1 asynchronous transfer mode u developed as part of broadband isdn u used in private non-isdn...
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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
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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
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ATM Protocol Structure
Higher Layerprotocols
Higher Layerprotocols
ATM Adaptation Layer
ATM Layer
Physical Layer
Management plane
Control Plane User Plane
Layer management
Plane management
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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
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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
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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
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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
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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)
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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
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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
Generic flow control Virtual Path Identifier
Virtual Path Identifier Virtual channel ident
Virtual channel Identifier
Payload type CLPVirtual channel ident
Header error control
Information Field(48 octets)
User-network interface
8 7 6 5 4 3 2 1
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ATM Cell Format
Virtual Path Identifier
Virtual Path Identifier Virtual channel ident
Virtual channel Identifier
Payload type CLPVirtual channel ident
Header error control
Information Field(48 octets)
Network-network interface
8 7 6 5 4 3 2 1
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Header Error Control
CorrectionMode
DetectionMode
Single-bit error detected (correction)
no error detected(no action)
Multi-bit error detected(cell discarded)
no error detected(no action)
error detected(cell discarded)
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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
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Cell Based Physical Layer
No framing is imposed Some form of synchronising is needed This is performed by header error control
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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
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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
Timing relationbetween sourceand destination
Bit rate
Connection mode
AAL protocol
Class A Class B Class C Class D
Required Not Required
Constant variable
Connection oriented Connectionless
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
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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
SN SNP SAR-PDU payload
Header, 1 octet 47 octets
AAL Type 1
ST SN SAR-PDU payload
Header, 2 octets 44 octets
AAL Type 3/4
MID LI CRC
Trailer, 2 octets
SAR-PDU payload 48 octetsAAL Type 5
SN= Sequence no. (4 bits)SNP = Sequence no. protectionMID = multiplexing identification
LI= Length identificationCRC = Cyclic redundancy checkST = Segment type
Legend:
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AAL Type 5
Increasingly popular in ATM LANS Provides streamlined transport for higher-
layer connection-oriented protocols
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CPCS PDU for AALType 5
CPCS-PDU trailerPADCPCS-PDU payload
CPCS-UU = user to user indicationCPI = common-part indicatorLength = length of CPCS-PDU payloadCRC = cyclic redundancy checkPAD = pads out payload so PDU is a multiple of 48 octets
CRCLengthCPICPCS-UU
1 Octet 1 Octet 2 Octets 4 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
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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
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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
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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
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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
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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
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Cell Delay Variation at UNITime
48 octets, X Mbps
48 octets, Y Mbps
H
H
Connection A, X Mbps
Connection B, Y Mbps
OAM H
OAM
HHH H
H
ATM layer SAP
AAL layer
PHYlayer SAP
ATM layer
PHY layer
H H H H H H H H HH
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
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Control of Cell Delay VariationLet 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
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ATM Traffic Control Functions
Network resource management Connection admission control Usage parameter control Priority control Fast resource management
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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
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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
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Connection Admission Control
These may be specified – At connection time– At subscription time– By signalling– By subscription– by default network-wide
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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
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Usage Parameter Control
Two methods of control are used– Non-compliant cells discarded– Non-compliant cells are tagged with
CLP=1and passed
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Priority Control
Low priority cells are discarded to protect network performance for higher priority cells before congestion occurs
Cells with CLP=1 may be discarded Cells may have been tagged earlier either
by the network or by the user
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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
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Congestion Control
Selective cell discarding occurs when congestion is experienced
Network is free to discard all CLP=1 cells and may even discard CLP=0 cells in circuits which are not compliant with traffic contract