Frame Relay Equipment

In order to connect to a Frame Relay network, both DTE and DCE equipment needs to be located at the customer premises. This DTE equipment is usually a router, whose serial interface connects to a DCE device. In the past, customers required a completely separate DTE device known as a Frame Relay Access Device (FRAD) to connect to a Frame Relay network. However, almost all routers sold today (with an appropriate serial interface) are capable of handling Frame Relay encapsulation and communication. The DCE device is usually a CSU/DSU that provides clocking functions and the connection to the provider’s physical circuit. Ultimately, the physical link from the customer premises connects to the Frame Relay switching equipment of the service provider. This switching equipment is not the responsibility of the customer. The figure below illustrates the interconnections of equipment on a Frame Relay network.

Figure: Connections through a Frame Relay network are made using DTE, DCE, and PSE equipment.

Frame Relay

Frame Relay is a packet-switching technology that exists at the Data Link layer of the OSI model and is one that has become increasingly common as a WAN solution since the early 1990s. Unlike with leased lines and circuit-switched networks, the available bandwidth on a provider’s Frame Relay network is shared amongst many subscribers. This sharing of resources leads to significantly lower costs than traditional leased lines.

Many people tend to be confused by packet-switching technologies like Frame Relay. Mostly this is a result of trying to understand how data actually gets from one location to another. On packet-switched networks (like Frame Relay), data streams are separated through the use of “virtual” rather than dedicated hardware circuits. In other words, a logical path is defined between endpoints, through a provider’s packet-switched network. Many virtual circuits will be defined for different customers, and will be multiplexed over the shared physical links of the network. As an example, consider the figure below. It shows two different companies, each connecting two offices over a provider’s Frame Relay network. Notice that between Frame Relay switches X and Y, both of their virtual circuits traverse a common physical link. The data that one company passes between their own offices is completely separate from the data of the other company – all data stays within each company’s dedicated virtual circuit only.

Figure: PVCs of two different companies traveling over the same Frame Relay network, and at times, common links.

Two main types of virtual circuits can be defined on a Frame Relay networks – permanent virtual circuits (PVCs) and switched virtual circuits (SVCs). A PVC functions somewhat similar to a leased line, in that a service provider defines a path through the packet switched network to each customer location. In cases where companies wish to have “always-on” connectivity between locations using Frame Relay, PVCs are usually defined.

A switched virtual circuit (SVC) functions somewhat differently, almost like a circuit-switched connection. SVCs are not permanent, and can instead be created as required across a packet-switched network. For example, an SVC could be created between a company’s head office and a remote location. For the duration of the connection, data would travel across the path defined by the SVC. However, if the circuit was terminated and a new SVC established at a later time, data might travel over a completely different path.

Frame Relay networks are referred to as being non-broadcast multi-access (NBMA). What this means is that, by default, broadcast traffic will normally not be passed over a virtual circuit without explicit configuration. This is an important consideration when dealing with the use of broadcast-based routing protocols like RIP or IGRP in a Frame Relay environment. You’ll look at how broadcast traffic can be handled on Frame Relay networks later in this section.

Monitoring ISDN

There are a variety of commands available for the purpose of monitoring ISDN connectivity, as outlined below.

  • show isdn active. This command displays information about the current ISDN call, including the number dialed and whether a call is in progress.
  • show isdn status. Displays the status of all ISDN interfaces, providing information relating to OSI layers 1 through 3.
  • debug isdn q921. Displays link access information about ISDN D channels at the Data Layer. To make this command easier to remember, just relate the “2” in the q921 keyword as the associated OSI layer.
  • debug isdn q931. Displays information about ISDN call setup and teardown on ISDN D channels at the Network layer. To make this command easier to remember, just relate the “3” in the q931 keyword as the associated OSI layer.
  • debug dialer. Displays information relating to call setup and teardown.

Configuring ISDN Dial-on-Demand Routing (DDR)

In cases where a branch office doesn’t require a permanent connection to a company’s head office, ISDN is often the best choice amongst WAN technologies. Because ISDN is circuit switched, a branch office router using ISDN could dial into a head office, as necessary, and then terminate the connection once a data transfer is complete. This can lead to significant cost savings, especially when compared to the expense associated with permanent connections or leased lines.

