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Predetrmined VCIs are used for cells serving special purposes. A VCI is mapped at a VC link termination, such as a switch.
A Virtual Channel Connection VCC is a concatenation of VC links. It preserves the cell sequence. At the UNI, a VCC may be establishedreleased using
different methods: through user-network signaling, through meta signaling, or as a result of subscription without signaling.
12.4.4. Payload Type
This field is 2 bits wide and denotes the type of the information stored in the cell. A PT value of 00 is used for cells carrying user information. Other values can be used
for representing control information to provide an inband signaling capability.
12.4.5. Cell Loss Priority
This is a single-bit field and hence may assume a value of 0 or 1. When set to 0, it indicates to the network that this cell should not be discarded as long as possible. In
a congestion situation, therefore, the network will give preference to the discarding of cells with a CLP of 1. This field may be set by the user or the network.
12.5. ATM Adaptation Layer
The AAL supports the use of non-ATM protocols e.g., LAP-D for the transfer of information. It does so by mapping information from other protocols e.g., LAP-D
frames into ATM cells before transmission and their reassembly upon arrival.
Four classes of AAL services have been identified on the basis of the timing relation between source and destination, the bit rate, and the connection mode.
These are summarized by Figure 12.147. The AAL provides four protocol types types 1-4 to, respectively, support these service classes.
Figure 12.147 AAL service classes. Service Class
Timing Relation Bit Rate
Connection Mode Example
Class A Required
Constant Connection-oriented
Circuit emulation Class B
Required Variable
Connection-oriented VideoAudio
Class C Not required
Variable Connection-oriented
Data transfer Class D
Not required Variable
Connectionless Data transfer
As mentioned earlier, the AAL is divided into two sublayers: the Segmentation And Reassembly SAR sublayer and the Convergence Sublayer CS. These are
separately discussed below.
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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
