Download (direct link):
1, 2, ..., 7) are all non-P format CS-PDUs; these use the entire 47 bytes to pack the 20-byte blocks back-to-back. For instance, the CS-PDU with sequence number 1 contains the 14 remaining bytes of the third block (a full block) and 13 bytes of the fifth block. Note that the CS-PDU with sequence count 7 contains a partial block at the end with 14 bytes. The remaining bytes of this block are in the next CS-PDU, which starts a new cycle. This CS-PDU contains two complete blocks, and 1 byte of another block. In this case, the SDT pointer points to the beginning of the second block as shown in the figure.
Transfer of timing information
Some AAL users require that the clock frequency at the source be transferred to the destination. CS provides mechanisms for transferring such timing information.
If the sender’s clock and the receiver’s clock are phase-locked to a network’s clock, then there is no need to transfer the source’s clock frequency to the receiver, and AAL 1 is not required to transfer any timing information. However, if the two clocks are not phase-locked to a network’s clock, then AAL 1 is required to transfer the source clock frequency to the receiver. This can be done using the synchronous residual time stamp (SRTS) method, where AAL 1 conveys to the receiver the difference between a common reference clock derived from the network and the sender’s service clock. This information is transported over successive cells in the CSI bit of the SAR-PDU header. The common reference clock has to be available to both the sender and receiver. This is the case, for instance, when they are both attached to a synchronous network like SONET.
When a common reference clock is not available, then the adaptive clock method can be used. In this method, the receiver writes the received information into a buffer and then reads out from the buffer with a local clock. The fill level of the buffer is used to control the frequency of the local clock. To do this, the buffer fill is continuously measured; if it exceeds the median, then the local clock is assumed to be slow, and its speed is increased. If the fill level is lower than the median, then the clock is assumed to be fast and its speed is decreased.
3.7.2 ATM Adaptation Layer 2 (AAL 2)
This adaptation layer provides an efficient transport over ATM for multiple applications that are delay sensitive and have a low variable bit rate (such as voice, fax, and voiceband data traffic). AAL 2 is primarily used in cellular telephony. AAL 2 was designed to multiplex a number of such low variable bit rate data streams on to a single ATM connection. At the receiving side, it demultiplexes them back to the individual data streams. An example of AAL 2 is given in Figure 3.17. In this example, the transmitting AAL 2 multiplexes the data streams from users A, B, C, and D, onto the same ATM connection. The receiving AAL 2 demultiplexes the data stream into individual streams and delivers each stream from A, B, C, and D to its peer user A, B, C, and D, respectively.
CS, which provides the AAL 2 services, is further subdivided into the SSCS and the CPS. There is no SAR layer in AAL 2. The multiplexing of the different user data streams is achieved by associating each user with a different SSCS. Different SSCS protocols can be defined to support different types of service. Also, the SSCS might be null. Each SSCS receives data from its user and passes this data to the CPS in the form of short packets.
A B C D
Figure 3.17 AAL 2 can multiplex several data streams.
THE ATM ADAPTATION LAYER
Figure 3.18 Functional model of AAL 2 (sender side).
The CPS provides a multiplexing function, whereby the packets received from different SSCS are all multiplexed onto a single ATM connection. At the receiving side of the ATM connection, these packets are retrieved from the incoming ATM cells by CPS and delivered to their corresponding SSCS receivers. Finally, each SSCS delivers its data to its user. The functional model of AAL 2 at the sender’s side is shown in Figure 3.18.
A transmitting SSCS typically uses a timer to decide when to pass on data to CPS. When the timer expires, it passes the data that it has received from its higher-level layer application to CPS in the form of a packet, known as the CPS-packet. Since the applications that use AAL 2 are low variable bit rate, the CPS-packets are very short and can have a variable length. Each CPS-packet is encapsulated by CPS and then is packed into a CPS-PDU. As mentioned above, AAL 2 has been designed to multiplex several SSCS streams onto a single ATM connection. This is done by packing several CPS-packets into a single CPS-PDU, where each CPS-packet belongs to a different SSCS stream. A CPS-packet might potentially straddle two successive CPS-PDUs (see Figure 3.19). Note that CPS-packets 1 and 2 fit entirely in a CPS-PDU, whereas CPS-packet 3 has to be split between the first and second CPS-PDU. The point where a CPS-packet is split can occur anywhere in the CPS-packet, including the CPS-packet header. The unused payload in a CPS-PDU is padded with 0 bytes.