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.
Chapter 4: Performing Call Signaling over Digital Voice Ports 187
No D Channel Required
Analog Signaling
V 30 B Channels (Voice)
V
No D Channel Required
Analog Signaling
24 B Channels (Voice)
T1 CAS
E1 R2
Figure 4-1 Voice Ports