A Made IT project http://www.made-it.com info@made-it.com

The T1 (trunk) digital lines were developed to replace the analog lines that interconnected the local telephone offices (CO, central office) in the USA. The CO's were connected using 4-wire lines, and frequency division multiplexing to get enough bandwidth to handle enough calls between local offices. Each call was allocated 3kHz of bandwidth, 0-3, 3-6, 6-9, etc. on the trunk line. Because of the variation with time and temperature of the characteristics of the discrete components used, these lines needed a lot of maintenance and still were not free of cross talk due to overlapping bands. Every 1830m/ 6000ft an amplifier was needed that also amplified noise. T1 was decided to sample a 4kHz bandswidth at an 8k (2x highest frequency) sample rate taking 8 bit samples. That resulted in a 64kbps digital signal. Combining 24 channels results in 1.536Mbps. Adding a framing bit gives the 1.544Mbps that T1 is known for. The old analog amplifiers were replaced by T1 repeaters or digital regenerators. The digital signal was regenerated not amplified, thus discarding any noise that was picked up.
The T1 trunk lines are changed over now to carry the ISDN network signals. The 2B+D Basic Rate Interface uses 64k for each B-channel plus 16k for the D-channel, the 23B+D Primary Rate Interface uses 64k for each B-channel plus 64k for the D-channel, which equals 1.544 Mbps, T1 bandwidth. The PRI in Europe uses 31B+D because the 2.048Mbps E1 lines there allow for 32 64k channels.

How they invented the speeds
T1 and up is the terminology for digital, two-way transmission of voice, data, or video over a single highspeed circuit. The transmission rate is based on the bandwidth for one voice channel in digital form. This channel is called DS-0 and consists of 64 kbps of bandwidth.
By Time Division Multiplexing 24 DS-0 channels, T1 is formed. But there is more. To separate the different channels a framing bit is used. For framing 8000 bps are used.
T1 gives you 24 analog voice channels plus the framing rate. This makes the T1 speed: 24 x 64000 + 8000 = 1.544 Mbps. For the other T-versions an equal equasion is used ending everything up to the specs given in the table below.

The DS-1 interface
There are several different possible interfaces for T1. For copper the DS-1 interface can transmit signals 1828 meter (6000 feet) over 24 AWG twisted pair cable. The actual connectors are a Sub-D15 female or a RJ45 female.

Sub-D15 Female	RJ45 Female

Framing
For a receiving multiplexer to recognize what is what in the incoming bitstream some smart things are done. First of all from every channel only 8 bits at a time are send. Each of those 8 bits is called a time slot, so T1 consists of 24 time slots in the frame. The entire frame is thus 192 (24 x 8) bits and 1 control bit (the framing bit) which makes up 193 bits in total. At a repeat speed of 8000 times per second you get the T1 rate of 1.544 Mbps.
The modern T1 lines use one of two framing schemes: SuperFrame (SF) or Extended SuperFrame (ESF).

SF

Also known as D3/D4 frame (CCITT: G.733). Each superframe consists of 12 frames. The 193rd bit has different meanings. In the odd-numbered frames it acts as a terminal framing bit (FT), but in the even-numbered frames it acts as a signaling framing bit (FS).

FT frames are used for synchronization.

FS frames may be used for synchronization or for data link communication between terminals or in SLC-96 applications.

The framing bits (FT and FS togather) have a sequence of 1000 1101 1100. One out of 8 bits is robbed for signaling from each DS-0 through DS-24 channel in the 6th and 12th frame of each superframe. This way of bit robbing is refered to as A/B signaling: 2 bit codes for on-hook, off-hook, busy or idle.

Frame Number	FT	FS	Information bits	Signaling bit	Signaling channel
1	1	-	1-8
2	-	0	1-8
3	0	-	1-8
4	-	0	1-8
5	1	-	1-8
6	-	1	1-7	8	A
7	0	-	1-8
8	-	1	1-8
9	1	-	1-8
10	-	1	1-8
11	0	-	1-8
12	-	0	1-7	8	B

ESF

Also known as D5 frame or Fe. Each extended superframe consists of 24 frames. The ESF knows 3 different framing types:

Synchronization: bit sequence 001011 in frames 4, 8, 12, 16, 20, 24

Data Link: frames 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23

CRC-6: frames 2, 6, 10, 14, 18, 22

One of 8 bits of each DS-0 channel is robbed for signaling in 6th, 12th, 18th, and 24th frame. This is also called A/B/C/D signaling. The options define up to 16 features.

6 out of 24 S-bits are used for synchronization. The are called Fe bits. The remaining bits are used for a 4 kbps data link and an ESF block check as shown in the table below.

Frame Number	S-bits			Bit use in each channel time slot		Signaling-bit Use options
	Fe	DL	CRC	Traffic	Signaling	T	2	4	16
1	-	m	-	1-8
2	-	-	C1	1-8
3	-	m	-	1-8
4	0	-	-	1-8
5	-	m	-	1-8
6	-	-	C2	1-7	8	-	A	A	A
7	-	m	-	1-8
8	0	-	-	1-8
9	-	m	-	1-8
10	-	-	C3	1-8
11	-	m	-	1-8
12	1	-	-	1-7	8	-	A	B	B
13	-	m	-	1-8
14	-	-	C4	1-8
15	-	m	-	1-8
16	0	-	-	1-8
17	-	m	-	1-8
18	-	-	C5	1-7	8	-	A	A	C
19	-	m	-	1-8
20	1	-	-	1-8
21	-	m	-	1-8
22	-	-	C6	1-8
23	-	m	-	1-8
24	1	-	-	1-7	8	-	A	B	D

This way only 25% of the 8000-bit bandwidth for frame synchronization, 2 kbps is used for framing, 2 kbps is used for error checking and 4 kbps is used for diagnostics and performance characteristics, which is called the Facility Data Link.