T3 technology has two applications in wide use in corporate networking: as a backbone or Internet link, and as a means of aggregating multiple T1 lines into one physical access point at the core of your network.

The first application treats the T3 as one big pipe (a single channel with 43-Mbps throughput) between two points, most commonly between a Web server farm and the Internet. Alternatively, the T3 can be used as part of the corporate backbone that links two sites requiring that level of available bandwidth.

The second application is more interesting. Channelized service was introduced with T1 lines. T1s are used to deliver multiple 64-Kbps DS-0 circuits on one physical connection to be groomed out either individually or collectively to remote locations. This is done by allocating time slots on the T1 to each remote destination, with each time slot equating to one 64-Kbps channel. This makes it easy to carve out 64 Kbps, 128 Kbps or any other multiple of 64 to any remote site. A channelized T3 works similarly, but instead of the 64-Kbps DS-0 channel as the unit of bandwidth, a full T1 is groomed out to remote locations.

A third, less common option is to interface a T3 directly to a remote access controller. This makes it possible to service 672 simultaneous dial-up connections via the one physical link. This works by breaking down the T3 into its component T1s, then accessing the individual DS-0 circuits within those T1s. However, few companies really need this number of dial-up connections available in one location.

The problem with T3 lines is that they are betwixt and between in terms of the end-user equipment that can interface to them at the physical level. The serial interfaces typically found on a router to support line speeds up to T1 (1.536 Mbps) are no good for T3 applications. Likewise, optical interfaces that support the high data rates available via Sonet don't accept the copper-based T3 connections.

A higher-speed serial interface that will work with copper-based connections is needed. On Cisco equipment, that need was met by HSSI (High-Speed Serial Interface), which is now a standard, EIA/TIA-612/613, and supports data rates up to 52 Mbps. HSSI cards initially were only available for the 7x00 series routers, whereas there is now a HSSI module for the lower-cost 3640 series.

More recently, Cisco Systems' Port Adapter Module (PAM) arrived, specifically for T3 connectivity. Among its advantages is PAM's ability to access individual channels on the T3, whereas a HSSI interface just sees 43.008 Mbps throughput as one channel. The PAM therefore gives you the flexibility to take in a T3 feed from the phone company, and groom out individual T1 circuits (see "Making T3 Connections With PAM," to the right).

Other suppliers offer alternatives. Ascend Communications' MAX TNT product supports DS-3 access via 75-ohm BNC connectors into cards that are specific either to Frame Relay or ATM encapsulation types. The Cisco HSSI offers additional flexibility in that it will support the STS-1 rate of 51.2 Mbps, whereas the Ascend device is limited to 44.736 Mbps. STS is the Synchronous Transport Signaling mechanism used by Sonet.