ISDN
Another protocol used for digital trunks is ISDN. ISDN is a circuit-switched telephone network system 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 comprises digital telephony and data-transport services offered by regional telephone carriers. 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. The emergence of ISDN represents an effort to standardize subscriber services, user/network interfaces, and network and internetwork capabilities.
ISDN Services
In contrast to the CAS and R2 signaling, which provide only DNIS, ISDN offers additional supplementary services such as Call Waiting and Do Not Disturb (DND). ISDN applications include high-speed image applications (such as Group IV facsimile), additional telephone lines in homes to serve the telecommuting industry, high-speed file transfer, and video conferencing. Voice service is also an application for ISDN.
ISDN Media Types
Cisco routing devices support ISDN BRI and ISDN PRI. Both media types use B channels and D channels. The B channels carry user data. The D channel, in its role as signal carrier for the B channels, directs the CO switch to send incoming calls to particular timeslots on the Cisco access server or router. It also identifies the call as a circuit-switched digital call or an analog modem call. Circuit-switched digital calls are relayed directly to the ISDN processor in the router. Analog modem calls are decoded and then sent to the onboard modems. Figure 4-5 illustrates three sample ISDN installation options.
Figure 4-5 ISDN Installation Options
ISDN BRI, referred to as "2 B + D," has the following characteristics:
■ Two 64-kbps B channels carry voice or data for a maximum transmission speed of 128 kbps.
■ One 16-kbps D channel 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 kbps.
The BRI physical layer specification is ITU-T I.430. BRI is very common in Europe and is also available in North America. BRI allows up to two simultaneous calls.
ISDN PRI, referred to as "23 B + D" or "30 B + D," has the following characteristics:
■ 23 B channels (in North America and Japan) or 30 B channels (in the rest of the world) carry voice or data, yielding a total bit rate of 1.544 Mbps and 2.048 Mbps, respectively.
■ One 64-kbps D channel carries signaling traffic.
The PRI physical layer specification is ITU-T Standards Section I.431.
Note The PRI interface is economically preferable to BRI because an interface card supporting PRI is usually already in place on modern PBXs.
Following are worldwide standards for PRI:
■ T1-PRI: Use this interface to designate North American ISDN PRI with 23 B channels and one CCS channel.
■ E1-PRI: Use this interface to designate European ISDN PRI with 30 B channels, one CCS channel, and one framing channel.
■ ISDN-PRI Nonfacility Associated Signaling (NFAS): ISDN NFAS enables a single D channel to control multiple ISDN PRIs on a chassis. This D channel functions as the primary channel with the option of having another D channel in the group as a backup. After you have configured the channelized controllers for ISDN NFAS, you need to configure only the NFAS primary D channel. Its configuration is distributed to all the members of the associated NFAS group. The benefit of PRI NFAS is it frees the B channel by using a single D channel to control multiple PRI interfaces. One B channel on each additional interface is free to carry other traffic.
■ Fractional PRI: The term fractional PRI has different meanings in different parts of the world. One meaning indicates multiple PRI groups (bearer channels [B channel] and associated D channel) on the same T1/E1 interface. Because the NM-HDV supports only a single D channel per T1/E1, the PRI feature does not support this definition of fractional PRI. However, the other version of the term indicates the capability to define a single D channel for each interface with less than 23/31 B channels associated with it. This definition of fractional PRI is supported on Cisco voice gateways.
BRI and PRI Interfaces
Table 4-2 compares the capabilities of BRI and PRI interfaces.
Table 4-2 BRI and PRI Interfaces
|
Capability |
BRI |
T1 PRI |
E1 PRI |
|
B-Channels |
2 x 64 kbps |
23 x 64 kbps |
30 x 64 kbps |
|
D-Channels |
1 x 16 kbps |
1 x 64 kbps |
1 x 64 kbps |
|
Framing |
16 kbps |
8 kbps |
64 kbps |
|
Total Data Rate |
160 kbps |
1.544 Mbps |
2.048 Mbps |
|
Framing |
NT, TE Frame |
SF, ESF |
Multiframe |
|
Line Coding |
2B1Q or 4B3T |
AMI or B8ZS |
HDB3 |
|
Country |
World |
North America, Japan |
Europe, Australia |
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 kbps with no robbed bits.
■ ISDN has a built-in call control protocol known as ITU-T 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 Group 4 (G4) fax and audio channels.
With ISDN, user data is separated from signaling data. User data, such as the payload from a digitized phone call, goes to a 64-kbps B channel, and signaling data, such as a call setup message, goes to a D channel. A single D channel supports multiple B channels, which is why ISDN service is known as CCS.
The drop and insert capability allows for dynamic multiplexing of B channels between different interfaces. This feature is available only if all interfaces use a common clock source, as is the case with Integrated Service Routers (ISRs).
