HEC Generation and Verification

174 Communication Networks Copyright © 2005 PragSoft

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