174 Communication Networks
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12.5.1. Segmentation and Reassembly Sublayer
SAR handles the segmentation of information from CS into ATM cells, and their subsequent reassembly upon arrival. Figure 12.148 illustrates the SAR PDU format
for the four protocol types.
The Type 1 SAR PDU is intended for transmissions involving a constant bit rate source see Class A in Figure 12.147. It consists of 8 bits of overhead and a
payload of 47 octets. The overhead is comprised of two fields: a 4-bit Sequence Number SN and a 4-bit Sequence Number Protection SNP. The SN is used for
the detection of cell sequence violation e.g., lost or misinserted cells. The SNP serves as a CRC for error detection and single-bit error correction related to the
overhead.
The Type 2 SAR PDU as well as Type 3 and 4 supports transmissions involving a variable bit rate source. It consists of a payload and four overhead fields.
The SN is as before. The Information Type IT field is used to denote the relationship of the SAR PDU to the CS PDU i.e., one of: beginning of message,
continuation of message, end of message, or single-segment message. The Length Indicator LI denotes the actual number of CS PDU octets carried in this SAR
PDU. The CRC field protects the SAR PDU against errors. Type 2 is suitable for the transmission of analog data, such as audio and video, using PCM.
Figure 12.148 SAR PDU formats. Type
PDU Format
1 SN
4 bits SNP
4 bits Payload
47 octets 2
SN 4 bits
IT 4 bits
Payload 45 octets
LI 6 bits
CRC 10 bits
3 ST
2 bits SN
4 bits RES
10 bits Payload
44 octets LI
6 bits CRC
10 bits 4
ST 2 bits
SN 4 bits
MID 10 bits
Payload 44 octets
LI 6 bits
CRC 10 bits
The Type 3 SAR PDU consists of 32 bits of overhead and a payload of 44 octets. The Segment Type ST field is similar to the IT field of Type 2. The SN, LI,
and CRC fields are as before. The remaining 10 bits are reserved for future use. The Type 4 SAR PDU is very similar to Type 3, except that here a 10-bit Multiplexing
Identifier MID is used. It supports the multiplexing of multiple CS PDUs over the same connection. Type 3 and 4 are suitable for data transfer applications.
Two modes of service are provided for Type 3 and 4: stream mode and
messaging mode. The stream mode service involves the transfer of fixed-size
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blocks, where each block is mapped to one cell. It is intended for continuous data at low rates. The message mode service is suitable for framed data e.g., from LAP-
D. Each frame is mapped to one or more cells.
12.5.2. Convergence Sublayer
The convergence sublayer is responsible for a variety of functions depending on which of the AAL protocol types described above is used, including the following:
•
Indication of the information contained by CS PDUs.
•
Segmenation and reassembly of higher layer PDUs into CS PDUs.
•
Handling of lost of misinserted cells following their detection by the SAR sublayer.
•
Detection of corrupted CS PDUs and their proper handling.
•
Explicit time indication through time stamps as required by some services.
•
Clock recovery as required by some services.
12.6. B-ISDN Standards
12.7. Further Reading
An introductory coverage of B-ISDN and ATM can be found in Minzer 1989, De Prycker 1991, Van Duuren et al 1992, Stallings 1992, and Onvural 1994.
Kessler 1993 and Handel and Huber 1994 provide a wide coverage of ATM protocols and standards. Ballart and Ching 1989, Stallings 1992, Spohn 1993
describe SONET. De Prycker et al 1993 relate B-ISDN to the OSI reference model. Ahmadi and Denzel 1989 and Rooholamini et al 1994 describe a number
of ATM switching techniques and products. Onvural 1994 presents an in-depth treatment of ATM network performance issues. Viniotis and Onvural 1993 is a
useful collection of paper on ATM networks.
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