ISDN E1

In an occasional connection needed such as sending and receiving data for only short periods of time, the use of dialed connections including ISDN (either ISDN BRI; or ISDN PRI) is required. Telco companies typically base its networks on T1 services, and some other use E1 services. Cisco hardware routers used for ISDN PRIs consist mostly of chips and software used by ISDN T1 and ISDN E1 lines.

As the main reason of using ISDN by a site is just for instant access to data only in short period of time, the use of ISDN mostly to create a backup link when their primary leased line or Frame Relay connection is lost. You might include the use of ISDN in your business continuity and disaster recovery plan as backup link. Depending on the network’s business goals, long outages might not be acceptable, so ISDN (either BRI or PRI) could be used to dial back to the main site. See detail about ISDN BRI here. ISDN PRI that based on T1 services are mostly used in US regions, while mostly ISDN E1 services are used in Europe.

As discussed before (see also ISDN network), the router creates a B channel call to another device using either BRI or PRI, it sends the phone number it wants to connect to inside a message sent across the D channel. At the Central Office (CO) of the phone company, switch receives the message and sets up the circuit. The different types of ISDN lines are often described based on the numbers of B and D channels used. ISDN BRI is described as 2B+D which mean two B channels and one D channel. PRIs in US based on ESDN T1 framing, and are referred to as 23B+D, and PRIs based on ESDN E1 framing are referred to as 30B+D. the following table lists the number of channels for each type of ISDN line and the terminology used to describe them. See also Diva ISDN USB by Eicon.

In some parts of the world, the Telephone Company bases its networks on ISDN T1 services, and other parts of the world use ISDN E1 services. The Cisco router hardware used for PRIs consists mostly of chips and software used by T1 and E1 lines. Configuring these cards, knowing the encoding and framing options is required. If the code value configured is different from the Telco, the line will not work.

PRI Encoding

ISDN PRI in North America is based on a digital ISDN T1 circuit. ISDN T1 circuits use two different encoding schemes—Alternate Mark Inversion (AMI) and Binary 8 with Zero Substitution (B8ZS). You will configure one or the other for a PRI; all you need to do is make the router configuration match what the Telco is using. For PRI circuits in Europe, Australia, and other parts of the world that use ISDN E1, the only choice for line coding is High-Density Bipolar 3 (HDB3).

PRI Framing

In ISDN Physical layer, the framing is used to define how a device can decide which bits are part of each channel. ISDN PRI framing is based on the underlying T1 or E1 specifications. The two T1 framing options define 24 different 64-kbps DS0 channels, plus an 8-kbps management channel used by the Telco to provide a total speed of 1.544 Mbps.

Framing used in ISDN E1 defines 32 64-kbps channels, for a total of 2.048 Mbps, regardless of the type of framing used. The two options for framing on T1s are to use either Extended Super Frame (ESF) or the older option—Super Frame (SF). In most cases today, new T1s use ESF. For PRIs in Europe and Australia, based on E1s, the line uses CRC-4 framing or the original line framing defined for E1s. You simply need to tell the router whether to enable CRC-4 or not.

As soon as the framing details are known, the PRI can assign some channels as B channels and one channel as the D channel. For PRIs based on T1s, the first 23 DS0 channels are the B channels, and the last DS0 channel is the D channel, giving you 23B+D. With PRIs based on E1 circuits, the D channel is channel 15. The channels are counted from 0 to 31. Channel 31 is unavailable for use because it is used for framing overhead. That leaves channels 0 through 14 and 16 through 30 as the B channels, which results in a total of 30B+1D. Table 10-8 summarizes the key concepts behind framing and encoding, along with the options for each with T1 and E1 circuits.

The following table describes the definitions for Encoding and framing

Suggested reading:

• Frame relay network
• Point-to-Point protocols
• WAN Technologies
• Firewall network security
• IP routing guide