Transcript Lecture 5 - Egan Family

ECS5365 Lecture 5
ATM Protocols and Networks
Philip Branch
Centre for Telecommunications and Information
Engineering (CTIE)
Monash University
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ATM Protocols and Networks
Overview
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ATM
Virtual Circuits
ATM Cell
Services
ATM Adaptation Layers
Examples
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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
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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
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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
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Virtual Circuits in ATM
• Non Broadcast Multiple Access (NBMA)
• Virtual circuits across same medium
• Order preserving
– simplifies switch design
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Types of Virtual Circuits
• Permanent Virtual Circuits
– Established once manually
• Semi-permanent Virtual Circuits
– Time based
• Switched Virtual Circuits
– On demand
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The ATM connection hierarchy
• Virtual Channel
• Virtual Path
• Physical Medium
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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
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Tunnelling SVCs through PVPs
• Carriers may offer only Permanent Virtual
Paths
• Can ‘tunnel’ Switched Virtual Channels
through them
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ATM Layer Functions
• Cell multiplexing and demultiplexing
• Virtual Path and Virtual Circuit Identifier
translation
• Cell header generation and extraction
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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
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Fields in an ATM Cell
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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
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Generic Flow Control
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Present in UNI but not in NNI
Use not fully specified
Intended for priority scheme
Rarely used
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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
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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
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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
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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
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Problems with ATM UNI Header
• Generic Flow Control
– Better done at higher layer
• GFC limits number of VCI values
• User-network interface and networknetwork interface distinction artificial
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Quality of Service
– Traffic parameters
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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
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Classes of Service Defined in
UNI 3.1
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Class A - constant bit rate
Class B - variable bit rate, real time
Class C - connection oriented data
Class D - connectionless data
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ATM Services
• Constant Bit Rate (CBR)
• Variable Bit Rate (VBR)
– real time
– non-realtime
• Available Bit Rate (ABR)
• Unspecified Bit Rate (UBR)
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Constant Bit Rate
• Circuit emulation
– voice, H.320 Videoconferencing
• Parameters
– Peak Cell Rate
– Cell Delay Variation Tolerance
– Quality of service parameters
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Variable Bit Rate
• Variable encoded video and voice (rt)
• Bursty data (nrt)
• Parameters
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Peak Cell Rate
Cell Delay Variation Tolerance
Sustained Cell Rate (rt only)
Maximum Burst Size (rt only)
Quality of service parameters
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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
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Unspecified Bit Rate
• No parameters specified
• No QoS guarantees
• ATM Forum only (ITU-T not defined)
– VBR with SCR 0 and CLP 1
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ATM Adaptation Layer
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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
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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
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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
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AAL1
• Constant bit rate services
• Uses 1 byte per cell from the payload for
AAL Service Data Unit information
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convergence sublayer indicator
sequence count
CRC
parity
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AAL3/4
• Variable bit rate services
• 2 bytes per cell header
– type
– sequence number
– Multiplexing ID
• 2 bytes per cell trailer
– Length
– CRC
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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
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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
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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.
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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
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Example: Circuit Emulation
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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
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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
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Summary
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ATM
Virtual Circuits
ATM Cell
Services
ATM Adaptation Layers
Examples
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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?
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