Understanding Cisco Voice Gateways - Part 4. Digital Voice Ports

In the previous parts of our Cisco Voice Gateway story we identified what analog voice ports are. Also we talked about different types of analog supervisory signaling. Now let's see how we can use our Cisco voice gateways with digital lines such as T1, E1 and BRI. These types of connection are still widely available worldwide. This post maps out the various digital interfaces and explains the differences between them.

Digital voice ports are used in traditional PBX deployments and packet voice networks, mainly

to connect to the digital, circuit-switched telephone network - to PSTN or to traditional PBXs. The digital voice port interfaces that connect the voice gateway to digital T1, E1 or BRI lines pass voice data and signaling between the packet network and the circuit-switched network. Digital lines replaced analog lines in telephony for some reasons such as: 

• one phone number is associated with one line and one line can only support one conversation at a time;
• electrical interference from transformers, power cables and TV transmitters.

All digital telephony lines use Time-Division Multiplexing (TDM) as a method of transmitting and receiving voice and signaling. TDM allows to transmits two or more voice channels converted to digital signals with Pulse Code Modulation (PCM) over a common transmission medium (copper pair of wires, optical fiber or radio channel). You can see the operation of a TDM system below:

First of all, all PCM signals are combination of samples from initial audio signals. The transmitter on the left side takes digital samples from signals A, B, C and D and multiplexes them to a common stream. On the receiving side the stream is demultiplexed and the signals (channels) are restored back. One sample from a signal (A, B, C, D) occupies one time-slot, and 4 time-slots in this example form a frame. After forming a frame the originating side takes next samples from these channels, forms another slot and so on. That's how we can send several channels over one pair of copper wires. 

No need to say that the similar process occurs on backward direction, when we send voice information of these channels from right to left. So in order to establish full duplex communication path the TDM systems typically uses two copper wire pairs - one pair for transmitting information and the second one to receive the information back. 

The T1, E1, or BRI lines that connect a telephony network to the digital voice ports on a voice gateway or PBX contain channels for voice calls; a T1 line contains 24 full-duplex channels or time slots, an E1 line contains 32 timeslots (30 for voice, 1 for signaling and 1 for synchronization bits - clocking). The signal on each channel is transmitted at 64 kb/s, a standard known as digital service level 0 (DS0); the channels are known as DS0 channels. BRI lines can be considered as two 64 kb/s timeslots for voice, 16 kb/s for signaling and 48kb/s for overheads. 

Digital lines use two types of signaling on T1 or E1 connections:

• CAS (Channel Associated Signaling): Takes place within the voice channel itself
• CCS (Common Chanel Signaling): Sends signaling information down a dedicated channel

Nowadays the CAS is used very rare, while CCS is still widely used. Therefore, we will discuss the CCS type of digital signaling in this post. And an example of CCS is ISDN (Integrated Services Digital Networks) protocol. BRI interfaces doesn't support CAS, they work only with CCS (ISDN). 

ISDN is a circuit-switched telephone network system that is designed to allow digital transmission of voice and data over ordinary telephone copper wires, resulting in better quality and higher speeds than is available with the PSTN system. ISDN involves the digitization of the telephone network, which permits voice, data, text, graphics, music, video, and other source material to be transmitted over existing telephone wires. 

Cisco voice gateways support ISDN BRI and ISDN PRI (T1\E1). Both types of connections use B-channels and D-channels. The B (bearer) channels carry user media (voice, video or digital data) . The D (data) channel, in its role as signal carrier for the B-channels, directs the CO switch to send calls to particular time slots on the digital voice ports. 

ISDN BRI is known as 2 B + D connection. It means the following:

• Two 64-kb/s B channels that carry voice or data for a maximum transmission speed of 128 kb/s 
• One 16-kb/s D channel that carries signaling traffic, that is, instructions about how to handle each of the B channels, although it can support user data transmission under certain circumstances

The D channel signaling protocol comprises Layers 1 through 3 of the Open Systems Interconnection (OSI) reference model. BRI also provides for framing control and other overhead, bringing its total bit rate to 192 kb/s.

The BRI physical layer specification is ITU-T Recommendation I.430. BRI is very common in Europe and is also available in North America. BRI interface allows up to two simultaneous calls and can be used in small companies as connection lines to PSTN. For example, if you have 40 telephony users (employees) in you company, then you require four connection channels to PSTN. So you can you two BRI line to satisfy this requirement. 

ISDN PRI may have 23 B + D or 30 B + D frame structure:

• 23 B channels in T1 PRI (in North America and Japan) or 30 B channels in E1 PRI (in the rest of the world) that carry voice or data yielding a total bit rate of 1.544 Mb/s and 2.048 Mb/s respectively
• One 64-kb/s D channel that carries signaling traffic

The PRI physical layer specification is ITU-T Recommendation I.431. PRI interfaces are widely used in middle-size or big companies. Sometimes you can see a PRI interface configuration containing less B channels (timeslots) then 23 or 30. Such situation can occur when the voice gateways doesn't have enough DSP resources to convert all 23 or 30 channels to IP packets. This configuration is called Fractional PRI. 

The figure illustrates the different capabilities of ISDN BRI and PRI interfaces.

Using ISDN for voice traffic has these benefits:

• ISDN is perfect for G.711 pulse code modulation (PCM) because each B channel is a full 64 kb/s with no robbed bits.
• ISDN has a built-in call control protocol known as ITU-T Recommendation Q.931.
• ISDN can convey standards-based voice features, such as speed dialing, automated operator services, call waiting, call forwarding, and geographic analysis of customer databases.
• ISDN supports standards-based enhanced dial up capabilities, such as G4 fax and audio channels.

To be continued...