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ATM provides a mechanism for communication between two end points within the context of a generalized protocol between the communicating entities. One example of such a communications protocol is the Open Systems Interconnection (OSI) reference model . This model consists of seven layers, each layer providing a set of services to the next higher layer:
The B-ISDN protocol reference model is defined in ITU-T Recommendation 1.321 and is shown in Figure 3-1 . The B-ISDN PRM is a three dimensional model consisting of planes in the vertical dimension:
(Figure 3.1: B-ISDN protocol reference model.)
Figure 3-2 presents a conceptual depiction of the B-ISDN Protocol. Since we are focusing on the portion of the protocol that involves ATM specifically, higher layers are not presented. The same conceptual notions, however, extend to the higher layer protocols.
(Figure 3-2: B-SISN protocol operation.)
In Figure 3-2, the Higher Layer Protocol sends a packet of data, or Protocol Data Unit (Higher Layer PDU), to the AAL by accessing a Service Access Point (SAP) for the AAL (the AAL-SAP). The AAL-SAP defines a set of functions and data structures (or, "primitives") that are passed between the AAL and the Higher Layer The SAP is uniquely addressed such that it corresponds to a specific connection between communicating Higher Layer entities. There may be one or more n-SAPs at the boundary between the Layer-n, and Layer-(n + 1) protocols.
At the AAL, the Higher Layer PDU becomes an AAL Service Data Unit (AAL- SDU) to which is appended an AAL-Header (and/or Trailer) containing information to support the functions performed by the AAL. Together the AAL-Header and/or Trailer, and AAL-SDU form the AAL-PDU The maximum size of the AAL-PDU, which may be of variable length, is determined at the time that a connection is established between communicating endpoints.
In a generic layered protocol discussion, this AAL-PDU is passed as a unit to the next lower protocol layer which, in turn, appends its own header. The ATM layer accepts PDUs of a fixed size of 48 octets. Thus, Figure 3-2 shows that the AAL divides the AAL-PDU into smaller PDUs, which are then passed to the ATM Layer across the ATM LayerSAP. Thus, the AAL Layer consists of two sublayers: a Convergence Sublayer (CS), which processes the Higher Layer PDU and appends an AAL Header; and a Segmentation and Reassembly (SAR), which divides the AAL-PDU into 48-octet SAR-PDUs at the originating end and, on the receiving end, reassembles the 48-octet units into the original AAL-PDU.
At the ATM Layer, a fixed 5-octet header is appended to SAR-SDU received across the ATM-SAP This forms a 53-octet ATM-PDU commonly referred to as a "cell."
At the Physical Layer the cell is encoded into a bit stream for transport across a physical transmission medium.
The Physical Layer is defined in ITU-T Recommendation 1.432, and in the ATM User-Network Interface Specification published by the ATM Forum . At the time of this writing the ATM Forum had defined physical layer ATM interfaces for transmission rates ranging from 25.6 Mb/s to 622.08 Mb/s. The physical layer is functionally divided into two sublayers: the Physical Media Dependent (PMD) Sublayer, and the Transmission Convergence (TC) Sublayer.
The PMD sublayer deals with characteristics, which are dependent upon the physical medium used. These include:
The TC sublayer addresses physical layer characteristics, which are independent of the physical medium used. The TC is dependent upon the transmission format used for sending information across the physical medium. Regardless of transmission format, certain characteristics are common to all formats:
(Table 3-1: Transmission Convergence Sublayer Functions)...
|Ch. 1||The Evolution of ATM||3|
|Ch. 2||ATM Technology Concepts||17|
|Ch. 3||ATM Communications Protocols||51|
|Ch. 4||ATM Signalling||91|
|Ch. 5||Operations Administration, Maintenance, and Provisioning||123|
|Ch. 6||Traffic Management||141|
|Ch. 7||ATM Service Interworking||173|
|Ch. 8||IP Over ATM||215|
|Ch. 9||TCP Over ATM||255|
|Ch. 10||ATM and the Internet||277|
|Ch. 11||Voice Over ATM||319|
|Ch. 12||Future Directions for ATM||339|