Time-division multiplexing (TDM) Digital signaling Systems Channel associated signaling (CAS) Common channel signaling (CCS) T1 E1

1. CCNA Voice 640-461
Part 4 - Historic Voice: Digital Connectivity- Part 2
www.amir-jafari.com
amirjafari17@gmail.com

2. Historic Voice: Digital Connectivity
Solving the writing problem: TDM
Understanding T1 and E1 CAS specifics
Understanding T1 and E1 CSS specifics

3. Historic Voice: Digital Connectivity
Time-division multiplexing (TDM)
- Digital voice uses a technology known as time-division multiplexing (TDM)
-TDM allows voice networks to carry multiple conversations at the same time
over a single, four-wire path.
- Because the multiple conversations have been digitized, the numeric values
are transmitted in specific time slots (thus, the “time division”) that
differentiate the separate conversations.

4. Historic Voice: Digital Connectivity
- Each of the voice conversations been digitized and assigned a numeric
value and transmitted over the digital PSTN connection.
- Based on the time the voice data was sent, the PSTN carrier is able to
distinguish and reassemble the voice conversations.
- Corporations use digital voice connections to the PSTN as T1 circuits in
the United States, Canada, and Japan.
- A T1 circuit is built from 24 separate 64-kbps channels known as a digital
signal 0 (DS0).
- Each one of these channels is able to support a single voice call.
- Corporations in areas outside the United States, Canada, and Japan use
E1 circuits, which allow you to use up to 30 DS0s for voice calls.

5. Historic Voice: Digital Connectivity
Signaling Systems
- With analog circuits, supervisory signals were passed by connecting
the tip and ring wires together. The phone company generated
informational and address signals through specific frequencies of
electricity.
- By solving the problems associated with analog signaling, digital
signaling also removed the typical signaling capabilities.
-To solve this, two primary styles of signaling were created for digital
circuits:
•Channel associated signaling (CAS): Signaling information is transmitted
using the same bandwidth as the voice.
•Common channel signaling (CCS): Signaling information is transmitted
using a separate, dedicated signaling channel.
- PBXs and Cisco devices use T1 and E1 interfaces to convey voice.

6. Historic Voice: Digital Connectivity
T1 Channel Associated Signaling
- T1 combines 24 separate voice channels onto a single link
- The T1 data stream is broken into frames consisting of a single framing bit plus 24
channels of 8-bit bytes (1 framing bit per frame + 24 channels per frame X 8 bits per
channel = 193 bits per frame)
- The frames must repeat 8,000 times per second in order to properly recreate the
voice signal. Thus, the required bit rate for T1 is 1.544 Mbps (8,000 frames per second
X 193 bits per frame)
T1 Framing Bit

7. Historic Voice: Digital Connectivity
T1 Channel Associated Signaling
-T1 has two major framing and format standards:
Super Frame (SF) or D4, specifies 12 frames in sequence.
-D4 defines a 12-bit framing sequence which is sent as the 193rd bit in 12
consecutive frames.
-The framing pattern is defined as 100011011100. This pattern repeats
continuously and the receiving equipment locks onto it in order to properly
synchronize with the incoming data. (12 bits are used for synchronization)
Extended Superframe (ESF)
- The Extended Superframe Format (ESF) extends the D4 superframe from 12
frames to 24 frames.
- ESF also redefines the 193rd bit location in order to add additional
functionality.
- In ESF the 193rd bit location serves three different purposes:
 Frame synchronization(6 bits)
 Error detection(6 bits)
 Maintenance communications (Facilities Data Link - FDL) (12 bits)

8. Historic Voice: Digital Connectivity
T1 Channel Associated Signaling

9. Historic Voice: Digital Connectivity
T1 Channel Associated Signaling
-Channel associated signaling (CAS) is a signaling method commonly used
between PBXs or between a PBX and a Cisco voice device
- T1 digital connections that use CAS actually “steal” binary bits that would
typically have been used to communicate voice information and use them for
signaling.
- Although the voice quality does drop some, the number of binary bits stolen
for signaling information is small enough that the change in voice quality is not
noticeable.
- Because T1 CAS steals bits from the voice channel to transfer signaling
information, it is often called robbed bit signaling (RBS).
- The voice device running the T1 line uses the least significant bit on every
sixth sample in each T1 channel (DS0)

10. Historic Voice: Digital Connectivity
-This process occurs for every sixth frame after this (12th, 18th, 24th, and so on)
- This stolen bit relays the signaling information for each respective DS0 channel

11. Historic Voice: Digital Connectivity
T1 Channel Associated Signaling
-In order to send supervisory information over a D4(Super Frame ) link "bit robbing" is
used.
-SF framing makes use of this characteristic of voice and uses the least significant bits in
each channel of the 6th (A Bit) and 12th (B Bit) frames to send signaling information;
on-hook, off-hook, dialing and busy status.
-ESF utilizes 4 frames per superframe for this signaling. The 6th (A bit), 12th (B
bit), 18th (C bit), and 24th (D bit) frames are used for the robbed bits.
-Different signaling scheme make different use of the A and B Bits
In E&M signaling:
A = B = 1 CIRCUIT BUSY
A = B = 0 CIRCUIT IDLE

12. Historic Voice: Digital Connectivity
E1 Channel Associated Signaling
-In E1 framing and signaling, 30 of the 32 available channels, or time slots, are
used for voice and data
- Framing information uses time slot 1, while time slot 17 (E0 16) is used for
signaling by all the other time slots.
-This signaling format is also known as CAS because the use of the bits in the 17th
time slot is exclusively reserved for the purpose of signaling each respective
channel.

