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Weighted round-robin scheduling can be used to serve a different number of cells from each queue. For instance, let us assume that there are five connections, each with the following weights: 0.1, 0.2, 0.4, 0.7, and 1, respectively, for each queue. Each weight is converted into an integer number by multiplying them all by a common number. For example, 0.1, 0.2, 0.4, 0.7, and 1 are each multiplied by 10 to get 1, 2, 4, 7, and 10, respectively. These integer numbers indicate how many cells should be served from each queue. Thus, the scheduler serves one cell from the first queue, two from the second queue, four from the third queue, seven from the fourth queue, and ten from the fifth queue.
Consider the case where one of the queues, say queue 5, becomes idle for some time. If that happens, then queues 1 to 4 will be served normally, and queue 5 will be skipped each time its turn comes up. The ten slots that would have been used for queue 5 are now allocated to queues 1 to 4, proportionally to their weights.
3.7 THE ATM ADAPTATION LAYER
The ATM adaptation layer (AAL) is sandwiched between the ATM layer and the higher-level layers (see Figure 3.12). AAL converts the traffic generated by a higher-level layer to ATM payloads and provides different types of services to the higher-level layer.
AAL consists of two sublayers: the convergence sublayer (CS) and the segmentation and reassembly sublayer (SAR). (See Figure 3.13.) The convergence sublayer provides service-specific functions. It is further subdivided into the service-specific convergence sublayer (SSCS) and the common part sublayer (CPS). SAR, on the other hand, has two
THE ATM ADAPTATION LAYER
Figure 3.12 The ATM adaptation layer.
Service Specific Convergence Sublayer (SSCS)
Common Part Sublayer (CPS)
Segmentation and Reassembly
Figure 3.13 The ATM adaptation sublayers.
related functions, depending on where the ATM cell is currently located: at the transmitting side or at the receiving side. At the transmitting side, SAR segments higher-level layer PDUs into a suitable size for the information field of an ATM cell. At the receiving side, it reassembles the information fields of ATM cells into higher-level layer PDUs.
Four ATM Adaptation Layers have been standardized: ATM adaptation layer 1 (AAL 1), ATM adaptation layer 2 (AAL 2), ATM adaptation layer 3/4 (AAL 3/4), and ATM adaptation layer 5 (AAL 5). Of these, all are currently used except for AAL 3/4. An additional ATM adaptation layer, the signaling ATM adaptation layer (SAAL), was defined to support the ATM signaling protocols. SAAL is described in Chapter 5.
3.7.1 ATM Adaptation Layer 1 (AAL 1)
AAL 1 can be used for applications such as circuit emulation services, constant-bit rate video, and high-quality constant-bit rate audio. It provides transfer of constant-bit rate data, delivery at the same bit rate, and transfer of timing information between the sending and receiving applications. Also, it can handle cell delay variation and detect lost or misrouted cells.
AAL 1 consists of a SAR sublayer and a CS. The SAR sublayer is responsible for the transport and bit error detection, and possibly correction, of blocks of data received from CS. The CS performs a variety of functions. These functions include handling cell delay variation, processing the sequence count, transferring structured and unstructured data, and transferring timing information.
The AAL 1 SAR sublayer
The SAR sublayer accepts blocks of 47 bytes from the CS and adds a 1-byte header to form the SAR-PDU. The SAR-PDU is then passed on to the ATM layer, where it gets encapsulated with a 5-byte ATM header. The ATM cell is then passed on to the physical layer, which transmits it out. At the receiving SAR sublayer, the 1-byte header is stripped and the payload of the SAR-PDU is delivered to the receiving CS.
The encapsulation of the SAR-PDU is shown in Figure 3.14. The header consists of two fields: the sequence number (SN) field and the sequence number protection (SNP) field. Both fields are 4 bits long. The SN field contains the subfields:
• Convergence sublayer indication (CSI): It carries an indication that is provided by the CS. The default value of the CSI bit is 0.
• Sequence count: Provided by the transmitting CS, this field is associated with the block of data in the SAR-PDU. The count starts at 0 and is increased sequentially modulo 8. The receiving CS uses the sequence count to detect lost or misinserted cells.