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October 19, 1998

Strengthen Your Backbone

Gigabit Ethernet and ATM can help keep traffic moving smoothly over your network backbone

By Logan Harbaugh

s network bandwidth requirements increase, and as more users spend more of their time accessing resources beyond their LAN, the demand for high-speed network backbones is increasing even more rapidly than network growth in general. Backbones carry traffic between LAN segments and may also connect some servers that need to be generally available across a large network.

Though many existing backbones are 100Base-T (100-Mbps Ethernet) or Fiber Distributed Data Interface, the two new technologies most often thought of for backbone use are Gigabit Ethernet and OC-12 asynchronous transfer mode (622-Mbps ATM). Each has the capacity to grow with networks over the next few years and provide sufficient bandwidth to accommodate expanding LANs.

Each has its advantages. ATM is more mature, has established quality-of-service standards, and integrates well with the ATM-based carrier networks used for WAN access. Gigabit Ethernet is less expensive, less complex, and more easily understood by the majority of network administrators who are already familiar with Ethernet's foibles.

My recent participation in Gigabit Ethernet testing at the Information and Computer Sciences School's Advanced Network Computing Lab at the University of Hawaii at Manoa made me realize how much difference there is between the two technologies. While the actual Gigabit testing will be covered more fully in a later article, here I will explore the underlying differences between Gigabit Ethernet and ATM, and the implications for network managers and designers.

The biggest long-termdifference administrators will see between Gigabit Ethernet and ATM is quality of service, while the important short-term differences include cost, ease of installation, ease of configuration, and their staffs' familiarity with the technology.

Quality of service is important in a couple of specific applications. It's intended to let administrators determine which kinds of network traffic have priority--for example, letting them give top priority to video streams for remote learning or videoconferencing because they are most sensitive to delays.

This kind of prioritization is important for anyone who supports video over a network, but it's also very useful for WAN traffic, allowing administrators to set priorities for which traffic will get the most bandwidth over the relatively slow WAN links.

As video over the network becomes commonplace, more administrators are interested in quality-of-service features. The popularity of voice traffic on the network, including voice over IP, is another reason administrators want quality of service. The ATM protocol has established quality-of-service features that let administrators set standards between switches from different vendors, as well as over WAN links, through a carrier's ATM network. There are occasional problems in getting quality-of-service features to work between equipment from a few different vendors, but in general, ATM quality of service should work with most equipment.

Though Gigabit Ethernet vendors offer quality-of-service features, those features are not standards-based; each vendor has its own solution and their devices will not interoperate. In a large enterprise, being limited to products from a single vendor is not often encouraged. In addition, implementing quality of service over a WAN link becomes problematic.

A standard for Ethernet quality of service, 802.1q, has been proposed, but it is still being defined. Until it's been ratified and implemented by vendors, there is little chance that switches or routers from different vendors will interoperate and provide quality of service, or that quality of service over WAN links will be simple to set up. Some vendors are implementing 802.1q now, but there is the chance that the standard will change, and that software upgrades at least will be required to ensure compatibility with the amended standard.

Integration With Existing Networks
Both ATM and Ethernet will integrate well with existing networks. ATM integrates well with token-ring networks and with ATM-based carrier networks, while Gigabit Ethernet integrates well with existing Ethernet networks. This is not to say that ATM cannot be integrated with Ethernet, or Ethernet with token ring, just that the process would be more complex, requiring routers or placing more load on a router or layer-3 switch than with the complementary products.

Token ring is a relatively difficult thing to connect to Ethernet. It's a token-based network protocol that maintains quite a bit of information about where a network packet originated and where it's supposed to go. Ethernet has a very simple packet that can't contain all the same information. ATM, on the other hand, integrates well with token ring; translation between the two is relatively straightforward and puts a relatively small burden on a router.