Demand-dial Routing (DDR) is the term used to describe such a connection. Setting up DDR involves configuring branch office routers to dial into a central corporate office (or vice versa), allowing data to be routed between locations. DDR isn’t limited to ISDN – it can also be configured using slower technologies like traditional analog phone lines. However, the speeds at which connections are made with ISDN (often 1-2 seconds versus almost 30 seconds for analog calls) make it the better choice for demand-dial routing.

Perhaps the most important consideration when configuring a DDR connection is determining which types of traffic will initiate the connection. For example, will any traffic destined for the remote network initiate the connection, or will only certain types of traffic, such as HTTP or SMTP initiate it? These are very important decisions. If not specified, any type of traffic destined for the remote network could initiate the connection, which might it turn leave it almost permanently connected. This would certainly have an impact on any expected cost savings. The solution to this issue involves defining what traffic the router will find “interesting”. If correctly configured, a router will only initiate a DDR connection when it comes across interesting traffic destined for the remote network.

A second key element to consider when configuring DDR is how routing will take place. As a general rule, DDR connections should use static or default routing only. If a dynamic routing protocol like RIP were to be used on a DDR link, update traffic would reinitiate connections every 30 seconds. Remember that static routes will need to be defined to all networks (subnets) that have to be reached by the branch office. In cases where the branch office is a stub network (as shown in the figure below), a default route is usually the best option.

Figure: For DDR connections from a stub network, using a default route is usually the easiest configuration option.

For this example, let’s assume that our network is configured as shown in the figure below. Our goal is to configure the branch office ISDN router such that it will initiate a connection to our head office location, but only for SMTP and HTTP traffic. Conversely, the head office ISDN router could also be configured to initiate connections to the branch office if necessary.

Figure: Network diagram for the ISDN configuration example.

As a first step, you’ll configure the branch office router with an IP address on its BRI interface, define the ISDN switch type in use at the local CO, and configure a static route to the head office location. You’ll also configure the interface to use PPP encapsulation, which will allow you to add CHAP authentication (a very good idea) for these DDR connections.

Branch1604#config t
Enter configuration commands, one per line. End with CNTL/Z.
Branch1604(config)#isdn switch-type basic-ni1
Branch1604(config)#int bri0
Branch1604(config-if)#ip address
Branch1604(config-if)#encapsulation ppp
Branch1604(config)#ip route

Notice that I chose to use a static route to connect to the head office location. Since the branch office is a stub network, it would also have been reasonable to create a default route, using the destination network address

The next step involves specifying which traffic the router will consider interesting. This is accomplished using the dialer-list command. Recall that our goal was to limit DDR connections to SMTP and HTTP traffic only. In order to accomplish this, you need to use an extended IP access list. For illustration purposes, initially configure the router to view all IP traffic as interesting:

Branch1604(config)#dialer-list 1 protocol ip permit
Branch1604(config)#int bri0
Branch1604(config-if)#dialer-group 1

The dialer-list command specified above tells the router that all IP traffic is “interesting”, and should initiate the link. However, this dialer list does nothing until actually applied to an interface using the dialer-group command from interface configuration mode. To remove this dialer list from the interface, use the no dialer-group 1 command.

Branch1604(config-if)#no dialer-group 1

In order to narrow the list of traffic that the router finds “interesting”, you can use access lists. Recall that in order to filter traffic according to port number, an extended IP access list would be required. This is illustrated below.

Branch1604(config)#access-list 150 permit tcp any eq 25
Branch1604(config)#access-list 150 permit tcp any eq 80
Branch1604(config)#dialer-list 1 list 150
Branch1604(config)#int bri0
Branch1604(config-if)#dialer-group 1

With the “interesting” traffic now specified, you still need to configure the number to be dialed. The command to do this is dialer map, followed by the ip address of the remote router, its hostname, and the phone number to connect to the remote location.

Branch1604(config-if)#dialer-map ip name Cisco2620 4165551111

One additional capability that you might be interested in is the ability to define when the second BRI interface should be connected. This is accomplished using the dialer load-threshold command. The number specified after the command is an integer value between 1 and 255, where the number specified is used as a percentage. For example, if you were to configure the interface with a load threshold of 255, the router wouldn’t bring up the second B interface until the load on the first had reached 100% utilization. If a lower number such as 128 were specified, the second B channel would be connected once utilization on the first had reached just over 50%. An associated direction is also specified with the command – for example, you can configure the load-threshold to consider only inbound, outbound, or traffic in both directions in its calculations. To specify both, the either keyword is added to the command as shown below.

