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Integrated Services Digital Network Primer

Chapter Description

This chapter covers Integrated Services Digital Network (ISDN) basics, ISDN physical attributes, ISDN specifications, BRI, and PRI.

BRI

The good thing about the deployment of ISDN BRI is that no one tried to reinvent the wheel. Well, at least for the cabling. Your analog circuits, also known as POTS lines, are deployed over 24-26 gauge copper wiring. ISDN BRI service can be deployed over the same copper wiring to your premises.

Hardware Considerations

When deployed to your premises, they are normally installed in RJ-45 jacks using pins 4 and 5. There is no polarity with the center two pins, you just need to make sure that you have them installed properly at the jack. BRI service doesn't even have to be deployed with RJ-45 interfaces, but most service providers prefer this so as to differentiate that jack from standard analog service.

If you were to plug an ISDN TA into an analog line, there is a pretty good chance that you would short out that equipment if an analog call with –48 vdc came in. Therefore, if the installer deploys a different looking jack, it cuts down on that becoming a possibility. Most ISDN TAs must be powered from an external power source because there isn't any line voltage on a BRI circuit (at least not enough for your equipment).

What this tells you is that if you lose power to your premises, your ISDN goes with it unless you have a significant power backup in place. For this reason, most service providers advise against using the ISDN circuit as the only line at your location. There are two caveats to this issue:

  • If you replace analog with ISDN entirely, you lose all phone service in a power outage (no 911 among other problems).

  • In most areas, ISDN is not governed by the same tariffs as analog. With analog service, the service provider must repair it within a short time period. As a WAN service, ISDN can take up to a week to repair, leaving you without phone service for that entire length of time.

The service repair window is only a problem in the United States because most European countries are ahead of the U.S. in ISDN deployment and because analog and ISDN circuits are typically treated the same internationally.

BRI Operation

ISDN BRI circuits are always active and use a type of heartbeat to maintain circuit status. On a standard BRI installation, if you plug an analog phone into your ISDN jack you can hear an intermittent click. That click is a good way to see if you are getting an ISDN signal. If you can hear the click, your service is turned on. That doesn't mean that there isn't a problem with the service, just that you have it.

The main selling points for BRI service are that there is increased bandwidth over analog service (128 kbps compared to 56 kbps), you can use multiple devices at the same time, and the dialing time is dramatically decreased. The increased bandwidth comes from the bandwidth provided with the B channels. Even if your BRI circuit only has a single 64-kbps DS0, it still has more bandwidth than a 56-kbps analog device.

However, in some locations only 56-kbps DS0s are available because of service provider equipment restraints. This is becoming less and less common because most service provider infrastructures are now 64 kbps clear (can provide 64 kbps end-to-end). Also, many service providers cannot provide conditioning services to speed up analog connections, stating that analog circuits were never intended for data transmission. Conditioning services include the removal of bridge taps and line loading (where possible) to provide faster analog service.

The typical circuit rate for a BRI service is 160 kbps (with other network overhead, it can be as high as 192 kbps). In this 160 kbps, there are 2 64-kbps B channels, a 16-kbps D channel, and approximately 16 kbps of overhead, as shown in Figure 8-21. BRI combines two B channels to form a larger pipe of bandwidth through a process known as channel bonding. A call is placed on the first B channel, after that a second call is placed on the other B channel. When both B channels are connected, they are bonded together to form the full bandwidth rate.

Figure 8-21Figure 8-21 BRI Circuit Composition

BRI Capabilities and Features

ISDN BRI can support up to eight devices on the S/T bus, but you are limited by the number of B channels that your circuit has. Most service providers don't tell you this, but if you have two B channels with your ISDN service, you can only use two devices at any one given time. After both B channels are in use, you cannot add any other services without disconnecting something. So if your ISDN TA is using both B channels for a data call and you want to place or receive a voice call, you have to drop one of the B channels to facilitate that function.

Most modern ISDN TAs have a feature known as bandwidth allocation control protocol (BACP). BACP allows your device to dynamically drop a B channel to allow you to take an incoming voice or fax call and then reallocate that B channel back to the data call after it is finished. This feature is implemented through the bandwidth allocation protocol (BAP), which works in tandem with BACP. Working in tandem with BAP and BACP, is bandwidth on demand (BOD). Hypothetically, if you only have a limited amount of ISDN usage per month, you do not want to use more than absolutely necessary.

To save money, you want to use your ISDN circuit wisely. BOD allows you to set up your equipment to dial up with a single B channel and then add the second B channel only if necessary. This feature is normally set with a series of thresholds such as 50 kbps for 10 seconds. When using BOD, make sure that your channel addition threshold and subtraction threshold are not too close together. Otherwise, you will constantly add and subtract your secondary channel, which negates the BOD feature.