13. Historic Voice: Digital Connectivity
E1 Channel Associated Signaling
- Slot 17 of the first frame
declares the beginning of
a 16-frame multiframe
(M).
- Slot 17 of the second
frame carries ABCD for
voice slot 2 (X) and
ABCD for voice slot 18
(Y).
- Slot 17 of the third
frame carries ABCD for
voice slot 3 (X) and
ABCD for voice slot 19
(Y).
This process continues for all the remaining frames.

14. Historic Voice: Digital Connectivity
Common channel signaling (CCS)
-Common channel signaling (CCS) differs from CAS in that all channels use a
common channel and protocol for call setup.
- CCS dedicates one of the DS0 channels from a T1 or E1 link for signaling
information.
-This is often called out-of-band signaling because the signaling traffic is sent
completely separate from the voice traffic.
- When using CCS configurations with T1 lines, the 24th time slot is always the
signaling channel. As a result, a T1 connection using CCS has only 23 usable
DS0s for voice.
- When using CCS configurations with E1 lines, the 17th time slot is always the
signaling channel
- Allows the use of a signaling protocol rather than just four bits of signaling per
channel. A full signaling protocol sends the necessary information for all voice
channels.

15. Historic Voice: Digital Connectivity
Common channel signaling (CCS)
-CCS is the most popular connection used between voice systems worldwide
because it offers more flexibility with signaling messages, more bandwidth for the
voice bearer channels, and higher security (because the signaling is not embedded
in the voice channel)
- CCS also allows PBX vendors to communicate proprietary messages (and
features) between their PBX systems using ISDN signaling, whereas CAS does not
offer any of these capabilities.

16. Historic Voice: Digital Connectivity
Common channel signaling (CCS)
-Examples of CCS signaling are as follows:
-Proprietary implementations Some PBX vendors choose to use CCS for T1 and
E1 and implement a proprietary CCS protocol between their PBXs.
-Integrated Services Digital Network (ISDN) ISDN uses Q.931 in a common
channel to signal all other channels.
-Q Signaling (QSIG) Like ISDN, QSIG uses a common channel to signal all other
channels.
-Digital Private Network Signaling System (DPNSS) DPNSS is an open standard
developed by British Telecom for implementation by any vendor who chooses to
use it. DPNSS also uses a common channel to signal all other channels.
-Signaling System 7 (SS7) SS7 is an out-of-band network implemented and
maintained by various telephone companies and used for signaling and other
supplemental services.

17. Historic Voice: Digital Connectivity
ISDN
-Integrated Services Digital Network (ISDN) is an all-digital phone line
connection that was standardized in the early 1980s
-ISDN is actually a set of communication protocols proposed by telephone
companies that allows them to carry a group of digital services that
simultaneously convey data, text, voice, music, graphics, and video to end
users, and it was designed to achieve this over the telephone systems already in
place.
-ISDN provides greater bandwidth and lower latency compared to dialup
analog technology
-Although ISDN has been around for many years, the industry is moving toward
using broadband technologies such as cable, DSL and public wireless with
IPsec VPNs.
-ISDN remains as an effective WAN solution only if broadband is not available.

18. Historic Voice: Digital Connectivity

19. Historic Voice: Digital Connectivity
ISDN
ISDN comes in two service types
ISDN Basic Rate Interface (BRI) Service
ISDN BRI consists of two B channels and one D channel (2B+D). Both of the BRI
B channels operate at 64 kbps and carry user data. The D channel handles the
signaling and control information and operates at 16 kbps. Another 48 kbps is
used for framing and synchronization, for a total bit rate of 192 kbps.
ISDN Primary Rate Interface (PRI) Service
ISDN PRI service offers 23 B channels and 1 D channel (23B+D) in both North
America and Japan. Each channel (including the D channel) operates at 64 kbps,
for a total bit rate of 1.544 Mbps, including overhead. In other parts of the world,
such as Europe and Australia, the ISDN PRI service provides 30 B channels and 1
64-kbps D channel.

20. Historic Voice: Digital Connectivity
ISDN
-ISDN Protocols
ISDN protocols are defined by the ITU, and there are several series of protocols
dealing with diverse issues:
Protocols beginning with the letter E deal with using ISDN on the existing
telephone network
Protocols beginning with the letter I deal with concepts, aspects, and services
Protocols beginning with the letter Q cover switching and signaling

21. References
Cioara, J., Valentine, M. (2012). CCNA Voice 640-461 Official Cert Guide,
Cisco Press, USA
Davidson, J., Peters, J., Bhatia, M., Kalidindi, S., Mukherjee, S. (2006). Voice
over IP Fundamentals, Second Edition, Cisco Press, USA
Froehlich, A. (2010). CCNA Voice Study Guide, Wiley Publishing, Inc.,
Indianapolis, Indiana
Kaza, R., Asadullah, S. (2005). Cisco IP Telephony: Planning, Design,
Implementation, Operation, and Optimization, Cisco Press, USA
Wallace, K. (2005). Voice over IP First-Step, Cisco Press, USA
Wallace, K. (2006). Authorized Self-Study Guide Cisco Voice over IP
(CVoice), Cisco Press, USA