Branch1604(config-if)#dialer load-threshold 128 either

By default, a router will terminate a demand dial connection after 120 seconds have passed without it coming across any interesting traffic. This number can be changed using the dialer idle-timeout command, and specifying a new value in seconds.

Branch1604(config-if)#dialer idle-timeout 90

Although not explicitly required, it’s always a good idea to configure CHAP authentication on DDR links for the extra security it provides. In order to do this, you need to specify a username and password from global configuration mode, following the exact same steps used in the PPP section of this chapter. Configuring this router to use CHAP will force authentication when the other router initiates a session. Similar commands would need to be issued on the 2620 router at the head office location to force CHAP authentication when our 1604 router attempts to connect. As the last step, don’t forget to issue the no shutdown command on the bri0 interface!

Branch1604(config)#username Cisco2620 password isdn-is-fun
Branch1604(config)#int bri0
Branch1604(config-if)#ppp authentication chap
Branch1604(config-if)#no shutdown

Configuring ISDN

For the purpose of your exams, you will need to be familiar with the configuration of ISDN. While many Cisco router models include an integrated ISDN BRI interface, many do not. In cases where an ISDN BRI port is not provided, you have two options. First, you can add a modular BRI port to your router using a WAN interface card. If one is not available (or your router’s chassis isn’t modular), you also have the option of using one of your existing synchronous serial interfaces in conjunction with an ISDN Terminal Adapter (TA).

If you’re located in North America, it’s generally a good idea to purchase a router whose BRI interface already includes an NT1 (this is usually marked next to the interface). If your model includes only an S/T interface, remember that you will require an external NT1 device. In Europe, this is obviously not an issue – you’ll simply need an S/T interface, as NT equipment is located at the service provider’s office.

BRI ports are referenced in the Cisco IOS using the same numbering scheme as other interfaces – the first BRI port will be bri0, the second bri1, and so forth. Configuring a router for ISDN is not terribly difficult, but does require that you have information provided by your service provider. For example, you’ll need to know the ISDN switch type that you’re connecting to, and potentially any Service Profile Identifier (SPID) numbers assigned to you. I’ll discuss both in detail shortly.

To begin, I’m going to assume that your ISDN connection will be permanent. In other words, you are not using demand-dial routing (DDR) to connect locations. We’ll look at DDR connections a little later in this section.

The first step in configuring ISDN on a Cisco router involves specifying the ISDN switch type in use at the service provider’s facilities. A variety of different ISDN switch types exist, and you certainly don’t need to memorize the keywords associated with them all. However, you do need to contact your service provider to find out which switch type they are using in order for your router to connect to their facilities properly. If a switch type is configured from global configuration mode, it will apply to all ISDN interfaces on a router. If configured from interface configuration mode, it will apply to that particular interface only. In this example, you’ll configure our router to use a National ISDN-1 switch, identified by the keyword basic-ni1 (note that the last character in this command is the number “1” rather than a lowercase letter “l”). Common ISDN switch types are listed below. The router used in this example is a Cisco 1604, a model with a built-in BRI interface that includes an NT1 – in other words, it has a “U” interface.

Common ISDN switch types:

AT&T Basic Rate Switch    basic-5ess

Nortel DMS-100 Basic Rate Switch    basic-dms100

NET3 ISDN and Euro ISDN Switch (BRI)    basic-net3

National ISDN-1 Switch (BRI)    basic-ni1

AT&T 4ESS (PRI)    primary-4ess

AT&T 5ESS (PRI)    primary-5ess

Nortel DMS-100 (PRI)    primary-dms100

Cisco1604#config t
Enter configuration commands, one per line.  End with CNTL/Z.
Cisco1604(config)#isdn switch-type basic-ni1

After configuring the ISDN switch type, the next step involves configuring the BRI interface. To access interface configuration mode, use the command int bri0.

Cisco1604(config)#int bri0

The default encapsulation type used on an ISDN BRI interface is HDLC. PPP encapsulation can also be used on ISDN interfaces by issuing the command encapsulation ppp.