Increased dialing speed comes from several different things. There is no dial tone generated by the CO on BRI circuits. On an analog circuit, dial tone notifies you that you can place a call. Because the BRI circuit is always active, dial tone is not necessary. If the ISDN TA has POTS ports on the back for analog phones and you can hear dial tone when you pick your phone up, that dial tone is being generated from the equipment.

When you place an ISDN call, you do not hear the handshaking tones (those nasty screeching noises with analog dialup) because ISDN is a synchronous digital network medium. This provides a much faster connection, usually in three to five seconds.

Internationally, BRI service is deployed by using an S/T interface. The U interface belongs to the service provider, and the customer does not need to purchase an NT1 for the circuit. For this reason, most European countries allow you to plug TE1 equipment directly into an ISDN outlet. At that point, you are plugging directly into the S/T bus.

In Japan, Nippon Telephone and Telegraph (NTT) provides two main types of BRI service, INS Net 64 and INS Net 64 Lite. The difference between these two services is that the INS Net 64 service requires a monthly subscription fee. Therefore, most users prefer the INS Net 64 Lite service. The device deployed on these circuits is a data service unit (DSU/TA), which provides the same functions as the NT1/TA devices described above. You can't just buy an ISDN TA in the United States and expect it to work anywhere in the world. For instance, if you purchase an ISDN TA that does not have the Japanese Approvals Institute for Telecommunications Equipment (JATE) approval, there is a good chance that equipment will not work in Japan. Likewise, equipment destined for use in the United States should have the proper Underwriters Laboratories (UL) and FCC Part 65/68 certifications. Certification by these entities does not necessarily mean that the equipment will absolutely not work in other regions, but it is a good idea to make sure your equipment has the necessary hardware approvals for your location. In the field, you are more likely to run into problems with devices that do not support the proper switch protocols for ISDN rather than problems with the hardware certifications.

ISDN BRI Line Coding

ISDN BRI is a digital service that requires a line-coding scheme, similar to any other digital medium. The line coding that exists on BRI circuits is considered to be block line coding rather than linear line coding (such as alternate mark inversion [AMI] and bipolar 8-zero substitution [B8ZS]) because the line-coding schemes use a series of lookup tables for values associated with binary bit combinations. The line-coding type differs in North America. North America uses the 2 Binary 1 Quaternary (2B1Q) line-coding method.

2B1Q Line Coding

2B1Q is composed of four different discrete voltage states, and it transmits two binary digits during each pulse. This is in contrast with most other coding standards such as B8ZS and High Density Bipolar of Order 3 (HDB3), which only transmit one binary digit per pulse. As shown in Table 8-7, each voltage state has a specific value attached to it that does not change.

Table 8-7 2B1Q Binary Voltage Values

Binary Representation

Voltage

1 0

+3v

1 1

+1v

0 1

–1v

0 0

–3v


When 2B1Q codes binary digits, it looks a bit different than the other line-coding schemes you have learned about. 2B1Q does not employ the use of the 0 voltage state, and it does not have to adhere to any bipolar violation (BPV) specifications.

Figure 8-22Figure 8-22 2B1Q Line Coding

4B3T Line Coding

International BRI circuits, commonly called ISDN-2 or ISDN Basic Access (IBA), use a different type of line coding that is called 4 Binary 3 Ternary (4B3T). This coding scheme is a bit more complicated than most and is also referred to as Modified Monitoring State Block Code 43 (MMS43). The basic premise is to take four binary digits and, using a lookup table, convert them to be used in a strict three voltage state environment of negative, zero, and positive voltage. This function creates an approximately 25 percent reduction in baud rate as transmitted on the BRI circuit. Refer to Table 8-8 for a listing of the binary conversion.

Table 8-8 Lookup Table for 4B3T Line Coding

Binary Digits

Bit #

Voltage

Bit #

Voltage

Bit #

Voltage

Bit #

Voltage

0001

1

0-+

2

0-+

3

0-+

4

0-+

0111

1

-0+

2

-0+

3

-0+

4

-0+

0100

1

-+0

2

-+0

3

-+0

4

-+0

0010

1

+-0

2

+-0

3

+-0

4

+-0

1001

1

+0-

2

+0-

3

+0-

4

+0-

1110

1

0+-

2

0+-

3

0+-

4

0+-

1011

2

+-+

3

+-+

4

+-+

1

---

0011

2

00+

3

00+

4

00+

2

--0

1101

2

0+0

3

0+0

4

0+0

2

-0-

1000

2

+00

3

+00

4

+00

2

0--

0110

2

-++

3

-++

2

--+

3

--+

1010

2

++-

3

++-

2

+--

3

+--

1111

3

++0

1

00-

2

00-

3

00-

0000

3

+0+

1

0-0

2

0-0

3

0-0

0101

4

0++

1

-00

2

-00

3

-00

1100

4

+++

1

-+-

2

-+-

3

-+-


Binary representation of 4B3T is shown in Figure 8-23.

