T1 CAS
T1s have been around since early voice networks. They were developed as a means of carrying
multiple calls across one copper loop. Because the copper loop could carry much
more bandwidth than the 4000 Hz required for voice transmission, they first used
frequency-division multiplexing (FDM) to transmit 24 calls across a single copper loop.
Currently, T1 circuits use TDM to transmit digital data (1s and 0s) instead of the old analog
signals.
A single digital voice channel requires 64 kbps of bandwidth. This is calculated using the
following formula:
64 kbps = 8000 samples/sec ¥ 8 bits/sample = 64,000 bits/sec
This 64 kbps voice channel is also known as DS-0. With 24 voice channels at 64 kbps per
channel, a T1 represents 1.536 Mbps of data. Add an additional 8 kbps for framing, and
the total speed of a T1 circuit comes to 1.544 Mbps.
T1 CAS uses a digital T1 circuit together with in-band CAS. This is done by using bits in
the actual voice channel to transmit signaling information. CAS is sometimes called
robbed-bit signaling because user bandwidth is robbed by the network for signaling. A
bit is taken from every sixth frame of voice data to communicate on- or off-hook status,
wink, ground start, dialed digits, and other information about the call.
T1 CAS uses the same signaling types available for analog trunks: loop start, ground
start, and E&M variants such as wink-start, delay-start, and immediate-start. There are
also various feature groups available when you use E&M. Here are some common feature
groups:
■ E&M FG-B: Inbound and outbound DNIS, inbound ANI (only on Cisco AS5x00)
■ E&M FG-D: Inbound and outbound DNIS, inbound ANI
■ E&M FG-D EANA: Inbound and outbound DNIS, outbound ANI
Figure 4-2 shows CAS with the T1 Super Frame (SF) format. The top row of boxes represents
a single T1 frame with 24 time slots of 8 bits each. An additional bit is added at the
end of each frame that is used to synchronize the SF. A sequence of 12 T1 frames makes
up one SF. CAS is implemented by bit-robbing in frames 6 and 12 in this sequence. The
bottom row of boxes represents T1 frames 6 and 12. The least significant bit of each
voice channel is robbed, leaving 7 bits for voice data.
Extended Super Frame (ESF) format, as depicted in Figure 4-3, was developed as an
upgrade to SF and is now dominant in public and private networks. Both formats retain
the basic frame structure of one framing bit followed by 192 data bits. However, ESF
repurposes the use of the F bit. In ESF, of the total 8000 F bits used in T1, 2000 are used
for framing, 2000 are used for cyclic redundancy check (CRC) for error checking only,
and 4000 are used as an intelligent supervisory channel to control functions end to end
(such as loopback and error reporting).
Chapter 4: Performing Call Signaling over Digital Voice Ports 189
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
4 5 6 7 8 9 10 11 12
12 Frames = Super Frame
24 * (7 Bits + 1 Robbed-Bit) + 1 Bit = 1 Frame (193 Bits)
24 * 8 Bits + 1 Bit = 1 Frame (193 Bits)
Time Slot 8 Bits
1 Bit Sync.
1 2 3
Time Slot 7 Bits
+ 1 Robbed-Bit
Figure 4-2 T1 CAS Super Frame Format
Time Slot 8 Bits
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
24 Frames = Extended Super Frame
24 * (7 Bits + 1 Robbed-Bit) + 1 Bit = 1 Frame (193 Bits)
24 * 8 Bits + 1 Bit = 1 Frame (193 Bits)
Time Slot 7 Bits
+ 1 Robbed-Bit
1 Bit Sync.
Figure 4-3 T1 CAS Extended Super Frame Format