Chapter 11 Asynchronous Transfer Mode

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William Stallings

Data and Computer

Communications

Chapter 11

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Protocol Architecture

 _{Similarities between ATM and packet} switching

 _{Transfer of data in discrete chunks}

 _{Multiple logical connections over single} physical interface

 _{In ATM flow on each logical connection is in} fixed sized packets called cells

 _{Minimal error and flow control}  _{Reduced overhead}

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Reference Model Planes

 _{User plane}

 _{Provides for user information transfer}  _{Control plane}

 _{Call and connection control}  _{Management plane}

 _{Plane management}

 _{whole system functions}  _{Layer management}

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ATM Logical Connections

 _{Virtual channel connections (VCC)}  _{Analogous to virtual circuit in X.25}  _{Basic unit of switching}

 _{Between two end users}  _{Full duplex}

 _{Fixed size cells}

 _{Data, user-network exchange (control) and} network-network exchange (network

management and routing)

 _{Virtual path connection (VPC)}

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Advantages of Virtual Paths

 _{Simplified network architecture}

 _{Increased network performance and} reliability

 _{Reduced processing}

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Call

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Virtual Channel Connection Uses

 _{Between end users}

 _{End to end user data}  _{Control signals}

 _{VPC provides overall capacity}

 _{VCC organization done by users}  _{Between end user and network}

 _{Control signaling}

 _{Between network entities}

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VP/VC Characteristics

 _{Quality of service}

 _{Switched and semi-permanent channel} connections

 _{Call sequence integrity}

 _{Traffic parameter negotiation and usage} monitoring

 _{VPC only}

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Control Signaling - VCC

 _{Done on separate connection}  _{Semi-permanent VCC}

 _{Meta-signaling channel}

 _{Used as permanent control signal channel}

 _{User to network signaling virtual channel}

 _{For control signaling}

 _{Used to set up VCCs to carry user data}

 User to user signaling virtual channel

 _{Within pre-established VPC}

 _{Used by two end users without network}

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Control Signaling - VPC

 _{Semi-permanent}

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ATM Cells

 _{Fixed size}

 _{5 octet header}

 _{48 octet information field}

 _{Small cells reduce queuing delay for high} priority cells

 _{Small cells can be switched more} efficiently

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Header Format

 _{Generic flow control}

 _{Only at user to network interface}  _{Controls flow only at this point}

 _{Virtual path identifier}

 _{Virtual channel identifier}  _{Payload type}

 _{e.g. user info or network management}  _{Cell loss priority}

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Generic Flow Control (GFC)

 _{Control traffic flow at user to network interface} (UNI) to alleviate short term overload

 _{Two sets of procedures}

 _{Uncontrolled transmission}

 _{Controlled transmission}

 _{Every connection either subject to flow control} or not

 _{Subject to flow control}

 _{May be one group (A) default}

 _{May be two groups (A and B)}

 _{Flow control is from subscriber to network}

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Single Group of Connections (1)

 _{Terminal equipment (TE) initializes two} variables

 _{TRANSMIT flag to 1}

 _{GO_CNTR (credit counter) to 0}

 _{If TRANSMIT=1 cells on uncontrolled} connection may be sent any time

 _{If TRANSMIT=0 no cells may be sent (on} controlled or uncontrolled connections)  _{If HALT received, TRANSMIT set to 0 and}

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Single Group of Connections (2)

 _{If TRANSMIT=1 and no cell to transmit on} any uncontrolled connection:

