ISDN and Q.931

The ISDN protocol is where telephone calls to the outside world get started. ISDN is the digital telephone line standard, and is what the phone company provides to organizations that ask for digital lines. By itself, ISDN is not exactly a voice mobility protocol, but because a great number of voice calls from voice mobility devices must go over the public telephone network at some point, ISDN is important to understand.

With ISDN, however, we leave the world of packet-based voice, and look at tightly timed serial lines, divided into digital circuits. These circuits extend from the local public exchange—where analog phone lines sprout from before they run to the houses—over the same types of copper wires as for analog phones. The typical ISDN line that an enterprise uses starts from the designation T1, referring to a digital line with 24 voice circuits multiplexed onto it, for 1536kbps. The concept of the T1 (also known, somewhat more correctly, as a DS1, with each of the 24 digital circuits known as DS0s) is rather simple. The T1 line acts as a constant source or sink for these 1536kbps, divided up into the 24 channels of 64kbps each. With a few extra bits for overhead, to make sure both sides agree on which channel is which, the T1 simply goes in round-robin order, dedicating an eight-bit chunk (the actual byte) for the first circuit (channel), then the second, and so on. The vast majority of traffic is bearer traffic, encoded as standard 64kbps audio. The 23 channels dedicated for bearer traffic are called B channels.

As for signaling, an ISDN line that is running a signaling protocol uses the 24th line, called the D channel. This runs as a 64kbps network link, and standards define how this continuous serial line is broken up into messages. The signaling that goes over this channel usually falls into the ITU Q.931 protocol.

Q.931's job is to coordinate the setting up and tearing down of the independent bearer channels. To do this, Q.931 uses a particular structure for their messages. Because Q.931can run over any number of different protocols besides ISDN, with H.323 being the other major one, the descriptions provided here will steer clear of describing how the Q.931 messages are packaged.

Table 1 shows the basic format of the Q.931 message. The protocol discriminator is always the number 8. The call reference refers to the call that is being referred to, and is determined by the endpoints. The information elements contain the message body, stored in an extensible yet compact format.

Table 1: Q.931 Basic Format 

Protocol Discriminator	Length of Call Reference	Call Reference	Message Type	Information Elements
1 byte	1 byte	1-15 bytes	1 byte	variable

The message type is encompasses the activities of the protocol itself. To get a better sense for Q.931, the message types and meanings are:

• SETUP: this message starts the call. Included in the setup message is the dialed number, the number of the caller, and the type of bearer to use.
• CALL PROCEEDING: this message is returned by the other side, to inform the caller that the call is underway, and specifies which specific bearer channel can be used.
• ALERTING: informs the caller that the other party is ringing.
• CONNECT: the call has been answered, and the bearer channel is in use.
• DISCONNECT: the phone call is hanging up.
• RELEASE: releases the phone call and frees up the bearer.
• RELEASE COMPLETE: acknowledges the release.

There are a few more messages, but it is pretty clear to see that Q.931 might be the simplest protocol we have seen yet! There is a good reason for this: the public telephone system is remarkably uniform and homogenous. There is no reason for there to be flexible or complicated protocols, when the only action underway is to inform one side or the other of a call coming in, or choosing which companion bearer lines need to be used. Because Q.931 is designed from the point of view of the subscriber, network management issues do not need to be addressed by the protocol. In any event, a T1 line is limited to only 64kbps for the entire call signaling protocol, and that needs to be shared across the other 23 lines.

Digital PBXs use IDSN lines with Q.931 to communicate with each other and with the public telephone networks. IP PBXs, with IP links, will use one of the packet-based signaling protocols mentioned earlier.

Integrated Services Digital Network (ISDN)

The need for data communications services grew throughout the 1970s. These services were provided (mostly to the companies rather than individuals) by the X.25-based packet switched data networks (PSDNs). By the early 1980s it was clear to the industry that there was a market and technological feasibility for integrating data communications and voice in a single digital pipe and opening such pipes for businesses (as the means of PBX access) and households. The envisioned applications included video telephony, online directories, synchronization of a customer’s call with bringing the customer’s data to the computer screen of the answering agent, telemetrics (that is, monitoring devices—such as plant controls or smoke alarms—and automatic reporting of associated events via telephone calls), and a number of purely voice services. In addition, since the access was supposed to be digital, the voice channels could be used for data connections that would provide a much higher rate than had ever been possible with the analog line and modems.