Cisco1604(config-if)#encapsulation ppp

Many service providers also require that Service Profile Identifier (SPID – literally pronounced “spid”) numbers be specified in order to establish an ISDN connection with their switching equipment. A SPID is not a phone number, but is usually derived from the associated local dial number of the ISDN line. The purpose of a SPID is not only to identify you to the service provider’s ISDN switch, but is also used to define any additional services that you may have subscribed to (such as call waiting, etc). Not all service providers use SPIDs, so you’ll need to find out if yours does. If a SPID is not configured when required, you won’t be able to connect to the provider’s network. As a general rule, a SPID will be provided for each ISDN B channel, if necessary.

SPID numbers are added from interface configuration mode, using the commands isdn spid1 and isdn spid2 respectively. Many service providers also require that a local directory number (LDN) be provided with the SPID in order to use both B channels. If required, the LDN directly follows the SPID number.

Cisco1604(config-if)#isdn spid1 41655512120101 5551212
Cisco1604(config-if)#isdn spid2 41655512130101 5551213

Having configured your SPIDs, there are only two steps left – assigning the interface an IP address, and issuing the no shutdown command.

Cisco1604(config-if)#ip address
Cisco1604(config-if)#no shutdown

ISDN and the OSI Model

ISDN maps to the three lower layers of the OSI model – Network, Data Link, and Physical. However, different protocols exist at the Data Link and Network layers for B and D channels, as illustrated in the figure below. The functions handled at each OSI layer are described below.

  • Physical Layer. The ISDN Physical layer is concerned with the actual sending and receiving of bits over variety of interfaces. For example, the I.430 standard is responsible for providing communication over S/T reference points, while the ANSI T1.601 standard defines communication over U interfaces in North America.
  • Data Link Layer. ISDN D channels use the Link Access Procedure for D Channels (LAPD – Q.921) to frame signaling and control data at Layer 2. On the B channel, data can be framed in a variety of ways, including via PPP and HDLC.
  • Network Layer. ISDN D channels handle call setup, termination, and maintenance at the OSI Network layer using the Q.931 protocol, which implements common signaling standards. On B channels, ISDN uses common Network layer protocols like IP, IPX, AppleTalk, and so forth.

Figure: ISDN protocols and their relationship to the OSI model for B and D channels.

ISDN BRI and ISDN PRI Services

There are two main services associated with ISDN – Basic Rate Interface (BRI) and Primary Rate Interface (PRI). Both services consist of multiple channels over which data can be sent (known as B channels) and also include a signaling channel (the D channel). The D channel is used for control and signaling purposes, such as setting and tearing down ISDN call. Referred to as “out-of-band” signaling, this method ensures that other ISDN calls do not interfere with existing connections, that bandwidth on the B channels is reserved for data only, and ultimately results in quicker call setup and teardown.

Basic Rate Interface (BRI) ISDN

Basic Rate Interface ISDN is made up of two 64 Kbps B channels that are used for sending and receiving data in full duplex, and one 16K D channel for signaling. In total, an ISDN BRI interface provides 144K of bandwidth (64+64+16). ISDN BRI is often referred to as 2B+D. Many Cisco router models include built-in BRI interfaces, but they can also be added using modular WAN interface cards.

Primary Rate Interface (PRI) ISDN

For companies with higher bandwidth requirements, ISDN Primary Rate Interface (PRI) service exists. In North America, PRI service consists of 23 64 Kbps B channels, and one 64Kbps D channel, for a total possible bandwidth of 1.544Mbps (T1 equivalent). In Europe, PRI service consists of 30 B channels and 1 D channel, for a total bandwidth of 2.048Mbps (E1 equivalent). ISDN PRI interfaces are typically implemented as modular WAN interfaces on Cisco routers, although some models do include built-in PRI ports.

ISDN Protocols and Standards

Having looked at the various codes used to describe ISDN equipment and references, now is a good time to get the remaining codes out of the way – those associated with ISDN protocols and standards. Different codes are used by the ITU-T to differentiate between standards and protocols that relate to the ISDN network, ISDN concepts, and ISDN switching and signaling. These include:

E-series. E-series ISDN standards relate to the telephone network. For example, standard E.164 describes international ISDN addressing.

