ISDN PRI Configuration

The primary thing to remember with a PRI connection is its flexibility. You can use a single DS-0 in a T1 or E1 group for a constant data connection, one of a couple of forms of dial-up connections, or you can use it for a voice connection. It can run HDLC and PPP encapsulation, and with the right interface card, it can even be used as an Asynchronous Transfer Mode (ATM) circuit with a bandwidth of 1.544Mbs. The overall bundle of DS-0s is called a DS-1. Fortunately, the exam doesn't test you on all the different options, just the basics to get the WAN connection up and running in a simple configuration.

The connection to the telco switch requires that the router be configured for the correct switch type. This task is slightly different from the way it is configured with the BRI:

 Router(config)#isdn switch-type type 

Essentially, instead of a "basic" switch type, you need to select a "primary" switch type, as follows:

 Router(config)#isdn switch-type primary-5ess 

Configuring the Controller

The T1 or E1 controller is the interface for the WAN connection. You need to configure a few commands that are specific to controllers before the device is operational.

The first thing you need to do is go into controller configuration mode so you can set the rest of the parameters. The format is, from global configuration mode, to identify the type of controller (T1 or E1), the slot/port number, and the individual port, if a slot is being used, where the specific controller can be found:

 Router(config)#controller t1 slot/port 

The following is an example of identifying the first T1 controller in slot 0:

 Router(config)#controller t1 0/0 

Configuring Timeslots

Timeslots are the individual DS-0s that make up a PRI connection. Although it's possible to split up the timeslots into separate groups for different purposes, such as using 6 channels for voice and then bundling the remainder together for data, we keep it simple.

When configuring the timeslots, it's important to know what type of controller you're using. It's not just a matter of ordering a T1, using an E1 interface, and wasting channels. T1s and E1s use the same technology for some things but are different on others. One is knowing what channel will be used for signaling. A T1 uses channel 23, but an E1 uses channel 15. Although it is customizable in many cases, these defaults are what we use here. The following command shows the configuration:

 Router(config-controller)#timeslots beginning_range-end_range 

The following configures the timeslots on a T1 with the default D channel selection:

 Router(config-controller)#timeslots 1-23 

Layer 1 Communication

The communication that takes place between the router and telco switch uses very specific framing. We're not referring to PPP or HDLC; it is lower down than those. We're getting to voltage levels and how bits are represented.

Line Code

When people are asked to design a method for signaling, the first thing they usually think of is that a 1 has voltage attached to it and a 0 doesn't. This arrangement is fine if you have data constantly streaming alternately, but what happens when you have large strings of 0s? How does the router tell the difference between that and no signal at all? As technology has progressed, new line codes were created to "fix" flaws in older methods. The general standard is called Alternate Mark Inversion (AMI), and although a 0 is still zero voltage, it produces an alternately positive or negative voltage for each 1 bit. A string of 1s produces a signal that is alternately positive and negative.

The problem with AMI is that it doesn't address the issue of long strings of 0s. B8ZS for the T1 and HDB3 for the E1 do address this issue. Both of them insert deliberate errors in a prespecified form to show that a string of data 0s is real data as opposed to a dead line. Building on AMI, for example, let's assume a new line code sends three positive voltages in a row. The router knows that the rules of AMI say that 1s are either positive or negative and always alternate, so this is an error. If the routers can agree on what specific error is used, long strings of 0s can be accurately represented.

AMI is always a choice for a line code, but B8ZS and HDB3 are for the T1 and E1, respectively, provided the service provider supports their use. The command shows the options available:

 Router(config-controller)#linecode [ami |[b8zs | hdb3]] 

The following command shows how to configure a router for B8ZS support:

 Router(config-controller)#linecode b8zs 

With more advanced line codes, it is possible to compress data over the circuit without running compression software. What happens if, instead of using a rule detecting whether voltage exists, you use one that detects how much exists? No voltage can indicate 00, 10v can indicate 01, +10v can indicate 10, and +20v can indicate 11. Although such line codes exist, the problem is one of signal strength loss. Although too much resistance on the wire could cause a signal to be misinterpreted as a different bit pattern, it works very well in high-speed, short-haul communications.

Framing

Framing addresses the issue of "How do you get 24 separate channels to talk over a single pair of copper wire?" Using time-division multiplexing, the framing component puts the signal together for transmission.

Each frame in a T1 is 192 bits long, given that there are 24 channels and the framing process sends one byte of data from each one, but the frame itself is 193 bits. The data is placed into the frame from each channel in a round-robin fashion. Eight bits are taken from the first DS-0, eight bits from the second, and so on until each circuit is given eight bits, for a total of 192 bits. Each frame contains an extra framing bit that is used for synchronization. Over the course of 12 frames, a Superframe, these 12 extra bits should form a specific pattern that routers and switches look for, 100011011100. This pattern tells the receiving device that a total of 12 frames have been received and to realign its clocking mechanism. If the receiving device is just slightly off clocking with the sending device, misinterpretations can occur. This method is one way of making sure all the clocking is close.

Extended Superframe (ESF) takes this process a bit further. It used 24 frames instead of 12, and it only uses a 6-bit field for synchronization. Six more bits are used in each frame as a cyclic redundancy check (CRC) for the previous frame. This setup allows for error checking at Layer 1, reducing bandwidth consumption. The remaining 12 bits per frame are used for management purposes, effectively giving a 4Kbps communications channel.

CRC4 performs the same type of service for an E1 as Superframe does for a T1. The primary difference is that the CRC component can be enabled or disabled as desired.

Configuring the framing on a Cisco router is as easy as configuring the line code. Simply enter controller configuration mode and choose which one to use, as shown here:

 Router(config-controller)#framing esf 

Clocking

Clock timing can be received from a couple of sources. It can be generated internally, or it can be determined by the synchronization received with the frames as they arrive on the interface. One device on the connection must be using an internal clock, and the other should be getting the clocking from the line. To provide redundancy, Cisco also allows the secondary port on a T1 controller to provide clocking for the primary port. The following syntax shows the options available for this command, followed by an example:

 Router(config-controller)#clock source {line [primary | secondary] | internal] Router(config-controller)#clock source internal 

Configuring the Interface

Configuring the PRI interface is just like configuring a serial interface. In fact, it's a serial interface you'll be configuring. When you configure controller 0/0, it creates a logical serial interface with the same numbering. So controller 0/0 creates serial 0/0, controller 0/1 creates serial 0/1, and so on. The only thing you need to remember is that you aren't configuring everything about the serial interface; you're just configuring the D channel on the PRI. Rather than configure serial 0/0, you configure serial 0/0:23 for a T1 or serial 0/0:15 for an E1.

To begin configuring the management portion of an ISDN PRI, use the following command from global configuration mode:

 Router(config)# interface serial slot/port:D-channel 

Be sure to use the correct physical interface designation, as well as the correct D-channel indicator. The following command illustrates how to enter interface configuration mode for a T1:

 Router(config)#interface serial 0/0:23 

If the channels are used for people to call in via modem and you have Modem ISDN channel aggregation (MICA) modems installed in the router, you can direct the analog calls to the modems by using the command isdn incoming-voice modem in interface configuration mode.