Gigabit Ethernet and ATM


Discussion.  The following is excerpted from a section of a Cisco Technology Brief on Gigabit Ethernet and ATM.

Gigabit Ethernet and ATM

A few main factors drive network scalability on the campus. First, bandwidth and latency performance become more important as existing and emerging applications are and will be requiring higher bandwidth. The typical 80/20 rule (80 percent of the network traffic is local compared to 20 percent to the backbone) is being reversed such that 80 percent of the traffic is now destined for the backbone. This setup requires the backbone to have higher bandwidth and switching capacity.

Both ATM and Gigabit Ethernet solve the issue of bandwidth. ATM provides a migration from 25 Mbps at the desktop, to 155 Mbps from the wiring closet to the core, to 622 Mbps within the core. All this technology is available and shipping today. ATM also promises 2.4 Gbps of bandwidth via OC-48, which was available and standard at the end of 1997. Ethernet currently provides 10 Mbps to the desktop, with 100 Mbps to the core. Cisco has provided Fast EtherChannel® as a mechanism of scaling the core bandwidth and providing a migration to Gigabit Ethernet.

Second, a scalable campus networking architecture must account for existing desktops and networking protocols. This scenario forces compatibility with current desktop PCs, servers, mainframes, and cabling plants. Large enterprise networks have invested millions of dollars into this infrastructure. Also, in order to ensure a smooth migration, existing LAN protocols must be supported in some way in order to assure a smooth migration.

Quality of service (QoS) has increased in visibility, as network managers require some traffic to have higher-priority access to the network relative to other traffic, particularly over the WAN. The options for QoS include Guaranteed QoS, where a particular user or “flow” is guaranteed performance, and class of service (CoS), which provides best-effort QoS, and finally, increased bandwidth such that contention for that bandwidth is no longer an issue.

Ethernet promises to provide CoS by mapping priority within the network to mechanisms such as Resource Reservation Protocol (RSVP) for IP as well as other mechanisms for Internetwork Packet Exchange (IPX). ATM guarantees QoS within the backbone and over the WAN by using such mechanisms as available bit rate (ABR), constant bit rate (CBR), variable bit rate (VBR), and unspecified bit rate (UBR).

Both ATM and Ethernet attempt to solve similar application-type problems. Traditionally, Ethernet and Fast Ethernet have been utilized for high-speed backbone and riser connectivity. A common application, for example, is to provide switched or group-switched 10 Mbps to each desktop, with Fast Ethernet connectivity to and within the core. This scenario can be accomplished at a relatively low cost. Gigabit Ethernet promises to continue scaling that bandwidth further. Recently, ATM has also been utilized to build campus-wide backbones at a moderate price range. However, the key benefit of ATM has been seen in the metropolitan-area network and in the wide-area network. WAN integration and compatibility has been a significant driver in scaling campus networks. The importance of integrating data types such as voice, video, and data over a WAN has been a significant driver for service integration and will be key in reducing the cost of WAN services and maintenance.