Question 160 of 490 from exam 400-051: CCIE Collaboration

# Question 160 of 490 from exam 400-051: CCIE Collaboration

### Question

In Channel Associated Signaling on a T1 circuit using Extended Super Frame, how many signaling bits does each T1 timeslot have?

### Explanations

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A. B. C. D. E.

C.

Each T1 channel carries a sequence of frames.

These frames consist of 192 bits and an additional bit designated as the framing bit, for a total of 193 bits per frame.

Super Frame (SF) groups twelve of these 193 bit frames together and designates the framing bits of the even numbered frames as signaling bits.

CAS looks specifically at every sixth frame for the timeslot's or channel's associated signaling information.

These bits are commonly referred to as A- and B-bits.

Extended super frame (ESF), due to grouping the frames in sets of twenty-four, has four signaling bits per channel or timeslot.

These occur in frames 6, 12, 18, and 24 and are called the A-, B-, C-, and D-bits respectively.

References: Reference: http://www.cisco.com/c/en/us/support/docs/voice/digital-cas/22444-t1-cas-ios.html.

Channel Associated Signaling (CAS) is a technique used to signal between switches in a voice network over T1 or E1 circuits. In CAS, the signaling information is transmitted on the same channel as the voice traffic, in contrast to Common Channel Signaling (CCS) where signaling information is transmitted over a separate channel.

Extended Super Frame (ESF) is a framing format used in T1 circuits that provides improved synchronization and error detection capabilities over the older framing formats. ESF divides a T1 frame into 24 contiguous time slots, each with 8 bits, for a total of 192 bits per frame.

In ESF, there are several types of signaling bits that can be used for different purposes, including robbed-bit signaling (RBS), robbed-bit signaling with parity (RBSP), and clear-channel signaling (CSS).

For RBS and RBSP signaling, one signaling bit is allocated per time slot. This bit is "robbed" from the least significant bit of the time slot's voice channel, and is used to transmit signaling information between switches.

For CSS signaling, all 8 bits in the time slot are used for signaling, and no bits are "robbed" from the voice channel.

In the context of the exam question, it is asking specifically about CAS using ESF and how many signaling bits are present in each T1 timeslot. Based on the information above, we can determine that the answer is A) 1 signaling bit per timeslot for RBS and RBSP signaling. Therefore, each T1 timeslot in ESF with CAS has one signaling bit available for RBS or RBSP signaling.