» Digital Voice Ports

By | September 24, 2012

Here you will find answers to CVoice – Digital Voice Port Questions

Question 1

A customer needs to configure a CAS E&M circuit that will support inbound and outbound DNIS and inbound ANI. Which configuration will accomplish this task?

A. pri-group timeslots 1-24
B. ds0-group 0 timeslots 1-24 type none
C. ds0-group 0 timeslots 1-24 type e&m-fgd
D. ds0-group 0 timeslots 1-24 type fgd-eana
E. ds0-group 0 timeslots 1-31 type r2-digital r2-compelled ani

Answer: C


To define T1 channels for compressed voice calls and the channel-associated signaling (CAS) method by which the router connects to the PBX or PSTN, enter the ds0-group controller configuration command. Below is the syntax of this command:

ds0-group ds0-group-no timeslots timeslot-list type {e&m-immediate | e&m-delay | e&m-wink | e&m-fgd | fgd-eana}


ds0-group-no A value from 0 to 23 that identifies the DS0 group
timeslot-list timeslot-list is a single timeslot number, a single range of numbers, or multiple ranges of numbers separated by commas. For T1, allowable values are from 1 to 24. Examples are:

  • 2
  • 1-15, 17-24
  • 1-23
  • 2, 4, 6-12
type The signaling method selection for type depends on the connection that you are making. The E&M interface allows connection for PBX trunk lines (tie lines) and telephone equipment.

The options are as follows:

  • e&m-immediate-start specifies no specific offhook and onhook signaling.
  • e&m-delay specifies that the originating endpoint sends an offhook signal and then and waits for an offhook signal followed by an onhook signal from the destination.
  • e&m-fgd specifies E&M Type II Feature Group D.
  • e&m-wink-start specifies that the originating endpoint sends an offhook signal and waits for a wink signal from the destination.
  • fgd-eana specifies Group D Exchange Access North American (EANA) signaling.

(There are some more options but they are omitted)

(Reference: http://www.ciscosecure.net/en/US/docs/ios/12_1/12_1xd/feature/guide/hdv_fgd.html)

T1 CAS always provides the ANI/DNIS delimiter on incoming T1/CAS trunk lines. The customer wants E&M circuit so the answer should be C.


+ CAS signaling main feature is its use of user bandwidth to perform signaling functions. CAS signaling is often referred to as robbed-bit-signaling because user bandwidth is being “robbed” by the network for other purposes.

+ E&M signaling is typically used for trunks. It is normally the only way that a central office (CO) switch can provide two-way dialing with direct inward dialing.

+ ANI – Automatic number identification. SS7 (signaling system 7) feature in which a series of digits, either analog or digital, are included in the call, identifying the telephone number of the calling device. In other words, ANI identifies the number of the calling party.

+ DNIS – Dialed number identification service, also known as the called party number. The telephone number of the called party after translation occurs in the Public Switched Telephone Network. A given destination may have a different DNIS number based on how the call is placed (for example, 800 or direct dial).

Question 2

In T1 CAS, where are the signaling states and control features carried for Super Frame robbed-bit signaling?

A. 6th and 12th frame
B. 6th, 12th, 18th, and 24th frame
C. the first and seventeenth time slot
D. the first and sixteenth time slot

Answer: A


Each T1 has 24 channels ( or 24 DS0 – digital signal level 0) that can transmit 8 bits per channel each. This give us a total of 192 bits. One more bit is used for framing, bringing the total to 193 bits. Super Frame bundles 12 of these 193-bit frames for transport. The picture below shows the structure of a T1 Super Frame


The T1 CAS signaling looks at every 6th & 12th frames for signaling information, these bits are referred to as the A and B bits. The A and B bits can represent different signaling states or control features (on hook or off hook, idle, busy, ringing, and addressing)


According to Nyquist theory, we sample voice 8000 times per second, that means we need to send 8000 of these 193-bit frames every second. So 8000 x 193 = 1,544 Mbps.

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. So if the question asks about ESF, the answer should be B.