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BRI and 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. In some cases, you can just ignore those details. Unless you are building hardware for a networking vendor, you seldom think about those details.

However, to configure an ISDN PRI, you need to know something about encoding and framing at Layer 1, because you need to pick between two options for each when configuring a PRI on a Cisco router.

In some parts of the world, the telco bases its networks on T1 services, and other parts of the world use E1 services. In fact, the Cisco router hardware used for PRIs consists mostly of chips and software used by T1 and E1 lines. When configuring these cards, you need to know what to configure for the encoding and framing options. If you code a value different from what the telco uses, the line will not work.

PRI Encoding

For any physical layer specification, the line encoding defines which energy levels sent over the cable mean a 1 and which energy levels mean a 0. For instance, an early and simple encoding scheme simply used a +5 volt signal to mean a binary 1 and a –5 volt signal to mean a binary 0. Today, encoding schemes vary greatly from one Layer 1 technology to another. Some consider a signal of a different frequency to mean a 1 or 0. Others examine amplitude (signal strength), look for phase shifts in the signal, or look for more than one of these differences in electrical signals.

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 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 E1s, the only choice for line coding is High-Density Bipolar 3 (HDB3).

PRI Framing

PRI lines send and receive a serial stream of bits. So how does a PRI interface know which bits are part of the D channel, or the first B channel, or the second, or the third, and so on? In a word—framing.

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.

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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.

Table 10-8 Definitions for Encoding and Framing

BRI Framing and Encoding

ISDN BRI uses a single encoding scheme and a single option for framing. Because of this, there are no configuration options for either framing or encoding in a router.

ISDN Configuration and Dial-on-Demand Routing

This section covers ISDN configuration and the related DDR configuration that causes Cisco IOS software to use the BRI interface. You must understand DDR configuration and concepts for the ISDN configuration topics to make sense. ISDN configuration can be very brief. In spite of the noise surrounding all the protocols and terminology, you can configure just a few ISDN options in a router. However, the DDR configuration to tell the router when to dial and when to tear down the call can become quite involved.

This section first covers DDR configuration when using BRI interfaces, and then it covers the ISDN configuration for both BRI and PRI. After that, a different style of DDR configuration, DDR dialer profiles, is discussed. A feature called multilink PPP (MLP), which allows multiple B channels to be connected to the same remote site, closes the chapter.

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Table 10-9 ISDN Configuration Commands (Continued)

Table 10-10 ISDN-Related EXEC Commands

DDR Legacy Concepts and Configuration

You can configure DDR in several ways, including Legacy DDR and DDR dialer profiles. The main difference between the two is that Legacy DDR associates dial details with a physical interface, whereas DDR dialer profiles disassociate the dial configuration from a physical interface, allowing a great deal of flexibility. The concepts behind Legacy DDR apply to DDR dialer profiles as well, but Legacy DDR configuration is a little less detailed. Legacy DDR is covered first. Dialer profiles are covered later in this chapter. (Another very useful DDR feature, called dialer watch, is very interesting and useful if you have a requirement for DDR.)