 _{If GO_CNTR>0, TE may send cell on} controlled connection

 _{Cell marked as being on controlled} connection

 _{GO_CNTR decremented}

 _{If GO_CNTR=0, TE may not send on} controlled connection

 _{TE sets GO_CNTR to GO_VALUE upon} receiving SET signal

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Use of HALT

 _{To limit effective data rate on ATM}  _{Should be cyclic}

 _{To reduce data rate by half, HALT issued to} be in effect 50% of time

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Two Queue Model

 _{Two counters}

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Header Error Control

 _{8 bit error control field}

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Effect of

Error in

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Transmission of ATM Cells

 _622.08Mbps  _155.52Mbps  _51.84Mbps  _25.6Mbps

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Cell Based Physical Layer

 _{No framing imposed}

 _{Continuous stream of 53 octet cells}

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SDH Based Physical Layer

 _{Imposes structure on ATM stream}  _{e.g. for 155.52Mbps}

 _{Use STM-1 (STS-3) frame}

 _{Can carry ATM and STM payloads}  _{Specific connections can be circuit}

switched using SDH channel

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ATM Service Categories

 _{Real time}

 _{Constant bit rate (CBR)}

 _{Real time variable bit rate (rt-VBR)}  _{Non-real time}

 _{Non-real time variable bit rate (nrt-VBR)}  _{Available bit rate (ABR)}

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Real Time Services

 _{Amount of delay}

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CBR

 _{Fixed data rate continuously available}  _{Tight upper bound on delay}

 _{Uncompressed audio and video}  _{Video conferencing}

 _{Interactive audio}

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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

 _{So compressed data rate varies}

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nrt-VBR

 _{May be able to characterize expected} traffic flow

 _{Improve QoS in loss and delay}  _{End system specifies:}

 _{Peak cell rate}

 _{Sustainable or average rate}

 _{Measure of how bursty traffic is}  _{e.g. Airline reservations, banking}

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UBR

 _{May be additional capacity over and}

above that used by CBR and VBR traffic  _{Not all resources dedicated}

 _{Bursty nature of VBR}

 _{For application that can tolerate some cell} loss or variable delays

 _{e.g. TCP based traffic}

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ABR

 _{Application specifies peak cell rate (PCR)} and minimum cell rate (MCR)

 _{Resources allocated to give at least MCR}  _{Spare capacity shared among all ARB}

sources

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ATM Adaptation Layer

 _{Support for information transfer protocol} not based on ATM

 _{PCM (voice)}

 _{Assemble bits into cells}

 _{Re-assemble into constant flow}  _IP

 _{Map IP packets onto ATM cells}  _{Fragment IP packets}

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Adaptation Layer Services

 _{Handle transmission errors}

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Supported Application types

 _{Circuit emulation}

 _{VBR voice and video}  _{General data service}  _{IP over ATM}

 _{Multiprotocol encapsulation over ATM} (MPOA)

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AAL Protocols

 _{Convergence sublayer (CS)}

 _{Support for specific applications}  _{AAL user attaches at SAP}

 _{Segmentation and re-assembly sublayer} (SAR)

 _{Packages and unpacks info received from} CS into cells

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AAL Type 1

 _{CBR source}

 _{SAR packs and unpacks bits}

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AAL Type 2

 _VBR

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AAL Type 3/4

 _{Connectionless or connected}

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AAL Type 5

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Frame Relay

 _{Designed to be more efficient than X.25}  _{Developed before ATM}

 _{Larger installed base than ATM}

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Frame Relay Background - X.25

 _{Call control packets, in band signaling}  _{Multiplexing of virtual circuits at layer 3}  _{Layer 2 and 3 include flow and error}

control

 _{Considerable overhead}

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Frame Relay - Differences

 _{Call control carried in separate logical} connection

 _{Multiplexing and switching at layer 2}  _{Eliminates one layer of processing}  _{No hop by hop error or flow control}

 _{End to end flow and error control (if used)} are done by higher layer

 _{Single user data frame sent from source to} destination and ACK (from higher layer)

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Advantages and Disadvantages

 _{Lost link by link error and flow control}

 _{Increased reliability makes this less of a} problem

 _{Streamlined communications process}  _{Lower delay}

 _{Higher throughput}

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Control Plane

 _{Between subscriber and network}  _{Separate logical channel used}

 _{Similar to common channel signaling for} circuit switching services

 _{Data link layer}  _{LAPD (Q.921)}

 Reliable data link control  _{Error and flow control}

 _{Between user (TE) and network (NT)}  _{Used for exchange of Q.933 control}

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User Plane

 _{End to end functionality}

 _{Transfer of info between ends}

 _{LAPF (Link Access Procedure for Frame Mode}

Bearer Services) Q.922

 _{Frame delimiting, alignment and}

transparency

 _{Frame mux and demux using addressing field}

 _{Ensure frame is integral number of octets}

(zero bit insertion/extraction)

 _{Ensure frame is neither too long nor short}

 _{Detection of transmission errors}

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LAPF

Core

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User Data Transfer

 _{One frame type}  _{User data}

 _{No control frame}  _{No inband signaling}

 _{No sequence numbers}

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Required Reading