Enterprise networks often use digital circuits, in contrast to analog circuits, when interconnecting their Voice over IP (VoIP) network to traditional telephony environments, such as the public switched telephone network (PSTN) or a private branch exchange (PBX). One major advantage of using digital circuits is the economies of scale made possible by transporting multiple conversations over a single circuit. For example, a digital T1 circuit using Channel Associated Signaling (CAS) (which is described in this topic) can carry 24 simultaneous voice conversations on a single circuit.
Many enterprises also have the need to interconnect PBX systems, and these PBXs might be from different manufacturers. In many cases, two PBXs from different manufacturers can be interconnected via a digital circuit. However, a common signaling language needs to be spoken between the PBXs to communicate various call state information. Fortunately, if both PBXs support the Q Signaling (QSIG) protocol, they can use QSIG as their common signaling protocol. This topic explores QSIG theory and configuration.
Digital trunks are used to connect to the PSTN, to a PBX, or to the WAN and are widely available worldwide. In some areas, CAS trunks are the only connections available. Basic rate interface (BRI) and primary rate interface (PRI) trunks are very common when connecting a voice gateway to the PSTN. This section maps out the various digital interfaces and explains how to implement and verify digital trunks.
Digital voice ports are found at the intersection of a packet voice network and a digital, circuit-switched telephone network. The digital voice port interfaces that connect the router or access server to T1 or E1 lines pass voice data and signaling between the packet network and the circuit-switched network.
Three types of digital voice circuits are supported on Cisco voice gateways:
■ T1: Uses Time Division Multiplexing (TDM) to transmit digital data over 24 voice channels using CAS.
■ E1: Uses TDM to transmit digital data over 30 voice channels using either CAS or Common Channel Signaling (CCS).
■ ISDN: A circuit-switched telephone network system using CCS. Variations of Integrated Services Digital Network (ISDN) circuits include the following:
■ BRI: 2 B (Bearer) channels and 1 D (Delta) channel
■ T1 PRI: 23 B channels and 1 D channel
■ E1 PRI: 30 B channels and 1 D channel
Digital Trunks
Digital voice ports are used to interconnect gateways or PBX systems to other gateways, PBX systems, or the PSTN. A trunk is a single physical or logical interface that contains several logical interfaces and connects to a single destination.
There are two aspects to consider when signaling on digital lines. One aspect is the actual information about line and device states that is transmitted, and the second aspect is the method that is used to transmit the information on the digital lines.
The actual information about line and device states is communicated over digital lines using signaling methods that emulate the methods used in analog circuit-switched networks: Foreign Exchange Station (FXS), Foreign Exchange Office (FXO), and RecEive and TransMit (E&M).
For signaling to pass between a packet network and a circuit-switched network, both networks must use the same type of signaling. The voice ports on Cisco routers and access servers can be configured to match the signaling of most central offices (CO) and PBXs. Table 4-1 lists some of the common digital circuit options.
Table 4-1 Digital Trunks
|
Type |
Circuit Option |
Comments |
|
Digital |
T1/E1 CAS |
Analog signaling over digital T1/E1 |
|
E1 R2 |
Can provide Automatic Number Identification (ANI) |
|
|
ISDN T1 PRI |
More services than CAS |
|
|
E1 PRI |
Separate data channel (D channel) Common on modern PBXs |
|
|
PRI NFAS |
Multiple ISDN PRI interfaces controlled by a single D channel Backup D channel can be configured |
|
|
BRI |
Mostly for Europe, Middle East, and Africa |
|
|
QSIG |
Created for interoperation of PBXs from different vendors Rich in supplementary services |
The T1, E1, or ISDN lines that connect a telephony network to the digital voice ports on a router or access server contain channels for voice calls. A T1 or ISDN PRI line contains 24 full-duplex channels or timeslots, and an E1 line contains 30. The signal on each channel is transmitted at 64 kbps, a standard known as digital signal level 0 (DS0). The channels are known as DS0 channels. The ds0-group command creates a logical voice port (a DS0 group) from some or all of the DS0 channels, which allows you to address those channels easily, as a group, using voice-port configuration commands.
The method used to transmit the information describes the way that the emulated analog signaling is transmitted over digital lines.
Digital lines use two types of signaling:
■ CAS: Takes place within the voice channel itself.
■ CCS: Sends signaling information over a dedicated channel.
Two main types of digital trunks with channel associated signaling exist, as illustrated in Figure 4-1:
■ T1 CAS trunk: This type of circuit allows analog signaling via a digital T1 circuit. Many CAS variants operate over analog and digital interfaces. A common digital interface is used where each grouping of T1 frames (known as a super frame or an extended super frame) includes two or four dedicated signaling bits. The type of signaling most commonly used with T1 CAS is E&M signaling. In addition to setting up and tearing down calls, CAS provides the receipt and capture of dialed number identification (DNIS) and ANI information, which are used to support authentication and other functions. The main disadvantage of CAS signaling is its use of user bandwidth to perform these signaling functions.
■ E1 R2 trunk: R2 signaling is a CAS system developed in the 1960s that is still in use today in Europe, Latin America, Australia, and Asia. R2 signaling exists in several country versions or variants in an international version called Consultative Committee for International Telegraph and Telephone (CCITT-R2). The R2 signaling specifications are contained in International Telecommunication Union Telecommunication Standardization sector (ITU-T) Recommendations Q.400 through Q.490. R2 also provides ANI.
