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Q4. What is SLC-96 and how does it relate to T1?

A4. SLC-96 (pronounced "slick 96") is used in conjunction with T1 as a digital-loop carrier system, which provides service

for up to 96 subscribers over three to five separate T1 lines. SLC-96 was the first widely deployed digital-loop carrier

system. SLC-96 operation consists of two terminals: a central-office terminal (COT) and a remote terminal (RT). The COT

multiplexes the analog phone lines from a class 5 switch into four T1 lines with an optional fifth T1 line used for protection

switching. The RT is the termination point for the T1 lines and converts them into subscriber phone lines. SLC-96

revolutionized the telecommunication industry by introducing digital electronics into the local loop as a permanent

replacement for cable. The frame format of SLC-96 closely resembles that of the super-frame (SF or D4) format, except

that additional data-link information is transferred in the framing bits. This allows the deployment of advanced functions

such as single-party, multiparty, coin-telephone, and special-service circuits. For more information on SLC-96, please

consult Telcordia specification TR-008.

Q5. How are frames, channel time slots, and bits numbered in T1 and E1?

A5. According to the ITU-T specification G.704, the numbering structures for T1 and E1 digital transmission systems are

as follows:

In T1 systems, the frames are numbered 1 to 12 in the SF/D4 format, and 1 to 24 in the ESF. The channel time slots are

numbered 1 to 24. The bits are numbered 1 to 8, with the 8th bit being the least significant bit (LSB).

In E1 systems, the frames are numbered 0 to 15 in the CRC-4 and CAS multiframe formats. The channel time slots are

numbered 0 to 31. The bits are numbered 1 to 8, with the 8th bit being the least significant bit (LSB).

Q6. What are the differences between robbed-bit signaling (RBS), channel-associated signaling

(CAS), and common-channel signaling (CCS)?

A6. RBS is the original signaling system used by T1 and provides either 2 or 4 signaling bits per channel depending on the

multiframe format. In RBS, the LSB in every channel of every 6th frame is used as a signaling bit. Hence, in the SF

format there is an A/B bit for every channel, and in the ESF format there is an A/B/C/D bit. CAS is the original signaling

system used by E1 and provides 4 signaling bits for every channel. In CAS, channel 16 is reserved for signaling and

the A/B/C/D bits for each channel are divided among 16 frames. Frame 0 contains the alignment signal, alarm, and spare

bits. Frame 1 contains the A/B/C/D bits for channel 1 in the upper half of the channel, and the A/B/C/D bit for channel 16

in the lower half. The remaining 14 frames follow the frame 1 format accordingly. In recent years, the term RBS has been

replaced by CAS, which is now used to refer to bits that are associated with a specific channel whether it is in the T1 or

E1 format. CCS is used by either T1 or E1 and refers to a system that does not use a specific bit structure for signaling.

Instead, all or part of a channel is used to pass messages between two systems to indicate how a channel is being used.

This type of system is commonly found in ISDN, which uses a D channel to pass messages.

Q7. What are the red, blue, and yellow alarms in a T1 system?

A7. The three different alarms are used to indicate different problems in transmission or reception of data in the T1

system. To explain the different alarms, it is necessary to have a simple T1 system model that has a T1 signal sourced

from the central office (CO) through a repeater to the customer premises equipment (CPE) and back. The red alarm is

actually a CPE state resulting from the detection of an incoming signal failure (i.e., from a line break). The yellow alarm is

transmitted to indicate the loss of an incoming signal. In the case where the CPE is in the red alarm state, it will transmit

the yellow alarm to the repeater. The blue alarm is transmitted upon loss of the originating signal or when a signal is

actively disrupted. In the case where the repeater receives the yellow alarm, it will transmit the blue alarm back to the

CO. Upon receiving a blue alarm, the CO will transmit a yellow alarm to the repeater to indicate the loss of incoming

signal. It may seem confusing as to whether a piece of equipment should transmit a yellow or blue alarm. Generally, any

piece of equipment on the endpoint (i.e., a CPE or a CO) will transmit the yellow alarm to indicate the loss of signal, while

equipment in the middle of the path (i.e., a repeater) will transmit the blue alarm.

Q8. What are the alarm indication signal (AIS), remote alarm indication (RAI), and distant

multiframe alarm (DMA) in an E1 system?

A8. The AIS and RAI are essentially the same as the T1 blue and yellow alarm, respectively. The AIS is transmitted upon

loss of the originating signal or when the incoming signal is actively disrupted. The RAI is transmitted to indicate the loss

of an incoming signal. The DMA is specific to CAS signaling and is transmitted when the correct CAS multiframe alignment

signal is not found.

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