Figure 8-23Figure 8-23 4B3T Line Coding

Anatomy of BRI

When configuring an ISDN device for BRI service, you must have the proper information for communication on the service provider's network. First and foremost, you need to know what type of switch you are communicating with. The switch type is important because not all vendors have implemented ISDN in the same way. For this reason, if you tell your equipment that it is communicating with a Lucent 5ESS, but you are actually connected to a Nortel DMS-100, you might not be able to access the ISDN network. Refer to Cisco documentation for all the supported ISDN switch types.

Switch vendors began to diverge on compatibility in the United States in the early to mid-90's, so a set of standards was developed to help alleviate this problem. Contrary to what a lot of people say, NI-1 is not an actual switch, but a set of programming that allows many vendors to communicate with one another over the ISDN network. The National ISDN standards were developed as a three-stage process: NI-1, NI-2, and NI-3 to be deployed over a period of about 10 years. These standards allow customer ISDN equipment to internetwork with a larger number of vendors. Thus, Lucent doesn't have to figure out how to directly communicate with a Nortel switch in all aspects of ISDN. They merely have to program their switches to adhere to the National ISDN standards. The caveat with this is that if you are using a private ISDN configuration, you can use whatever switch type you want.

It is perfectly conceivable to use the primary-ni switch type on E1 circuits as long as they are consistent on both sides of the connection. On back-to-back configurations, it is also necessary to specify one of the two sides as the network. Cisco ISDN devices default to user, and two user devices on the same link do not work. If you have both sides set to user, Layer 2 never comes up.

Service Profile Identifiers (SPIDs) and Telephone Numbers

Possibly the most important single piece of information for a BRI subscriber in the United States is the SPID. SPIDs identify the BRI circuit on the switch along with the circuit ID, and the services that the circuit has assigned to it. For each B channel assigned to the circuit, there is a SPID associated with it, and for each SPID there is an associated telephone number. When you look at the format of a SPID, it looks a lot like a regular telephone number with an added extension. The most common format is 14 digits:

  • SPID Format: 91955512120101
  • Telephone Number: 9195551212

The last four numbers of the SPID (0101) indicate the sharing terminal ID and the terminal identifier as per the National ISDN Council. When you place a call to another ISDN device, you are placing the call to the telephone number assigned to the circuit rather than directly to the SPID. The SPIDs have local switch significance only.

Depending on how long you have had BRI service, you might not even know that you have SPIDs. By the time the United States reaches a full deployment of NI-3, SPIDs should be automatically downloaded and configured by your device. A lot of equipment already has this function built in, and it depends on the functionality of the switch you are communicating with as to whether or not it will currently work.

With the proper ISDN switch type and the correct SPIDs, your ISDN TA should enter a state that people call synching to the switch. When your ISDN device is synched to the switch, you can place and receive ISDN calls.

BRI Circuit Provisioning

Each BRI circuit is provisioned into the switch framework. The provisioning on the BRI circuits is completed through capability packages. The capability packages, also called ISDN Ordering Codes (IOCs), offer different combinations of bandwidth and service. Not only can the physical characteristics change (1B+D, 2B+D, just D), but the service types also can be modified. That means that a circuit can be provisioned for just voice, just data, or a combination of the two. These capability packages change so often that any direct discussion of them would be outdated by the time you read this.

Although BRI is classically referred to as a 128-kbps circuit, make sure that your capability package supports two B channels. Packages that offer strictly D channel configurations are typically used for applications such as credit card readers or automated gas pumps. In these instances, the D channel manages data transport.

All the services that you generally associate with analog circuits, such as voice mail, call forwarding, Caller ID Deluxe, and conference calling can be programmed for use in ISDN. Unfortunately, this is where most problems associated with ISDN are found. The issue isn't the vendor equipment or the software features, but the combination of the two.

Different vendor equipment reacts in a variety of ways to services offered by service providers. Moreover, not all service providers deploy the same services in the same manner. This combination creates a headache for switch programmers and customers alike. If you have ever been told that ISDN is complex and that It Still Does Nothing, it probably had something to do with a value-added service fiasco. Because of the issues associated with ISDN BRI service for Internet connectivity, it is being gradually replaced with higher bandwidth, and easier to use technologies such as digital subscriber line (DSL) and cable modems. ISDN switch provisioning has classically been a problem because of how many issues arise out of service programming. Not all services work with all the vendor equipment, and different vendor equipment can react differently to the same circuit provisioning. There also have been problems with different service provider switches communicating properly with vendor TAs.

