or average rate
- Measure of how ‘bursty’ traffic is
Nrt-VBR provides better QoS (in loss and delay) than ABR and UBR
E.g. Airline reservations, banking transactions (soft real-time aspects) 19
Richard Anthony, University of Greenwich

20. ATM Adaptation Layer (AAL) – Service Classes and QoS 5
ABR
For example, LAN interconnection.
User (end application) specifies QoS parameters:
- Peak Cell Rate (PCR) and
- Minimum Cell Rate (MCR)
Resources allocated to give at least MCR.
UBR
There may be additional capacity over and above that used by the other categories of traffic.
Not all resources can be dedicated, primarily because of the bursty nature of VBR (therefore statistical multiplexing is used within VBR service).
- UBR gets what is left – and thus can be starved out.
- UBR cells are forwarded on FIFO basis.
Only suited to applications that can tolerate some cell loss or variable delays, e.g. TCP based traffic (TCP performs recovery at higher layer). 20
Richard Anthony, University of Greenwich

21. ATM Adaptation Layer (AAL) – Bandwidth allocation
Bandwidth (cells) are allocated based on the highest priority service first (i.e. the real time service classes), then the non-real time services with minimum QoS guarantees.
UBR gets anything that is left – statistically there is always something due to VBR multiplexing, but may be no actual cells allocated at peak flows. 21
Richard Anthony, University of Greenwich

22. Service provided: ‘Route’ (switch) cells across the ATM network
ATM Layer - Overview
Service provided: ‘Route’ (switch) cells across the ATM network
Function is analagous to IP network layer
Services provided (implicit in operation) very different to IP network layer
Designed to flexibly meet a wide variety of QoS needs (see table):
Network
architecture
Internet
ATM
Service
model
best effort
CBR
VBR
ABR
UBR
Bandwidth
none
constant
rate
guaranteed
minimum
Loss no
yes
Order
Timing
Congestion
feedback
no (inferred
via loss)
congestion
Guarantees ? 22
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23. ATM Layer – Virtual Connections
VC transport: cells carried on VC from source to destination
call setup, teardown for each call before data can flow
each packet carries VC identifier (not destination ID)
every switch on source-destination path maintains “state” for each passing connection
link, switch resources (bandwidth, buffers) may be allocated to VC: to get circuit-like performance.
Permanent VCs (PVCs)
‘Always-on’ virtual connections
typically: “permanent” route between two IP routers
Switched VCs (SVC):
dynamically set up on per-call basis 23
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24. ATM Layer - Virtual Connections, Virtual Paths & Transmission Paths24
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25. ATM Layer - VPs and VCs in action25
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26. ATM Layer - VC-VP Identifiers
VPI + VCI needed for each connection
Allows hierarchical switching 26
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27. ATM Layer - UNI and NNI – connection identifiers
UNI=User Network Interface
NNI = Network-Network Interface 27
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28. ATM Layer - ATM switching
Switch keeps a table containing VC switching information 28
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29. STM: Synchronous transfer mode ATM: Asynchronous transfer mode
ATM Layer – ‘A’ means Asynchronous cell allocation
TS 3
TS 7
Source B
Source A
Periodic frame
The connection is identified by the time slot’s position in the frame
Multiplexer
STM: Synchronous transfer mode
Synchronous cell allocation is used in systems that pre-allocate fixed time-slots / channels.
Good for R/T traffic (e.g. if only
voice channels are used).
Poor for efficiency where mixed
services are provided.
ATM: Asynchronous transfer mode
Source A
Source B
labels
Assembler
empty
The connection is identified by the label contained in the cell’s header
ATM fills cells ‘on-demand’ at the ATM layer – I.e. Asynchronously.
Cells are actually placed on
the carrier synchronously.
Richard Anthony, University of Greenwich

