T1
A T1, T-1, or DS1 is a digital transmission circuit standardized in the United States and Canada that consists of twenty-four 64 Kbps channels that use pulse code modulation (PCM) signals with time-division multiplexing (TDM) at an overall concatenated to transmit (send and receive) data at a rate of 1.544 million bits per second (Mbps) across copper wire, optical, and wireless media.
"T1" is short for "Trunk Level 1," a standard for digital transmission under the DS (Digital Signal Level) hierarchy. Over copper, a T1 is delivered by connecting two pairs of unshielded twisted copper wires between the subscriber location and the provider's central office. One of the twisted copper pairs is for transmitting data and one pair is for receiving data. Working together, the four-copper-wire transmission media carrying the DS digital signalling standard combine to produce a full duplex symmetrical (bidirectional) circuit. When delivered on fiber, a fiber pair is used as opposed to the 4-wire copper configuration.
T1s may be provisioned as a channelized service (delivered as separate voice or data channels), or as an unchannelized raw bit stream of 1.536 Mbps in both direction plus .008 Mbps of framing bits. North American carriers typically deliver T1 circuits as a channelized service with 23 or 24 voice-grade channels that carry 56 Kbps (56,000 bits per second) or 64 Kbps (64,000 bits per second).
When used for data communication applications, a T1 is usually provisioned as unchannelized and provides 1.536 Mbps which you can split up any way you choose. For example, a T1 can be split, by using a multiplexer, into 12 voice grade channels and 12 concatenated channels that create 768 Kbps of bandwidth for data applications such as Internet access, Wide Area Network (WAN) connectivity, video conferencing, etc.
T1s are often used as channelized PBX tie trunks or "tie lines" that connect voice PBXs directly to each other over long distances such as between corporate offices in different states. Similarly, unchannelized T1 cicuits may be used to connect local area network (LAN) routers and data switches.
