ISDN PRI

Integrated Services Digital Network (ISDN) uses digital telephony services offered by regional telephone carriers to provide a global WAN connection which can be used to transmit network data. Two types of ISDN services exist, namely ISDN BRI (Basic Rate Interface) and ISDN PRI (Primary Interface). ISDN BRI offers connection speeds up to 128 Kbps while ISDN PRI services offer connections ranging up to 2.048 Mbps (or 1.554 Mbps).

In the United States, ISDN PRI network uses an entire T-1 line to supply twenty-three 64 Kbps B channels and one 64 Kbps D channel. ISDN PRI network is used in businesses that require an “always on” high speed connection. ISDN PRI network is sometimes called 23B+D. in Europe ISDN PRI provides a 30 B+D or ISDN E1.

ISDN PRI Encoding and Framing

The physical layer always includes some specifications about encoding and framing. Without those specifications, the devices attached to the network would not know how to send and receive bits across the medium. To configure an ISDN PRI, you need to know something about encoding and framing at Layer 1.

ISDN PRI in North America is based on a digital T1 circuit. 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 an ISDN PRI; all you need to do is make the router configuration match what the telco is using. For ISDN PRI circuits in Europe, Australia, and other parts of the world that use E1s, the only choice for line coding is High-Density Bipolar 3 (HDB3).

Framing, at ISDN’s physical layer, defines how a device can decide which bits are part of each channel. As is true of encoding, 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, which gives you a total speed of 1.544 Mbps. That’s true regardless of which of the two framing methods are used on the T1.

With E1s, framing 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 ISDN PRI 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 ISDN PRI can assign some channels as B channels and one channel as the D channel.

For ISDN PRI 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 ISDN PRI 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.

Configuring ISDN PRI

Configure the ISDN switch type
Configure T1 or E1 encoding and framing options (controller configuration mode)
Configure T1 or E1 channels range for the DS0 channels used on this ISDN PRI (controller configuration mode)
Configure any interface settings (IP and encapsulation) on the interface representing the D channel

When you add an ISDN PRI interface to the router, IOS calls the interface a serial interface 1/0. Controller configuration mode allows you to configure physical layer parameters-the encoding, framing, and T1 or E1 channels that are in use.

The things you configure under the controller can typically be learned simply by looking at the paperwork from where you ordered the ISDN PRI. Your service provider will tell you what encoding and framing to configure on the router. Also, in almost every case, you will use all 24 DS0 channels in the ISDN PRI-23 B channels and the D channel.

To configure these options, you must first enter controller configuration mode.

router(config)#controller t1 1/0

router(config-controller)#framing esf

router(config-controller)#linecode b8zs

router(config-controller)#pri-group timeslots 1-24

a complete configuration example is shown below between Guinea Smelter (guisml) and Hong Kong Headquarter office (hnkhq):

Controller t1 1/0

framing esf

linecode b8zs

pri-group timeslots 1-24

ip route 172.16.3.0 255.255.255.0 172.16.2.1

ip route 172.16.4.0 255.255.255.0 172.16.2.3

username guisml password Clark

username hnkhq password Bruce

isdn switch-type basic-ni1

access-list 101 permit tcp any host 172.16.3.1 eq 80

access-list 101 permit tcp any host 172.16.4.1 eq 21

dialer-list 2 protocol ip list 101

interface serial 1/0:23

ip address 172.16.2.2 255.255.255.0

encapsulation ppp

ppp authentication chap

dialer idle-timeout 300

dialer fast-idle 120

dialer map ip 172.16.2.1 broadcast name guisml 14045551234

dialer map ip 172.16.2.3 broadcast speed 56 name hnkhq 19999999999

dialer-group 2

router igrp 6

network 172.16.0.0

There are a couple of very important things to notice about this example. First, the blue color highlighted commands are those added specifically for ISDN PRI. The rest are exactly like the ones in the complete configuration for DDR using ISDN BRI. Also note that no SPIDs are listed under the interface, because ISDN PRI does not use SPIDs.

ISDN PRI configuration summary

Command	Description
isdn switch-type type	Configure the ISDN switch type in global or physical interface
linecode ami \| b8zs \| hdb3	Configure encoding for the T1/E1 circuits – controller configuration mode
framing sf \| esf \| crc4	Configures framing for the T1/E1 circuits – in controller configuration mode
pri-group timeslots starting_channel – ending_channel	Configures the DS0 channels used on this ISDN PRI – in controller configuration mode