Figure 4-1 Voice Ports
T1 CAS
T1s have been around since early voice networks. They were developed as a means of carrying multiple calls across one copper loop. Because the copper loop could carry much more bandwidth than the 4000 Hz required for voice transmission, they first used frequency-division multiplexing (FDM) to transmit 24 calls across a single copper loop. Currently, T1 circuits use TDM to transmit digital data (1s and 0s) instead of the old analog signals.
A single digital voice channel requires 64 kbps of bandwidth. This is calculated using the following formula:
64 kbps = 8000 samples/sec x 8 bits/sample = 64,000 bits/sec
This 64 kbps voice channel is also known as DS-0. With 24 voice channels at 64 kbps per channel, a T1 represents 1.536 Mbps of data. Add an additional 8 kbps for framing, and the total speed of a T1 circuit comes to 1.544 Mbps.
T1 CAS uses a digital T1 circuit together with in-band CAS. This is done by using bits in the actual voice channel to transmit signaling information. CAS is sometimes called robbed-bit signaling because user bandwidth is robbed by the network for signaling. A bit is taken from every sixth frame of voice data to communicate on- or off-hook status, wink, ground start, dialed digits, and other information about the call.
T1 CAS uses the same signaling types available for analog trunks: loop start, ground start, and E&M variants such as wink-start, delay-start, and immediate-start. There are also various feature groups available when you use E&M. Here are some common feature groups:
■ E&M FG-B: Inbound and outbound DNIS, inbound ANI (only on Cisco AS5x00)
■ E&M FG-D: Inbound and outbound DNIS, inbound ANI
■ E&M FG-D EANA: Inbound and outbound DNIS, outbound ANI
Figure 4-2 shows CAS with the T1 Super Frame (SF) format. The top row of boxes represents a single T1 frame with 24 time slots of 8 bits each. An additional bit is added at the end of each frame that is used to synchronize the SF. A sequence of 12 T1 frames makes up one SF. CAS is implemented by bit-robbing in frames 6 and 12 in this sequence. The bottom row of boxes represents T1 frames 6 and 12. The least significant bit of each voice channel is robbed, leaving 7 bits for voice data.
Extended Super Frame (ESF) format, as depicted in Figure 4-3, was developed as an upgrade to SF and is now dominant in public and private networks. Both formats retain the basic frame structure of one framing bit followed by 192 data bits. However, ESF repurposes the use of the F bit. In ESF, of the total 8000 F bits used in T1, 2000 are used for framing, 2000 are used for cyclic redundancy check (CRC) for error checking only, and 4000 are used as an intelligent supervisory channel to control functions end to end (such as loopback and error reporting).
Figure 4-2 T1 CAS Super Frame Format
Figure 4-3 T1 CAS Extended Super Frame Format E1 R2 CAS
An E1 circuit is similar to a T1 circuit. It is a TDM circuit that carries several DS-0s in one connection. E1 circuits are widely used in Europe, Asia, and Central and South America.
One big difference between an E1 and a T1 is that an E1 bundles 32 time slots instead of 24. This results in a bandwidth of 2.048 Mbps. With an E1, one time slot is used for framing and one is used for signaling. This leaves 30 time slots available for user data.
E1 digital circuits can be deployed using R2 signaling. These trunks are called E1 R2 trunks. To understand how E1 R2 signaling works, you need to understand the E1 multi-frame format, which is used with E1 R2.
A multiframe consists of 16 consecutive 256-bit frames. Each frame carries 32 time slots. The first time slot is used exclusively for frame synchronization. Time slots 2 to 16 and 18 to 32 carry the actual voice traffic, and time slot 17 is used for R2 signaling.
The first frame in an E1 multiframe includes the multiframe format information in time slot 17. Frames 2 to 16 include the signaling information, each frame containing the signaling for two voice time slots.
Using this signaling method, E1R2 supports inbound and outbound DNIS and ANI. Figure 4-4 shows the signaling concept used by E1 R2.
Figure 4-4 E1 R2 CAS
Time slot 17 is used for signaling, and each of its frames carries information for two voice time slots. This results in the following frame allocation for signaling:
■ 1. Frame, Time slot 17: Declares the multiframe
■ 2. Frame, Time slot 17: Signaling for time slots 2 and 18
■ 3. Frame, Time slot 17: Signaling for time slots 3 and 19
■ 4. Frame, Time slot 17: Signaling for time slots 4 and 20
■ 5. Frame, Time slot 17: Signaling for time slots 5 and 21
■ 6. Frame, Time slot 17: Signaling for time slots 6 and 22
■ 7. Frame, Time slot 17: Signaling for time slots 7 and 23
■ 8. Frame, Time slot 17: Signaling for time slots 8 and 24
■ 9. Frame, Time slot 17: Signaling for time slots 9 and 25
■ 10. Frame, Time slot 17: Signaling for time slots 10 and 26
■ 11. Frame, Time slot 17: Signaling for time slots 11 and 27
■ 12. Frame, Time slot 17: Signaling for time slots 12 and 28
■ 13. Frame, Time slot 17: Signaling for time slots 13 and 29
■ 14. Frame, Time slot 17: Signaling for time slots 14 and 30
■ 15. Frame, Time slot 17: Signaling for time slots 15 and 31
■ 16. Frame, Time slot 17: Signaling for time slots 16 and 32
