ATM Cell Structure (BBC)

ATM Cell Structure :-

ATM Cell Header Structure

At eithera private or a public user-network interface (UNI), an ATM cell always consistsof a 5-byte header followed by a 48-byte payload. The header is composed of sixelements, each detailed in Figure.

Generic Flow Control:

TheGeneric Flow Control (GFC) field is a 4-bit field that was originally added tosupport the connection of ATM networks to shared access networks such as aDistributed Queue Dual Bus (DQDB) ring. The GFC field was designed to give theUser-Network Interface (UNI) 4 bits in which to negotiate multiplexing and flowcontrol among the cells of various ATM connections. However, the use and exactvalues of the GFC field have not been standardized, and the field is always setto 0000.

VirtualPath Identifier:

TheVirtual Path Identifier (VPI) defines the virtual path for this particularcell. VPIs for a particular virtual channel connection are discovered duringthe connection setup process for switched virtual circuit (SVC) connections andmanually configured for permanent virtual circuit (PVC) connections. At theUNI, the VPI length of 8 bits allows up to 256 different virtual paths. VPI 0exists by default on all ATM equipment and is used for administrative purposessuch as signaling to create and delete dynamic ATM connections.

Virtual Channel Identifier:

TheVirtual Channel Identifier (VCI) defines the virtual channel within thespecified virtual path for this particular cell. Just as with VPIs, VCIs arealso discovered during the connection setup process for switched virtualcircuit (SVC) connections and manually configured for permanent virtual circuit(PVC) connections. The VCI length of 16 bits allows up to 65,536 differentvirtual channels for each virtual path. VCIs 0 to 15 are reserved by the ITUand VCIs from16 to 32 are reserved by the ATM Forum (for each virtual path).These reserved VCIs are used for signaling, operation and maintenance, andresource management.

Thecombination of VPI and VCI values identifies the virtual circuit for aspecified ATM cell. The VPI/VCI combination provides the ATM forwardinginformation that the ATM switch uses to forward the cell to its destination.The VPI/VCI combination is not a network layer address such as an IP or IPXnetwork address.

TheVPI/VCI combination acts as a local identifier of a virtual circuit and issimilar to the Logical Channel Number in X.25 and the Data Link ConnectionIdentifier (DLCI) in Frame Relay. At any particular ATM endpoint or switch, theVPI/VCI uniquely identifies a virtual circuit to the next ATM endpoint orswitch. The VPI/VCI pair need not match the VCI/VPI used by the finaldestination ATM endpoint.

TheVPI/VCI combination is unique for each transmission path (that is, for eachcable or connection to the ATM switch). However, two different virtual circuitson two different ports on an ATM switch can have the same VPI/VCI withoutconflict.

Payload Type Indicator:

ThePayload Type Indicator (PTI) is a 3-bit field. Its bits are used as follows:

The firstbit indicates the type of ATM cell that follows. A first bit set to 0 indicatesuser data; a bit set to 1 indicates operations, administration & management(OA&M) data.

Thesecond bit indicates whether the cell experienced congestion in its journeyfrom source to destination. This bit is also called the Explicit ForwardCongestion Indication (EFCI) bit. The second bit is set to 0 by the source; ifan interim switch experiences congestion while routing the cell, it sets thebit to 1. After it is set to 1, all other switches in the path leave this bitvalue at 1.

DestinationATM endpoints can use the EFCI bit to implement flow control mechanisms tothrottle back on the transmission rate until cells with an EFCI bit set to 0are received.

The thirdbit indicates the last cell in a block for AAL5 in user ATM cells. For non-userATM cells, the third bit is used for OA&M functions.

Cell Loss Priority:

The CellLoss Priority (CLP) field is a 1-bit field used as a priority indicator. Whenit is set to 0, the cell is high priority and interim switches must make everyeffort to forward the cell successfully. When the CLP bit is set to 1, theinterim switches sometimes discard the cell in congestion situations. The CLPbit is very similar to the Discard Eligibility (DE) bit in Frame Relay.

An ATMendpoint sets the CLP bit to 1 when a cell is created to indicate a lowerpriority cell. The ATM switch can set the CLP to 1 if the cell exceeds thenegotiated parameters of the virtual channel connection. This is similar tobursting above the Committed Information Rate (CIR) in Frame Relay.

Header Error Check:

TheHeader Error Check (HEC) field is an 8-bit field that allows an ATM switch orATM endpoint to correct a single-bit error or to detect multi-bit errors in thefirst 4 bytes of the ATM header. Multi-bit errored cells are silentlydiscarded. The HEC only checks the ATM header and not the ATM payload. Checkingthe payload for errors is the responsibility of upper layer protocols.