ccna voice 640-461- part 4 historic voice-digital connectivity-part 2

CCNA Voice 640-461

Part 4 - Historic Voice: Digital Connectivity- Part 2

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Historic Voice: Digital Connectivity

Solving the writing problem: TDM

Understanding T1 and E1 CAS specifics

Understanding T1 and E1 CSS specifics


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.


- 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.


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.


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



-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)



-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)


-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



-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 BUSYA = B = 0 CIRCUIT IDLE


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.


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.


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.

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-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.



-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.


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.


ISDN

ISDN comes in two service types

ISDN Basic Rate Interface (BRI) ServiceISDN 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) ServiceISDN 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.


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 networkProtocols beginning with the letter I deal with concepts, aspects, and servicesProtocols beginning with the letter Q cover switching and signaling

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

ccna voice 640-461- part 4 historic voice-digital connectivity-part 2

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