172 Communication Networks
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transmission rate, the transmitter inserts idle cells in the stream when it has no cells to send. The receiver identifies and discards idle cells upon their arrival.
Idle cells are similar to OAM cells in that they have their VPI set to 0 and their VCI set to a fixed value 1. The cell header conatins a valid HEC and the cells
information octets are set to the bit pattern 01101010.
12.4. ATM Layer
The ATM layer uses a cell as its basic unit of communication. Unlike the physical layer, at the ATM layer only the logical structure of a cell is of interest. This was
described in Section 12.2.2. Cells which are for the use of the physical layer only are distinguished by having the VPI and VCI set to 0, and the least significant bit of the
fourth octet in the cell header set to 1.
Except for the HEC field which is not used by the ATM layer, the remaining ATM cell fields are separately discussed below.
12.4.1. Generic Flow Control
The GFC field is 4 bits wide and defaults to 0. The GFC field provides a means of exercising UNI-based control over the flow of traffic to counter overload situations.
It only applies to traffic generated by the CPE as opposed to the network. The exact GFC details remain largely undefined.
The ATM network itself has no provision for flow control comparable to those provided by packet networks. This essentially refelects its streamlined nature.
12.4.2. Virtual Path Identifier
The VPI field is 8 bits wide for the UNI and 12 bits wide for the Network-Network Interface NNI. It is used to distinguish between VP links multiplexed into the same
physical connection. All cells belonging to the same VP are assigned the same VPI. As explained earlier, certain cells may be assigned predetermined VPIs. A VPI is
mapped at a VP link termination, such as a switch.
A Virtual Path Connection VPC is a concatenation of VP links, along which the VCIs may be mapped link-by-link. A VPC preserves the cell sequence for each
of the channels it contains.
12.4.3. Virtual Channel Identifier
The VCI field is 16 bits wide and is used to distinguish between VCs sharing the same VP connection. Combined with the VPI, it provides a complete routing
identifier. All cells belonging to the same virtual channel are assigned the same VCI.
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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.