The ISDN telephone (often called the ISDN terminal) is effectively a computer that runs a specialized application. The ISDN telephone always has a display; in some cases it even looks like a computer terminal, with a screen and keyboard in addition to the receiver and speaker. Several such terminals could be connected to the network terminator (NT) device, which can be placed in the home or office and which has a direct connection to the ISDN switch. Non-ISDN terminals (telephones) can also be connected to the ISDN via a terminal adapter. As far as the enterprise goes, a digital PBX connects to the NT1, and all other enterprise devices (including ISDN and non-ISDN terminals and enterprise data network gateways) terminate in the PBX.

These arrangements are depicted on the left side of Figure 1. The right side of the figure shows the partial structure of the PSTN, which does not seem different at this level from the pre-ISDN PSTN structure. This similarity is no surprise, since the PSTN had already gone digital prior to the introduction of the ISDN. In addition, bringing the ISDN to either the residential or enterprise market did not require much rewiring because the original twisted pair of copper wires could be used in about 70 percent of subscriber lines (Werbach, 1997). What has changed is that codecs moved at the ultimate point of the end-to-end architecture—to the ISDN terminals—and the local offices did need to change somewhat to support the ISDN access signaling standardized by ITU-T. Again, common channel signaling predated the ISDN, and its SS No. 7 version could easily perform all the functions needed for the intra-ISDN network signaling.

Figure 1: The ISDN architecture.

As for the digital pipe between the network and the user, it consists of channels of different capacities. Some of these channels are defined for carrying voice or data; others (actually, there is only one in this category) are used for out-of-band signaling. (There is no in-band signaling even between the user and the network with the ISDN.) The following channels have been standardized for user access:

• A. 4-kHz analog telephone channel.

• B. 64-kbps digital channel (for voice or data).

• C. 8- or 16-kbps digital channel (for data, to be used in combination with channel A).

• D. 16- or 64-kbps digital channel (for out-of-band signaling).

• H. 384-, 1536-, or 1920-kbps digital channel (which could be used for anything, except that it is not part of any standard combination of channels).

The major regional agreements support two combinations:

• Basic rate interface. Includes two B channels and one D channel of 16 kbps. (This combination is usually expressed as 2B+D.)

• Primary rate interface. Includes 23 B channels and 1 D channel of 64 kbps. (This combination is accordingly expressed as 23B+D, and it actually represents the primary rate in the United States and Japan. In Europe, it is 30B+D.)

The ISDN has been deployed throughout mostly for enterprise use. The residential market has never really picked up, although there has been a turnaround because of the demand for fast Internet access (it is possible to use the 2B+D combination as a single 144-kbps digital pipe) and because ISDN connections are becoming less expensive.

Even before the ISDN standardization was finished, the ISDN was renamed narrowband ISDN (N-ISDN), and work began on broadband ISDN (B-ISDN). B-ISDN will offer an end-to-end data rate of 155 Mbps, and it is based on the asynchronous transfer mode (ATM) technology. B-ISDN is to support services like video on demand—predicted to be a killer application; however, full deployment of B-ISDN means complete rewiring of houses and considerable change in the PSTN infrastructure.

Although the ISDN has recently enjoyed considerable growth owing to Internet access demand, its introduction has been slow. The United States until recently trailed Europe and Japan as far as deployment of the ISDN is concerned, particularly for consumers. This lag can in part be explained by the ever complex system of telephone tariffs, which seemed to benefit the development of the business use in the United States. Another explanation often brought up by industry analysts is leapfrogging: by the time Europe and Japan developed the infrastructure for total residential telephone service provision, the ISDN technology was available, while in the United States almost every household already had at least one telephone line long before the ISDN concept (not to mention ISDN equipment) existed

Configuring ISDN PRI Voice Ports

We discussed the analog VICs modules such as the E&M, FXO, and FXS, but larger organizations that need higher-density interfaces to the PSTN or PBX typically use digital T-1/E-1 modules. The following is a list of digital voice interfaces and supported platforms:

• 
  
  Digital T-1/E-1 Packet Voice Trunk Network Module, NM-HDV (VWIC-1MFT and VWIC-2MFT) Cisco 2600 and 3600 series.
• 
  
  Digital voice interface card (DVM) Cisco MC3810.
• 
  
  Octal or Quad T-1/E-1/PRI feature card Cisco AS5300 universal access server.
• 
  
  Channelized trunk card and voice feature card Cisco AS5800 universal access server.
• 
  
  T-1/E-1 high-capacity digital voice port adapter Cisco 7200 and 7500 series.

