Basic Concepts in ATM

Information Transfer

Routing

ATM Resources

ATM Cell Identifiers

Throughput

Quality Of Service

Usage Parameter Control

Flow Control

    Information Transfer: ATM is a fast packet oriented transfer mode based on asynchronous time division multiplexing and it uses fixed length(53 bytes) cells. Each ATM cell consists of 48 bytes for information field and 5 bytes for header. The header is used to identify cells belonging to the same virtual channel and thus used in appropriate routing. Cell sequence integrity is preserved per virtual channel. ATM Adaptation layers(AAL) are used to support various services and provide service specific functions. This AAL specific information is contained in the information field of the ATM cell. Basic ATM cell structure can be shown as follows.
    Routing: ATM is a connection oriented mode. The header values(i.e. VCI and VPI etc.) are assigned during the connection set up phase and translated when switched from one section to other. Signalling information is carried on a separate virtual channel than the user information. In routing, there are two types of connections i.e. Virtual channel connection(VCC) and Virtual path connection(VPC). A VPC is an aggregate of VCCs. Switching on cells is first done on the VPC and then on the VCC.
    ATM Resources: ATM is connection-oriented and the establishment of the connections includes the allocation of a VCI i.e. virtual channel identifier and/or VPI i.e. virtual path identifier and also includes the allocation of the required resources on the user access and inside the network. These resources, expressed in terms of throughput and quality of service, can be negotiated between user and network either before the call-set up or during the call.
    ATM Cell Identifiers:ATM cell identifiers, i.e. Virtual Path Identifier, Virtual Channel Identifier and Payload Type Identifier(PTI) are used to recognise an ATM cell on a physical transmission medium. VPI and VCI are same for cells belonging to the same virtual connection on a shared transmission medium.
    Throughput: Peak Cell Rate(PCR) can be defined as a Throughput parameter which in turn is defined as the inverse of the minimum interarrival time T between two consecutive basic events and T is the peak emission interval of the ATM connection. PCR applies to both constant bit rate(CBR) and variable bit rate(VBR) services for ATM connections. It is an upper bound of the cell rate of an ATM connection and there is another parameter sustainable cell rate(SCR) allows the ATM network to allocate resources more efficiently.
    Quality Of Service: Quality of Service (QOS) parameters include cell loss, the delay and the delay variation incurred by the cells belonging to the connection in an ATM network. QOS parameters can be either specified explicitly by the user or implicitly associated with specific service requests. A limited number of specific QOS classes will be standardized in practice.
    Usage Parameter Control In ATM, excessive reservation of of resources by one user affects traffic for other users. So the throughput must be policed at the user-network interface by a Usage Parameter Control function in the network to ensure that the negotiated connection parameters per VCC or VPC between network and subscriber is maintained by each other user. Traffic parameters describe the desired throughput and QOS in the contract. The traffic parameters are to be monitered in real time at the arrival of each cell. CCITT recommends a check of the peak cell rate(PCR) of the high priority cell flow(CLP = 0) and a check of the aggregate cell flow(CLP = 0+1), per virtual connection.
    Flow Control In order to control the flow of traffic on ATM connections from a terminal to the network, a Geniric Flow Control(GFC) mechanism is proposed by CCITT at the User to Network Interface(UNI). This function is supported by GFC field in the ATM cell header. Two sets of procedures are associated with the GFC field i.e. Uncontrolled Transmission which is for use in point-to-point configurations and Controlled Transmission which can be used in both point-to-point and shared medium configurations.

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