I-series. I-series ISDN standards deal with ISDN concepts and interfaces. For example, standard I.112 outlines vocabulary terms of ISDN.

Q-series. Q-series ISDN standards relate to ISDN switching and signaling. For example, standard Q.921 describes the details of the Link Access Procedure on the ISDN D channel, LAPD.

ISDN Reference Points

ISDN also specifies what are known as reference points between different types of ISDN equipment. These are letters that are used to define logical interfaces between equipment like an NT1 and TE1 device, for example. The letters R, S, T, and U identify the different reference points, but I’ve always found it easier to remember them using the term “RUST”. The reference points described below are also illustrated in the figure below.

  • The R reference point is used to describe the logical connection between non-ISDN devices (TE2) and a Terminal Adapter TA. If it helps you remember, just think of TE2 equipment as being “Retro”, since it predates ISDN standards.
  • The U reference point is used to describe the logical connection between the service provider’s network and an NT1 device. Since an NT1 device is only found at the customer premises in North America, I always find it easiest to remember by designating it “United States”.
  • The S reference point is used to describe the logical connection between ISDN-aware equipment (like TE1s and TAs) and an NT1 device. Because TE1s and TAs are ISDN-aware, I use the term “Standard” to remember these connections.
  • The T reference point is used to describe the logical connection between an NT1 and an NT2 device. In my mind, this has always looked like a type of trunk link, so I remember the interfaces using the term “Trunk”.

As mentioned earlier, many vendors implement both NT1 and NT2 capabilities with a single device, and their interfaces are electronically identical. As such, the S and T reference points are often referenced by the combination S/T.

Figure: ISDN Reference Points

ISDN Equipment

ISDN network equipment can generally be broken down into two groups – devices that understand and are capable of communication via ISDN, and those that do not natively support it. For example, a regular analog phone would not be capable of communicating over the ISDN network without some type of intermediate device converting its signals to the digital standard used by ISDN. However, some devices are already ISDN-capable, such as routers that ship with an ISDN interface. In the word of ISDN, devices are assigned a code that describes their capabilities.

The first type of device that you should look at is known as Network Termination 1 (NT1). In North America, an NT1 device sits at the customer premises, and connects to the service provider’s network, as shown in the figure below. The local loop wiring between the service provider and a customer is based on 2 wires. The purpose of an NT1 is to convert this 2-wire interface into the 4-wire interface used by ISDN. Once installed, the NT1 can be used to connect up to 8 additional ISDN-aware devices on the customer network. As such, any customer (in North America) that wants ISDN connectivity must have a local NT1 device. In Europe, NT1 equipment is located at the service provider’s facilities. Many Cisco router models provide an ISDN interface with a built-in NT1, allowing them to connect directly to a service provider’s ISDN network.

Figure: NT1 devices are located at the customer premises in North America and convert the standard 2-wire local loop interface to the 4-wire connection used by ISDN.

The next devices that you need to be aware of are known as Terminal Equipment (TE). Two types of Terminal Equipment exist, known as TE1, and TE2. TE1 devices are those designed to work with ISDN, using the 4-wire interface. An example might be an ISDN telephone, or a router with a built-in ISDN interface.

TE2 devices are those that pre-date ISDN standards, or are simply not ISDN capable. Examples would include a traditional analog phone, fax, or even a computer. In order for TE2 equipment to connect to the ISDN network, an additional piece of equipment is required. This device is referred to as a Terminal Adapter (TA). A Terminal Adapter is often an external device that connects to TE2 equipment over a serial port. It can also take the form of an expansion card, to install within a PC, for example. In either case, TE2 equipment always needs a TA to connect to an ISDN network. The figure below illustrates a small network with both TE1 and TE2 equipment connecting to an NT1 device.

Figure: TE1 devices can connect directly to the 4-wire ISDN connection. TE2 devices require a Terminal Adapter (TA) to connect.

Another ISDN device that you should be familiar with is known as Network Termination 2 (NT2). An NT2 device is much less common, but is used to provide switching, concentrating, and multiplexing services for ISDN lines. NT2 devices typically take the form of Private Branch Exchange (PBX) equipment. In reality, most NT1 devices also include NT2 capabilities in their design. The placement of an NT2 device is illustrated in the figure below.

Figure: Placement of an NT2 PBX device at a customer premises in North America.