1 The Physical Medium Dependent PMD sub-layer which provides the actual transmission of the bits, in the ATM cells.
2 The Transmission Convergence TC sub-layer which transforms the flow of cells into a steady stream of bits and bytes for the transmission over the physical medium.
A. PMD Sub-layer
The original transport medium for ATM was defined by ITU-T for B-ISDN architecture in the late 1980s. It was a fiber-based, scalable, high-speed network scheme referred to as Synchronous Digital
Hierarchy SDH. The SDH is standardized for use in the U.S. by the American National Science Institute ANSI and is known as the Synchronous Optical NETwork SONET
By 1992 the ATM forum declared that ATM did not have to be limited to high-speed fiber medium and other media such as coaxial cable, microwave links, etc. should be supported. Goralski 1995 and
Ginsberg 1996 discuss various mediums in detail with the emphasis on the fiber-based network. The overall philosophy of ATM network is that they should be fast and highly reliable. i.e., the transmission
errors should be fewer in magnitude. But it is also important that the transport media should be readily available. ATM is meant for providing unchannelized network support to a variety of users with
connections having different speeds. Thus it is more informative to group the allowed transport by physical medium rather than by technology. Figure 5-4 illustrates this framework where some transport
media are meant to be used in private ATM networks while others are anticipated to be used only in public ATM networks.
Previous Table of Contents Next
Copyr ight © CRC Pr ess LLC
by Abhijit S. Pandya; Ercan Sen CRC Press, CRC Press LLC
ISBN: 0849331390 Pub Date: 110198
Previous Table of Contents Next
B. ATM Transmission Convergence TC Layer
On the transmission side of the network the TC sub-layer is responsible for loading the ATM cells into the physical transport transmission frame, i.e., SONET or DS-3. In some cases it associates cells into blocks
4B5B, 8B10B, etc. before transmission. On the reception side it must delineate the individual cells in the received bit stream from the transmission frame format. It also provides error checking via the Header
Error Check field in the ATM cell header.
Table 5-1ATM layers and their functions LAYER
SUBLAYER FUNCTIONS
ATM Adaptation Layer AAL
Convergence Sublayer CS • Convergence functions
Segmentation and Reassembly SAR
• Segmentation and reassembly functions ATM Layer
• Cell multiplexing and demultiplexing • Cell VPIVCI translation
• Cell header generationextraction • Generic flow control
Physical Layer PL Transmission Convergence
TC • Cell delineation
• Cell rate coupling • HEC generationverification
• Transmission frame generationrecovery • Transmission frame adaptation
Physical Medium PM • Bit timing
• Physical medium management
Following are the major functions carried out by the TC sub-layer see Figure 5-5:
1 Transmission frame generation on the transmit side depending on the type of transport and recovery. It is also responsible for unpacking the cells at the receiver side.
2 Generation of Header Error Code HEC sequence at the time of transmission and using it to detect and correct errors on the receiver side.
Figure 5-4 Various forms of physical transports allowed by the ATM forum.
Figure 5-5 Major functions carried out by the TC sub-layer.
The cell rate decoupling feature provides the ATM network great flexibility in connection speeds over the same physical line rate. For the synchronous scheme it is essential that a fixed number of frames bits per
second are sent over a framed physical transmission path. Consider a situation where a SONET link STS- 3c 155Mbps is connected to a customer site. However the customer i.e., the transmitter is generating
data at a much slower rate. For example, the data bits are placed into cells at only 78 Mbps instead of the ATM network link rate of 155 Mbps. In this case, as illustrated in Figure 5-6, extra 77 Mbps of idle cells
are inserted by the TC sub-layer and removed by the receiver. The idle cells have a fixed format as shown in Figure 5-7 which makes it easy for the receiver to identify and discard them.
Before accepting a cell at the receiver node it is important to check for the correct header information. This is achieved by using the HEC to supply robust error correction and detection for errors in the header see
Figure 5-8. It is important to note that error detection and correction are performed only on the ATM cell header and not on the actual data or information in the cell itself.
As illustrated in Figure 5-8 only single bit errors are corrected, which is an optimal solution in the case of fiber media where most errors are single bit. However, it may lose its effectiveness on other media,
especially copper wires, with burst error characteristics.
Figure 5-9 shows the flow chart for an error correction algorithm while Figure 5-10 shows the corresponding state diagram.
III. The ATM Layer
The physical layer in the ATM protocol stack sends and receives cells. However, these cells are actually processed by the ATM layer. Typically, at the endpoints, the ATM layer, which deals with a stream of
cells, multiplexes and demultiplexes, while at the network nodes, it switches cells.