As with analog, the VICs are inserted into the VNMs to create the digital T-1 voice module for the Cisco 2600 and 3600 series routers. The Cisco 1760 router can use the individual VWIC-2MFT-T-1 card as an interface to a PBX or PSTN for digital connectivity. The card uses a RJ-48 crossover cable for connection to a PBX. The pinouts are listed below:

• 
  
  Pin 1 RX ring
• 
  
  Pin 2 RX tip
• 
  
  Pin 4 TX ring
• 
  
  Pin 5 TX tip

The items needed for configuration of the controller settings are the following:

• 
  
  Line interface T-1 or E-1
• 
  
  Signaling interface FXO, FXS, or E&M and ISDN PRI or BRI—Q.SIG or CCS
• 
  
  Line coding AMI or B8ZS for T-1, and AMI or HDB3 for E-1
• 
  
  Framing format SF (D4) or ESF for T-1, and CRC4 or no-CRC4 for E-1
• 
  
  Number of channels

The controller configuration steps for an ISDN PRI connection to a PBX follow:

router# config terminal
router(config)# isdn switch-type basic-ni1
router(config)# controller T-1 1/0
router(config-controller)# framing esf
router(config-controller)# clock source internal
router(config-controller)# linecode b8zs
router(config-controller)# pri-group timeslots 1-24

This produces the following configuration:

(Partial Router configuration)
!
hostname router
!
memory-size iomem 15
voice-card 1
!
isdn switch-type ni1
!
controller T-1 1/0
framing esf
clock source line
linecode b8zs
pri-group timeslots 1-24
!
interface Serial1/0:23
no ip address
no logging event link-status
isdn switch-type primary-ni1
isdn incoming-voice voice
isdn T310 60000
no cdp enable
!
voice-port 1/0:23
!

Configuring ISDN BRI Voice Ports

Cisco routers support several types of ISDN voice interface cards such as VIC-2BRI-S/T-TE and VIC-2BRI-NT/TE, which can provide connectivity to a PBX or PSTN. A benefit of using the ISDN BRI VIC rather than the analog VIC modules is the additional calling information that is passed.

Up to four calls are supported when the VIC-2BRI is installed in the NM-2V module. The BRI VIC needs to be installed in Slot 0 of the NM-2V for both ports to be active. If an additional VIC is installed in the second slot of the NM-2V, the second port on the first VIC will be disabled. This is based on the two ports of the BRI VIC requiring four DSPs. The VIC interface modules support both ISDN network and user-side configurations. The following are the steps necessary to configure ISDN BRI to a PBX:

1. Enter global configuration mode:

   router# configure terminal
   

2. Set the global ISDN switch type. The only NT supported type is basic-net or basic-qsig:

   router(config)# isdn switch-type switch-type
   

3. Set the ISDN BRI interface slot and port:

   router(config)# interface bri slot|port
   

4. Ensure that no IP address is configured for the ISDN interface (voice only):

   router(config-if)# no ip address
   

5. Specify incoming voice calls over ISDN:

   router(config-if)# isdn incoming-voice voice
   

6. Shut down the interface. Configure the physical layer type. Enable interface with no shutdown:

   • Enter user to configure the port as TE and to function as a clock slave. This is the default.

   • Enter network to configure the port as NT and to function as a clock master.

   router(config-if)# shutdown
   router(config-if)# isdn layer1-emulate {user | network}
   router(config-if)# no shutdown
   

7. Turn on or off the power supplied from an NT-configured port to a TE device:

   router(config-if)# [no] line-power
   

8. Configure the Layer 2 and Layer 3 port protocol:

   router(config-if)# isdn protocol-emulate {user | network}
   

9. Exit global configuration mode:

   router(config)# end
   

Figure 1 illustrates a scenario with ISDN BRI connectivity to a PBX and PSTN. A partial configuration lists the commands pertaining to the PSTN and PBX interfaces.

