Asynchronous Transfer Mode

1
Asynchronous Transfer Mode

• Developed as part of broadband ISDN
• used in private non-ISDN networks
• Also called Cell Relay
• More streamlined than Frame Relay
• Supports speeds at 155.52 Mbps and 622.08 Mbps
• Higher and lower speeds are possible

2
ATM

• Data is organised into fixed-size packets called
  cells
• This simplifies the process of switching data at
  each node
• Protocol is streamlined with minimal overhead

3
ATM Protocol Structure
Management plane
Control Plane
User Plane
Higher Layer protocols
Higher Layer protocols
ATM Adaptation Layer
ATM Layer
Physical Layer
Plane management
Layer management
4
ATM layer

• Logical connections are called virtual channel
  connections (VCC)
• Full-duplex, fixed-size cell, variable rate
  connection between users is established on a VCC
• VCC also used for user-network control signalling
  and network management and routing

5
ATM layer

• VCCs are bundled into groups that have same
  endpoints
• These are called virtual path connections (VPC)
• All cells in all VCCs in a VPC are switched
  together

6
Advantages of VPC

• Network transport functions can be split into
  those for individual VCCs and those for groups of
  VCCs in VPCs
• Network deals with smaller number of entities
• Increased network performance
• When VPC is established new VCCs can be set up
  with no transit node processing

7
VCC Uses

• Between end-users
• VPC between users provides overall capacity
• VCCs can be organised between the users up to VPC
  capacity
• End user - network used for control signalling
• Network-network used for network traffic
  management and routing

8
Characteristics of VPC/VCC

• Switched and semi-permanent connections
• Packet sequence is preserved
• Traffic parameters can be negotiated (e.g.peak
  and average rate)
• Quality of service is specified (cell loss ratio,
  delay variation)

9
Control Signalling for VCCs

• Not needed for semi-permanent connection
• Takes place on separate signalling channel
• Meta-signalling channel is a permanent channel
  used to set up signalling channel
• This can be used to set up a user-to-network
  signalling channel
• This can also be used to set up a user-to-user
  signalling channel within an existing VPC which
  users can use to set up user-user VCC

10
Control Signalling for VPCs

• Not needed for semi-permanent connection
• Customer controlled VPC
• Signalling VCC used by user to request/release
  VPC
• Network controlled VPC
• Network establishes VPC for own use. It may be
• Network-network
• User-to-network
• User-user

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ATM Cell Format
8 7 6 5 4
3 2 1
Generic flow control
Virtual Path Identifier
Virtual Path Identifier
Virtual channel ident
Virtual channel Identifier
Payload type
CLP
Virtual channel ident
Header error control
Information Field (48 octets)
User-network interface
12
ATM Cell Format
8 7 6 5 4
3 2 1
Virtual Path Identifier
Virtual Path Identifier
Virtual channel ident
Virtual channel Identifier
Payload type
CLP
Virtual channel ident
Header error control
Information Field (48 octets)
Network-network interface
13
Header Error Control
Multi-bit error detected (cell discarded)
no error detected (no action)
Correction Mode
Detection Mode
no error detected (no action)
error detected (cell discarded)
Single-bit error detected (correction)
14
Physical Layer

• Two rates specified - 155.52 Mbps or 622.08 Mbps
• Lower Rate Can use Synchronous Digital Hierarchy
  (SDH) interface or a cell-based physical layer
• The lower rate can support one or more video
  channels
• The higher rate can support multiple video
  channels

15
Cell Based Physical Layer

• No framing is imposed
• Some form of synchronising is needed
• This is performed by header error control

16
SDH-based Physical Layer

• Defined for 155.52 Mbps STM-1 service
• Advantages
• SDH can carry either ATM or STM traffic allowing
  mixed circuit and cell relay on common medium
• Four ATM streams can be combined to use a 622
  Mbps STM-4 interface if available

17
ATM Adaptation Layer

• Allows higher protocols not based on ATM
• PCM
• LAPF
• AAL handles
• Transmission errors
• Segmentation and re-assembly
• Lost and mis-inserted cells
• Flow and timing control

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Classes of Service Supported
Class A
Class B
Class C
Class D
Timing relation between source and destination
Required
Not Required
Bit rate
Constant
variable
Connection mode
Connection oriented
Connectionless
AAL protocol
Type 1
Type2
Type3/4,5
Type3/4
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ATM Adaptation Layer

• Two logical sub-layers defined
• Common part convergence sub-layer (CPCS)
• Provides the functions needed to support specific
  applications using AAL
• Segmentation and re-assembly sub-layer (SAR)
• Places data into cells for transmission and
  un-packs data from received cells

20
ATM Adaptation Layer

• Four Protocol Types are defined at both CPCS and
  SAR sub-layers to support different services
• Type 1
• Type 2 not yet defined
• Type 3 and four merged to form type 3/4
• Type 5

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SAR PDUs
AAL Type 1
SN
SNP
SAR-PDU payload
Header, 1 octet
47 octets
AAL Type 3/4
ST
SN
SAR-PDU payload
MID
LI
CRC
Header, 2 octets
44 octets
Trailer, 2 octets
AAL Type 5
SAR-PDU payload 48 octets
Legend
SN Sequence no. (4 bits) SNP Sequence no.
protection MID multiplexing identification
LI Length identification CRC Cyclic redundancy
check ST Segment type
22
AAL Type 5

• Increasingly popular in ATM LANS
• Provides streamlined transport for higher-layer
  connection-oriented protocols

