Summary

Chapter 8. WAN Protocols and Technologies: Asynchronous Transfer Mode (ATM)

Authoring contribution by Galina Diker Pildush

Asynchronous Transfer Mode (ATM) has been looked upon as the technology that can do it all ”carry voice, data, and video information, implying that both voice and data payloads, batch and real-time, can be transferred from one part of the world to another. This is implemented with guarantees in quality, such as integrity of the information and its throughput, toward different classes of services. In my book Cisco ATM Solutions , I elaborate on the whys and whats with respect to ATM. It is the technology that instilled in me a sense of the simplicity and beauty within it.

The concept behind ATM is quite simple: Push into the ATM cloud equal- size payloads (an ATM cell consists of a 48-byte payload and a 5-byte header) that consist of any type of applications above it. Perform no error checking of these payloads; do not waste any overhead on useless things such as sequence numbers (because ATM is connection-oriented anyway), just to perform error checking of the ATM header (which is done at the silicon layer); and zoom through those payloads as fast as you can at a low layer of the OSI reference model. ATM operates on 1.25 layers of the OSI reference model ”exactly, 1.25 layers ! Figure 8-1 provides the information about the ingredients of an ATM cell.

Figure 8-1. ATM Cell Format

Table 8-1 lists the meaning of the cell header fields.

Table 8-1. ATM Cell Header Fields

Field	Size	Meaning
GFC	4 bits	Generic Flow Control: Used at the UNI level.
VPI	8 or 12 bits	Virtual Path Identifier: Part of ATM locally significant address. It is 8 bits at the UNI and 12 bits at the NNI.
VCI	16 bits	Virtual Channel Identifier: Part of the ATM locally significant address.
PT	3 bits	Payload Type. The 3 bits are used as follows : The most significant bit is used to identify the data versus the OAM cell, the next bit is used to identify congestion on the network (it is called Explicit Forward Congestion Indication [EFCI]), and the last bit is used to identify the last cell of a higher-level packet/frame. This bit, called End of Message, is important in the congestion control algorithm within the ATM cloud.
CLP	1 bit	Cell Loss Priority: Used to indicate if a cell can be discarded by the network, in case there is congestion.
HEC	8 bits	Header Error Check. The ATM cloud does not perform payload error checking, only the header error checking.

The powerful concepts of ATM's Private Network-to-Network Interface (PNNI) helped in the development of such protocols as MPLS with traffic engineering, which introduces a connection-oriented flavor to connectionless IP. PNNI is a routing protocol deployed by the ATM signaling protocol to establish SVCs.

Recognizing ATM's impact on the customer's enterprise networks, Cisco introduced interconnections of various protocols over ATM into its Routing and Switching CCIE certification. This chapter discusses two methods of protocol interconnectivity over ATM that address implementation from the edge device perspective:

• RFC 2684, "Multiprotocol Encapsulation over ATM Adaptation Layer 5"

• RFC 2225, "Classical IP and ARP over ATM"

The implementations of the actual ATM cloud are not currently in the CCIE practical exam. See the Cisco ATM Solutions book for the implementations of the cloud itself.

The whole objective of ATM internetworking is to send upper-layer information over ATM. Four implementation methods exist for achieving that goal:

• Manual method for multiple upper-layer protocols by using RFC 2684. I call it a method without any magic "upstairs" in the upper layers of the OSI reference model.

• Dynamic method for IP implementation by using RFC 2225 (Classical IP). I call it a method with the magic of transporting IP over ATM.

• Dynamic method for Layer 2 protocols by using Local-Area Network Emulation (LANE).

• Dynamic method for all Layer 2 and Layer 3 protocols by using Multiprotocol over ATM (MPOA).

This chapter focuses on the first two methods ”RFC 2684 and RFC 2225.

The two labs presented in the chapter provide you with additional practical exercises. If you require information on other interconnection methods (for example LANE, MPOA, and so on) or more examples/labs, refer to the Cisco ATM Solutions book.