Figure 4-6 shows an example of the drop and insert feature. The channels of an ISDN PRI connection from an Internet service provider (ISP) are split up. Twenty-one channels are routed to another PRI interface of the router connected to a PBX, and two channels are routed to a BRI interface connected to an access server.
Figure 4-6 Drop and Insert
ISDN Signaling
ISDN uses Q.921 as its Layer 2 signaling protocol and Q.931 as its Layer 3 signaling protocol.
Q.921
Layer 2 of the ISDN signaling protocol is Link Access Procedure, D channel (LAPD). LAPD is similar to High-Level Data Link Control (HDLC) and Link Access Procedure, Balanced (LAPB). As the expansion of the LAPD acronym indicates, this layer is used across the D channel to ensure that control and signaling information flows and is received properly. The LAPD frame format is very similar to that of HDLC. Like HDLC, LAPD uses supervisory information and unnumbered frames. The LAPD protocol is formally specified in ITU-T Q.920 and ITU-T Q.921. The Terminal Endpoint Identifier (TEI) field identifies either a single terminal or multiple terminals. A TEI of all 1s indicates a broadcast.
Q.931
Two Layer 3 specifications are used for ISDN signaling: ITU-T I.450 (also known as ITU-T Q.930) and ITU-T I.451 (also known as ITU-T Q.931). Together, these protocols support user-to-user, circuit-switched (the B channels), and packet-switched (the D channel) connections. A variety of call-establishment, call-termination, information, and miscellaneous messages are specified, including SETUP, CONNECT, RELEASE, USER INFORMATION, CANCEL, STATUS, and DISCONNECT. These messages are functionally similar to those provided by the X.25 protocol.
Because ISDN message types might influence the function of a BRI or PRI trunk configuration, you should examine the messages that are part of the Q.931 packet structure and see how ISDN carries out the signaling function. Figure 4-7 illustrates the format of an ISDN frame.
Figure 4-7 ISDN Frame Format
ISDN signaling takes place in the D channel and uses a message-oriented protocol that supports call control signaling and packet data. In its role as signal carrier for the B channels, the D channel directs the CO switch to send incoming calls to particular time slots on the Cisco access server or router.
Following are the components of the ISDN frame that transmit these instructions:
■ Protocol discriminator: This is the protocol used to encode the remainder of the layer.
■ Length of call reference value: This defines the length of the next field. The call reference might be one or two bytes (octet) long, depending on the size of the value being encoded.
■ Flag: This is set to zero (0) for messages sent by the party that allocated the call reference value. Otherwise, it is set to one (1).
■ Call reference value: This is an arbitrary value that is allocated for the duration of the specific session. This value identifies the call between the device maintaining the call and the ISDN switch.
■ Message type: This identifies the message type (for example, SETUP) that determines what additional information is required and allowed. The message type might be one or more octets. When there is more than one octet, the first octet is coded as eight 0s.
■ ISDN Information Element (IE): Most D-channel messages include additional information needed for call processing, such as the calling party number, called party number, and CID. The additional information in a message is passed in information elements.
ISDN sends instructions in Layer 3 messages that are put into Layer 2 frames and are finally time-multiplexed onto a medium with either a BRI or a PRI Layer 1 line-coding specification.
A depiction of D-channel messages is shown in Figure 4-8. These messages allow complete control over call establishment and clearing, network maintenance, and the passing of other call-related information between switches.
The additional information required by an ISDN message is passed in IEs and varies depending on the message type, the action being performed, and the connected equipment. Mandatory and optional IEs for D-channel messages are defined in ITU-T Q.931.
An IE can be a single byte or several bytes, and by reading the message, the switch can determine this information. For example, in octet 1 of the IE, if bit 8, or the extension bit, is 0, the IE is of a variable length. If the bit is 1, the IE is a single byte.
The information contained in octet 3 is the coding standard and the location. Tables 4-3 and 4-4 provide the possible content of these fields.
Figure 4-8 ISDN Protocol Stack
Table 4-3 Coding Standard
|
Bit Sequence |
Meaning |
|
00 |
ITU standardized coding |
|
11 |
Standard specific to the location field |
Table 4-4 Location
|
Bit Sequence |
Meaning |
|
0000 |
User |
|
0001 |
Private network serving the local user |
|
0010 |
Public network serving the local user |
|
0011 |
Transit network |
|
0100 |
Public network serving the remote user |
|
0101 |
Remote private network |
|
0111 |
International network |
|
1010 |
Network beyond the interworking point |
A called number is passed to the PSTN by an IE. The IE contains bytes describing the numbering plan and the type of number. Typically, the numbering type is not changed. However, there might be times when a network administrator elects to have a specific gateway handle all international calls. If this connection to the PSTN is an ISDN PRI, the IE must tell the PSTN that the called number is in international format.