23
CPCS PDU for AALType 5
CPCS-PDU trailer
PAD
CPCS-PDU payload
CRC
Length
CPI
CPCS-UU
1 Octet
1 Octet
2 Octets
4 Octets
CPCS-UU user to user indication CPI
common-part indicator Length length of
CPCS-PDU payload CRC cyclic redundancy
check PAD pads out payload so PDU is a
multiple of 48 octets
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CPCS PDU Trailer AALType 5

• CPCS User to user indication (1 octet)
• Used to transparently transfer user information
• CRC (4 octets)
• Used to detect bit errors in PDU
• Common Part Indicator (1 octet) indicates
  interpretation of fields in trailer
• Length (2 octets)
• Length of payload field

25
SAR PDU for AALType 5

• No sequence number in PDU
• Assumes all SAR PDUs received are in sequence
• No MID field
• No interleaving of cells from different CPCS PDUs
• Each SAR PDU carries either part of current CPCS
  PDU or start of next CPCS PDU

26
SAR PDU for AALType 5

• AAU bit in ATM cell header is used to mark last
  block of a group of SAR PDUs to be reassembled
  into one CPCS PDU
• Padding is used before trailer in CPCS PDU so
  that last bit of trailer coincides with last bit
  of last SAR PDU

27
ATM Traffic and Congestion Control

• Traffic control describes methods used to avoid
  congestion
• Congestion control describes methods used to
  minimise intensity, spread and duration of
  congestion

28
ATM Traffic and Congestion Control

• Excessive traffic at nodes can cause buffer
  overflow and therefore data loss
• High speed and low number of header bits creates
  problems not found in slower systems
• Full strategy has not yet been developed

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Conventional Traffic and Congestion Control

• Most Packet switched networks carry non-real-time
  bursty data
• Each node does not have to replicate the timing
  pattern of the data at the exit node
• Statistical multiplexing can therefore be used
• This makes efficient use of link capacity
• Proven techniques are available for congestion
  control

30
ATM Traffic and Congestion Control

• Voice and data traffic is not amenable to flow
  control
• Wide range of traffic makes fair congestion
  control difficult
• Different applications require different services
  ( e.g. delay sensitive , loss sensitive)
• Very high speed switching and transmission makes
  stable ATM congestion control more difficult

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Cell Delay Variation

• Network
• Minimal due to
• Low overhead protocol, fixed frame size
• ATM switches are extremely fast acting
• Only occurs if network congested
• Most delay variation occurs at user-network
  interface

32
Cell Delay Variation at UNI
Time
48 octets, X Mbps
AAL layer
Connection A, X Mbps
48 octets, Y Mbps
Connection B, Y Mbps
ATM layer SAP
H
H
H
ATM layer
OAM
H
H
OAM
PHYlayer SAP
PHY layer
H
H
H
H
H
H
H
H
H
H
PHY layer overhead
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Control of Cell Delay Variation

• Cells with voice and standard video are inserted
  into the network at a constant rate
• They suffer variable delay in the network
• The destination node delays the first cell by an
  additional amount V equal to the estimated cell
  delay variation
• Subsequent cells are delayed by a variable amount
  so that they are delivered to the user at a
  constant rate
• Cells delayed by more than V are discarded

34
Control of Cell Delay Variation
Let R required constant delivery rate Required
time T between cell delivery 1/R Let t(0)
time of arrival of first cell First cell is
delayed by v(0) Next cell arrives at time
t(1) It is delayed by v(1) such that t(1)
v(1) t(0) v(0) T
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Traffic Control

• Strategy Based on
• Determining if new connection can be accommodated
• Agreeing a contract with subscriber on
  performance parameters that will be supported
• Functions are concerned with establishing and
  enforcing agreed parameters

36
ATM Traffic Control Functions

• Network resource management
• Connection admission control
• Usage parameter control
• Priority control
• Fast resource management

37
Network Resource Management

• At present deals with virtual path connections
• Primary parameters are cell loss ratio, cell
  transfer delay and cell delay variation
• VCCs with similar traffic characteristics are
  grouped into the same VPC
• Capacity can be allocated to VPC based on
• aggregate peak demand or
• statistical multiplexing

38
Connection Admission Control

• User selects the traffic characteristics required
  in both directions when requesting a new VPC or
  VCC
• Four parameters are defined
• Peak Cell Rate
• Cell Delay Variation
• Sustainable cell rate
• Burst tolerance

39
Connection Admission Control

• These may be specified
• At connection time
• At subscription time
• By signalling
• By subscription
• by default network-wide

40
Usage Parameter Control

• Monitors connections to ensure compliance with
  contract
• Can be performed at VCC and VPC level
• Two separate functions performed
• Control of peak cell rate and cell-delay
  variation
• Control of sustainable cell rate and associated
  burst tolerance

41
Usage Parameter Control

• Two methods of control are used
• Non-compliant cells discarded
• Non-compliant cells are tagged with CLP1and
  passed

42
Priority Control

• Low priority cells are discarded to protect
  network performance for higher priority cells
  before congestion occurs
• Cells with CLP1 may be discarded
• Cells may have been tagged earlier either by the
  network or by the user

43
Fast Resource Management

• These functions operate on same time-scale as
  round-trip propagation delay of ATM connection
• This is for further study
• An example would be ability of a user to request
  that agreed parameters be exceeded for a brief
  period
• If network resources are available network should
  oblige

44
Congestion Control

• Selective cell discarding occurs when congestion
  is experienced
• Network is free to discard all CLP1 cells and
  may even discard CLP0 cells in circuits which
  are not compliant with traffic contract

45
Congestion Control

• Explicit forward congestion notification
• Nodes experiencing congestion may set explicit
  forward congestion control indication in payload
  type field on passing cells
• The user may invoke actions in higher level
  protocols to lower cell rate