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T1 Basic’s – a Primer April 7, 2010

Posted by TelUS Consulting Services in Data Networking catagory.

As T1 and higher speeds become common everyday services, several people have asked for a basic tutorial on T1’s, so here it is. I hope this is useful.

T1 is a high speed digital network (1.544 mbps) developed by AT&T in 1957 and implemented in the early 1960’s to support long-haul pulse-code modulation (PCM) voice transmission. The primary innovation of T1 was to introduce “digitized” voice and to create a network fully capable of digitally representing what was up until then, a fully analog telephone system.

There is T1, which is, as we have discussed, a network that has a speed of 1.544 Mbps and was designed for voice circuits or “channels” (24 per each T1 line or “trunk”). There is T-3, operating at 44.736 Mbps which is a composite of 28 T1s, more commonly referred to as 45Mbps

The general T-Carrier hierarchy appears in Figure 1 and is detailed in Chart 1.

Figure 1 – T-Carrier Hierarchy

DS0 56/64Kbps 1/24 of T-1 1 Channel
DS1 1.544Mbps 1 T-1 24 Channels
DS3 44.736 Mbps 28 T-1 672 Channels

For mathematical reasons, a voice channel was selected to be at 64 Kbps. 24 of these channels is a composite of 1.536 Mbps, not 1.544 Mbps! Why is there a difference? The reason is that after a byte (8 bits) of data is sent from each channel (24 * 8 = 192 bits) there is an extra bit used for synchronizing called a Frame bit – hence 193 bits are sent and this increase of 1 bit per 192 causes the speed to increase to 1.544 Mbps.

So to paraphrase, only 1536Mbps (64Kbps X 24 = 1536Mbps) is usable bandwidth, the other 8Kps is used for overhead.


Time Division Multiplexing is what makes the T1 work the way it does.  Each channel on a T1 is actually a time slot and data is given an address in a header and sent to its destination on the next available time slot on the carrier.


There are currently only 2 types of framing utilized in today’s T1 networks, Superframe (SF) and Extended Superframe (ESF).  SF is very commonly referred to as D4  by many of those in the local facility arena.

Line Coding

There are two major types of line coding B8ZS (Binary 8 zero suppression)  and AMI (Alternate Mark Inversion).


This means that a plus voltage, a zero voltage, and a minus voltage are important to the coding of the signal. The code, which is used in T1, is call AMI for Alternate Mark Inversion. This means that if a “1” or Mark is coded as a positive voltage, the very next “1” must be a minus voltage or the result will be a Bipolar Violation (BPV).


An improvement to this technique was developed known as B8ZS with stands for Binary Eight Zero Substitution. This technique takes advantage of BPV’s in the data stream to be decoded as a signal.  With B8ZS coding, each block of 8 consecutive zeros is replaced with the B8ZS code word. Because of the ones density requirement, a one must be inserted for every seven consecutive zeros. B8ZS circumvents this requirement by inserting two code violations on a physical link instead.

This is the standard for “Clear Channel Capability” 64kbs.

*Note the maximum bandwidth on a single channel on a T1 with an AMI carrier is only 56kbs.  The other 8Kbs is utilized with B8ZS line coding to allow for a full 64Kbs per time slot or channel.

T1 Connectors

ANSI T1-403-1989 specification calls out for “one of four Universal Service Ordering Code (USOC) connectors (RJ48C, RJ48X, RJ48S, and RJ48M)” with pin assignments as follows:

1 Receive (ring)
2 Receive (tip)
3 Not Used
4 Transmit (ring)
5 Transmit (tip)
6 Not Used
7 Not Used
8 Not Used

Understanding USOC and what those (RJ) codes really mean. The Universal Service Ordering Code (USOC) system was developed by the Bell System and introduced by AT&TO in the 1970s to connect customer premises equipment to the public network. These codes, adopted in part by the FCC, Part 68, Subpart F, Section 68.502, are a series of Registered Jack (RJ) wiring configurations for telephone jacks that remain in use today.

Registered jack (RJ) numbers end with a letter indicating the wiring or mounting method being used.

  • C – Identifies a surface or flush mounted jack.
  • W – Identifies a wall mounted jack.
  • S – Identifies a single line jack
  • M – Identifies a Multiple line jack
  • X – Identifies a complex Multiline or series type jack.

RJ48C – RJ48S – & RJ48X are the most commonly used jacks.  The only one that is truly different is the X jack, because it offers a loopback towards the network when there is nothing plugged into it.  The loopback automatically clears when jack is plugged into it.

As it goes, the above pin-out and connectors is also the “de facto” standard vis-a-vis how currently available hardware is configured.

For more reference information, see http://www.arcelect.com/RJ48C_and_RJ48S_8_position_jack_.htm

In the next blog I will discuss a little about T1 applications…

Joe Buck, N.C.E.

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