Figure 1: ISDN BRI PBX and PBX Scenario

(Partial Cisco 1760 configuration)
!
hostname 1760
!
 isdn switch-type basic-net3
!
interface BRI 1/0
no shutdown
description connected to PBX
no ip address
 isdn switch-type basic-net3
 isdn incoming-voice voice
shutdown
isdn layer1-emulate user
no shutdown
isdn protocol-emulate user
!
interface BRI 2/0
no shutdown
description connected to PSTN
no ip address
 isdn switch-type basic-net3
 isdn incoming-voice voice
 isdn overlap-receiving
shutdown
isdn layer1-emulate network
no shutdown
isdn protocol-emulate network

System : Integrated Digital Services Network (ISDN)

Integrated Digital Services Network (ISDN)
A structured all digital telephone network system that was developed to replace (upgrade) existing analog telephone networks. The ISDN network supports for advanced telecommunications services and defined universal standard interfaces that are used in wireless and wired communications systems.

ISDN provides several communication channels to customers via local loop lines through a standardized digital transmission line. ISDN is provided in two interface formats: a basic rate (primarily for consumers) and high-speed rate (primarily for businesses). The basic rate interface (BRI) is 144 kbps and is divided into three digital channels called 2B + D. The primary rate interface (PRI) is 1.54 Mbps and is divided into 23B + D. The digital channels for the BRI are carried over a single, unshielded, twisted pair, copper wire and the PRI is normally carried on (2) twisted pairs of copper wire.

The “B” channels operate at 64kb per second digital synchronous rate and the “D” channel is a control channel. The D channel is used to coordinate (signal) the communication with the telephone network. When used on the BRI line, the D channel is 16kbps and when provided on the PRI channel, the D channel is 64 kbps. Because the amount of telephone system control signaling is relatively small, the D channel can also be used for low speed packet data messaging. The 64 kbps “B” channels can be used for voice and data. On the BRI system, the two B channels can be combined for 128 kbps data connection.

ISDN telephone lines exclusively use digital transmission. This requires a customer to replace their analog telephones with ISDN digital telephone equipment if they upgrade to ISDN service. ISDN service is typically provided using modular plugs. These plugs include a RJ45 interface (8 pin) for data equipment (called a BRI-S/T) and the other physical connection type is a two-wire, RJ11 type standard (called the BRI-U).

The maximum distance for a BRI-S/T line is approximately 3,000 feet and the maximum distance for the BRI-U is 18,000 feet. Beyond these distances, the service provider may install repeaters to provide service. However, repeaters are expensive to install and setup.

The ISDN BRI allows the user to change the use of the B channels whenever desired. For example, an ISDN user may be sending data using the two B channels at 128 Kbps. If a voice call comes in or is initiated, the data transmission is not interrupted; but is automatically reduced to one B channel at 64 Kbps. When the voice call ends, the data transmission returns to 128 Kbps on the two B channels.

Figure 1 provides the different interfaces that are available in the integrated services digital network (ISDN). The two interfaces shown are BRI and PRI. These are all digital interfaces from the PSTN to the end customers network termination. 1 (NT1) equipment. devices that are ISDN compatible can directly connect to the NT1 connection. Devices that require other standards (such as POTS or data modems) require a terminal adapter (TA).


Figure 1: Integrated Digital Services Network (ISDN)

Carrier System : Integrated Services Digital Network (ISDN)

Integrated services digital network (ISDN) is a structured all digital telephone network system that was designed to replace (upgrade) existing analog telephone networks. The ISDN network supports for advanced telecommunications services and defined universal standard interfaces that are used in wireless and wired communications systems. There are two key user interfaces defined for ISDN networks: basic rate interface (BRI) and primary rate interface (PRI).

The basic rate interface (BRI) is the smallest transmission system (or interface) available through ISDN. BRI provides for two 64 kbps bearer channels (B channels) and a 16 kbps signaling (data) channel (D channel). This configuration is also is also referred to as 2B+D.

The primary rate interface (PRI) is a standard high-speed data communications interface that is used in the ISDN system. This interface provides a standard data rates for T1 1.544 Mbps and E1 2.048 Mbps. The interface can be divided into combinations of 384 kbps (H) channels, 64 kbps (B) channels and includes at least one 64 kbps (D) control channel.

Integrated Services Digital Network (ISDN)