Get The Most Out Of ATM Networks With Multicasting

Asynchronous Transfer Mode (ATM) has evolved from limited field trials to early volume deployment since the introduction of the first commercial product in 1993. There is a growing recognition in the industry that ATM connections to individual desktop computers will provide significant benefits to users including:

* A single network access point for all services

* Improved access to multimedia and video conferencing services

* Quality and simplified management of services

To increase the pace of ATM deployment, perceived barriers such as high cost and the lack of stable specifications for ATM services need to be overcome. The ATM Forum is moving rapidly to finalize key specifications that will make ATM the best option for multimedia applications. The specifications already finalized or soon-to-be finalized are:

* LAN Emulation

* Private Network Interfaces

* Multiprotocol Over ATM

* Traffic Management

* Switched Virtual Circuit

* Voice Over ATM

One common service that has become a business driver for the deployment of ATM networks today is the Internet. Recent dramatic increases in Internet access and usage have pushed up bandwidth demand and produced delays and congestion. ATM is an ideal solution for these applications, because it is inherently capable of providing the right quality of service (QoS) depending on the requirements.

An approach that will enable ATM networks to be the most efficient and cost-effective solution is to use the switching capability of the network more efficiently. Such a scheme, multicasting, is discussed in this article.

The major advantage of an ATM network is its ability to transport different types of signals through a single network using a standard cell format. Different classes of service imply various levels of cell priority, such as constant bit rate (CBR), for the transport of delay-sensitive traffic such as voice and interactive video; variable bit rate (VBR), for delay-insensitive traffic such as data; available bit rate (ABR), for non-time-critical traffic; and unknown bit rate (UBR) traffic.

Different modes of transmission demand support for unicast (point-to-point) and multicast (point-to-multipoint) transmission. Multicasting can be implemented as a series of multiple unicast links if the network is substantially under-used. But the recent dramatic increases in Internet usage will most likely overwhelm networks using the the unicast approach. It is more economical to use true multicasting than to add more bandwidth and use unicasting. Therefore, the ATM switching nodes will need to provide both priority queueing and multicasting features.

Multicasting is an efficient way of reducing the demand on the ATM network and switching bandwidth, thereby reducing the cost per connection. For example, if the ATM nodes do not have multicasting, a user sending an e-mail to coworkers 1, 2, 3, 4, and 5 must transmit five unicast copies of the same e-mail to five different destinations (Fig. 1a).

With multicast functionality, there are different ways the e-mails can be sent. One way is to send a copy to node B, from which it is multicasted to nodes C and E (Fig. 1b). Node C delivers the e-mail to coworker 5. Node E sends it to node D, where it is delivered to coworker 4, and passed down to node G, which delivers it to coworkers 1, 2, and 3.

Comparing Figures 1a and 1b, the unicast method uses 13 segments of the network while the multicast method uses only five (Table 1). Therefore, using multicasting in the above example results in a 62% savings on network bandwidth usage. Different amounts of network bandwidth would be saved if other paths are used. The trade-offs would be in the amount of delay and the probability of congestion.

Delay in a network is defined as the time it takes for the message to go from the sender to the receiver. The minimum delay is the sum of the time it takes for the message to travel across the links and the queueing time in the nodes. If any of the links in the path become congested, additional delay will be added until the link becomes available, assuming the message is not discarded due to congestion.

In normal operation, there should be no congestion, and the minimum delay is the absolute time it takes to traverse a path. In the e-mail example above, the use of multicasting sends the e-mail to coworker 1 through a four-node path versus a two-node path using unicast. As a result, the multicasting incurs more node and link delays. If L is the average delay through one node and one link, the average delay using multicast would be 4L. This is the minimum possible delay assuming congestion-free transmission.

The total delay is dependent on how many times the e-mail stalls in a node, and the length of the congestion. Today, there is not enough experience with ATM traffic patterns, especially in the Internet service area, to give a definitive figure. However, a first-order estimate is given here to show the potential impact of multicasting.

If P_CM is the average probability of congestion for any node in multicast mode, and P_CU is the average probability of congestion for any node in unicast mode, the probability of congestion-free transmission through the four-node path (P_4F) using multicast is:

P_4F = (1-P_CM)⁴ (1)

The probability of congestion-free transmission through the two-node path (P_2F) using unicast is:

P_2F = (1-P_CU)² (2)

The probability of congestion in a network with multicast capability is smaller than with unicast only--in this example 62% lower. Therefore:

P_CM = 0.38 x P_CU (3)

Combining equations 1 and 3 yields:

P_4F = (1 - 0.38P_CU)⁴ (4)

A comparison of equations 2 and 4 is shown in Table 2.

For any given probability of congestion P_CU, the chance of having a congestion-free transmission is always higher using multicast than unicast, even though with multicast, the path is twice as long.

Therefore, using multicasting increases the probability of congestion-free transmission. Alternatively, the network can support more users for a given desired probability of congestion. The trade-off is longer absolute minimum delay.

Within an ATM switching node, the cell processing is usually partitioned between the switching fabric and the interfacing line cards. Cells received on an incoming link are either stored in input buffers waiting to be routed to outbound links, or routed directly to output buffers. There are several types of ATM switch architectures which deliver various levels of price performance for multicast services. One of the concerns in switch node design is how effectively multicast and broadcast functions are supported.