30. IP over ATM – Is ATM a network layer or link layer technology ?
Vision: end-to-end transport: “ATM from desktop to desktop”
ATM is a network technology (i.e. ATM functionality is analogous to ‘network’ layer)
Reality: used to connect IP backbone routers
“IP over ATM”
ATM is used as a switched link layer, connecting IP routers 30
Richard Anthony, University of Greenwich

31. Network host with FTP server running
IP over ATM – Routers pass IP traffic over ATM ‘Links’
Personal
Computer IP
Router
Network Layer (IP)
Transport Layer
Application Layer
Send File via FTP
Ethernet (layer 2)
Store received file
Token ring (lay 2)
Network host with FTP server running
Token
ring
Ethernet
Frame
Relay
ATM
Each ATM network is seen simply as a ‘link’ by the IP routing function
Network Design and Implementation Richard Anthony, Computer Science, The University of Greenwich

32. IP over ATM – Protocol layer view (ATM network serves as a Data Link)
ATM protocol stack
ATM Physical Layer
ATM Adaptation Layer
(AAL)
ATM Layer
TCP/IP protocol stack
Physical Layer
Internetwork Layer
Application Layer
Transport Layer
Link Layer
Richard Anthony, University of Greenwich

33. IP over ATM - Datagram Journey End-End - 1
At Source Host:
IP layer maps between IP and ATM destination address (using ARP)
(The ATM destination address is analogous to the MAC address when
generating a frame to pass a packet across an Ethernet network).
Passes datagram to AAL5.
AAL5 encapsulates data, segments cells, passes to ATM layer.
ATM network: Moves cell along VC to destination.
At Destination Host:
AAL5 reassembles cells into original datagram.
if CRC is OK, datagram is passed to IP. 33
Richard Anthony, University of Greenwich

34. End System ATM Switch ATM Switch End System
IP over ATM - Datagram Journey End-End - 2
Application Layer
Application Layer
Transport Layer
Transport Layer
Internetwork Layer
Internetwork Layer
ATM Adaptation Layer
(AAL)
ATM Adaptation Layer
(AAL)
ATM Layer
ATM Layer
ATM Layer
ATM Layer
ATM Physical Layer
ATM Physical Layer
ATM Physical Layer
ATM Physical Layer
End System ATM Switch ATM Switch End System
TCP /IP over ATM (actual communication path)
Richard Anthony, University of Greenwich

35. IP over ATM - Datagram Journey End-End - 3
Application Layer
Application Layer
Transport Layer
Transport Layer
Internetwork Layer
Internetwork Layer
Link and Physical layers
Link and Physical layers
End System End System
TCP /IP over ATM (logical communication path)
Richard Anthony, University of Greenwich

36. Adaptation / Encapsulation - AAL1 - CBR
CS: Convergence Sublayer
(interface with higher layer protocol)
SAR: Segmentation & Reassembly
(interface with ATM layer (cells) 36
Richard Anthony, University of Greenwich

37. Adaptation / Encapsulation – AAL2 – VBR
Note that, since the application layer provides short packets, CS PDUs are split across SAR PDUs for efficiency. 37
Richard Anthony, University of Greenwich

38. Further reading - see www.protocols.com/pbook/atm.htm
Adaptation / Encapsulation – AAL3/4 Connection Oriented / Connectionless services
Note. CS is split into service specific part (SSCS) and common part (CPCS).
Further reading - see 38
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39. Adaptation / Encapsulation – AAL5 - Data / ABR / UBR
Known as: Simple And Efficient AL (SEAL) 39
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40. Adaptation / Encapsulation – AAL5 continued
AAL5 encapsulation has low overhead:
AAL5 is used to carry IP datagrams, so the CS-level PDU payload size is exactly the maximum size of an IP packet.
- Large AAL5 data unit is fragmented into 48-byte ATM cells (SAR sub-layer)
- 4 byte cyclic redundancy check
- PAD ensures payload multiple of 48 bytes
CPCS-PDU payload PAD Channel ID Common part ID Length CRC
Bytes 40
Richard Anthony, University of Greenwich