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Physical Layer (PHY)

Physical Layer (PHY) functions are addressed through two sublayers: Physical Medium and Transmission Convergence. The ATM Forum’s specifications for various User Network Interfaces (UNIs) address the implementation of the physical layer. The B-UNI, or Public UNI, is the specification for carrier internetworks. The UNI and DXI are Private UNIs, describing the implementation specifics for user access to the ATM network.

Physical Medium (PM)
sublayer specifies the physical and electro-optical interfaces with the transmission media, on both the transmit and receive side. Timing functions are provided at this level.
Transmission Convergence (TC)
sublayer is responsible for frame generation, frame adaption, cell delineation, header error control, and cell rate decoupling. The frame generation function takes the frame of data presented by the transmitting device across the PM sublayer, for presentation to the ATM Layer and subsequent segmentation into cells. On the receive side, the TC sublayer receives data in cells, decouples them in order to reconstitute the frame of data, checking all the while for header errors before presenting the data to the PM sublayer which passes the data to the end user device across the UNI.

ATM Layer (ATM)

ATM functions include multiplexing of cells, selection of appropriate VPIs and VCIs, generation of headers, and flow control. It is at this layer that all multiplexing, switching and routing takes place for presentation to the appropriate Virtual Paths and Virtual Channels of the SONET fiber optic transport system, which interfaces through the Physical Layer.

ATM Adaptation Layer (AAL)

AAL functions are divided into two sublayers, the Convergence Sublayer (CS) and the Segmentation and Reassembly (SAR) sublayer.

Convergence Sublayer (CS)
functions are determined by the specifics of the service supported by that particular AAL. Service classes are designated as Class A, B, C and D.
Segmentation and Reassembly (SAR)
sublayer functions segment the user data into payloads for insertion into cells on the transmit side. On the receive side, the SAR extracts the payload from the cells and reassembles the data into the information stream as originally transmitted.
AAL Types
are supported by the functions of the Convergence Sublayer (CS). There exist defined AAL Types 1, 2, 3/4 and 5, each of which supports a specific class of traffic (Figure 11.10). The AAL Classes are not fully standardized at this writing. Additionally, not all carriers support all AAL Classes. By way of example, AT&T will offer AAL Classes A and C, while MCI likely will offer AAL Classes A, C and D [11-9].


Figure 11.10   AAL service classes. Source: ITU-T Recommendation I.362 (03/93)

AAL Type 1
supports Class A traffic, which is connection-oriented Constant Bit Rate (CBR) traffic timed between source and sink. Such traffic is stream-oriented and intolerant of delay. Digitized voice transmission (isochronous), T-carrier (synchronous) and uncompressed video traffic (synchronous) are supported via Class 1 AAL. All traffic is carefully timed and must depend on a guaranteed rate of network access, transport, and delivery. Such traffic would be marked as high-priority in the cell header, in order that it not be delayed in transmission—such delay would have considerable impact on presentation quality. Class A traffic would be transmitted over a Virtual Path (VP) and in a Virtual Channel (VC) appropriate for such high-priority traffic.
AAL Type 2
supports Class B traffic, which is connection-oriented, Variable Bit Rate (VBR), isochronous traffic timed between source and sink. Compressed audio and video are Class B; they are marked as high-priority in the cell header and transmitted over an appropriate VP and VC.
AAL Type 3/4
supports Class C or Class D traffic, which is Variable Bit Rate (VBR) data traffic with no timing relationship between source and sink. Class C traffic, such as X.25 packet data and Frame Relay data, is connection-oriented VBR traffic with no timing relationship between source and sink. Class D traffic, such as LAN and SMDS data, is connectionless VBR traffic which is sensitive to loss, but not highly sensitive to delay [11-9].
AAL Type 3/4
supports Message Mode and Streaming Mode Service. Message Mode Service is used for framed data where only one Interface Data Unit (IDU) is passed. In other words, it is a single-frame message of up to 65,535 octets(216 – 1). Streaming Mode Service is used for framed data where more than one IDU is passed. The IDUs can be separated in time; in other words, a stream of IDUs are sent. The IDUs can be up to 65,535 octets, with a 10-bit CRC added to the trailer at the SAR Layer.
AAL Type 5
supports Class C traffic in Message Mode only. AAL 5 traffic is Variable Bit Rate (VBR) traffic with no timing relationship between source and sink, and consisting of only 1 IDU (Figure 11.11). AAL Type 5 is also known as SEAL (Simple and Efficient AAL Layer), as some of the overhead has been stripped out of the Convergence Layer. AAL Type 5 is intended for use in signaling and control (e.g., NNI applications) and network management (e.g., Local Management Interface, LMI). The IDUs can be of variable length, up to 65,535 octets. A 32-bit CRC check is appended to the IDU at the Convergence Layer, as part of the trailer. Class X traffic, which is VBR and either connection-oriented or connectionless, is supported by AAL 5 [11-40].


Figure 11.11  AAL Type 5 operation.

Higher Layer Protocols and Functions

These relate to the specifics of the user Protocol Data Unit (PDU), such as a SDLC frame.

Control Plane

These functions include all aspects of network signaling and control.

User Plane

These functions deal with issues of user-to-user information transfer and associated controls (flow control and error control mechanisms).

Management Plane

These functions involve the management of the ATM switch or hub. The Management Plane is divided into Plane Management and Layer Management.

Plane management acts on the management of the switch as a whole, with no layered approach. Management of and coordination between the various planes is accomplished in Plane Management.

Layer management acts on the management of the resources at each specific layer of the model; Operation, Administration and Maintenance (OA&M) information.


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