Foreign Exchange (FX)

To deploy BRI circuits to customer sites, the service provider's switch must be able to accommodate the ISDN equipment. This typically includes a fairly large switch such as a Lucent 5ESS or Nortel DMS-100, with adequate port density and a series of line cards both at the CO and IDLCs. When deployed in IDLCs, BRI service typically takes up three full DS0s, so available bandwidth is also a factor. In areas that can't support these requirements, an FX of the circuit might be necessary. At its most basic level, an FX circuit reroutes the local loop through a switch at a CO that does not provide local ISDN service. Figure 8-24 shows a basic depiction of an ISDN circuit that is foreign exchanged to a remote office.

Figure 8-24Figure 8-24 'Foreign Exchanged BRI Circuits

In Figure 8-24, the FX ISDN service is deployed through a switch that does not necessarily provide local exchange service. In some areas, the facilities required are just not available. Rather than upgrade an entire switch for a couple of ISDN customers, the service provider would rather redirect you to a switch or IDLC that is already providing such service.

FX circuits are normally handled by a group called special services. They are typically a separate entity, which means that to test or repair this type of circuit you probably need to go through them directly.

There are cases where FX is not possible due to location or distance from the CO (if you live in the boonies). If this is the case, you might not be able to obtain ISDN service.

Always On Dynamic ISDN (AO-DI)

AO-DI is a relatively new addition to the ISDN scene. It allows you to maintain a permanent connection to the Internet by using the ISDN D channel. Remember that in BRI service the D channel is 16 kbps and that it is used for call-control and channel management signaling. AO-DI takes approximately 9.6 kbps of the 16 kbps for a link back to the service provider's ISDN switch. With this link, you can download stock tickers, e-mail, and other streaming desktop applications.

One of the main advantages of AO-DI is the fact that you do not need to connect to your service provider and use precious B channel minutes. Not only do you not need to worry about busy signals, but there is no waiting to connect (ISDN connection time only takes a couple of seconds). Instead, you can save your packaged time for use only when absolutely necessary.

Configuring ISDN BRI

The first of two types of ISDN that you will configure is ISDN BRI service. Make sure that before you go to set up your ISDN TA, you have the proper ISDN switch type, your SPIDs, and any telephone numbers that are associated with your circuit. You will be configuring a basic BRI application by using a Cisco router. Figure 8-25 shows a diagram of the network that you will configure in this example.

Figure 8-25Figure 8-25 BRI Configuration Example

The first step for the Raleigh router is to specify the ISDN switch type that you are using. This command has to be configured globally, but you can specify more than one switch type in the router by changing the switch type on the actual interface. The switch types supported are grouped as primary and basic switch types. These names refer to whether or not you are using a BRI or PRI service.

Raleigh#conf t
Raleigh(config)#isdn switch-type basic-ni

If you want to change the switch type on the BRI interface, you can use the same command with a different switch type.

Raleigh(config)#int bri 0
Raleigh(config-if)#isdn switch-type basic-5ess

Next, you need to configure any SPIDs that are associated with your ISDN circuit. The number following the SPID is the accompanying telephone number. It is important to remember how many digits are required for dialing. For example, if you are located within a 10-digit dialing area, be sure to enter a 10-digit number.

Raleigh(config-if)#isdn spid1 91955512120101 5551212
Raleigh(config-if)#isdn spid2 91955512130101 5551213

You have now completed a basic configuration for connectivity back to the ISDN switch. If you want to allow the Atlanta office to call Raleigh, but you also want callback capabilities, you can set up the ISDN callback feature. This ensures a specific level of secure caller identity for inbound ISDN calls.

Raleigh(config-if)#dialer caller 4045551212 callback
Raleigh(config-if)#dialer caller 4045551213 callback

The previous configuration starts callback for the phone numbers listed if they are detected as inbound ISDN calls. Caller ID is required from the service provider's switch for this option to function properly.

For basic dial out capability on BRI, you can set up the Raleigh router to dial to the Atlanta router if the proper IP address is detected. This can be started as a specific request from a user on the network or as a result of a default route setting on the Raleigh router.

Raleigh(config-if)#dialer map ip 10.15.1.1 name Atlanta 4045551212

For the dial out to work properly, you need to make sure that the proper username and password is set for authentication and that you enable Point-to-Point Protocol (PPP) and Challenge Handshake Authentication Protocol (CHAP).

After you have configured your BRI service, you can verify the circuit with the show isdn status command, as shown in Example 8-4. You are looking specifically for the layer status messages. At Layer 2 you should see MULTIPLE_FRAME_ESTABLISHED.

Example 8-4 The show isdn status Command

Raleigh#show isdn status
Global ISDN Switchtype = basic-ni
ISDN BRI0 interface
    dsl 0, interface ISDN Switchtype = basic-ni
  Layer 1 Status:
    ACTIVE
  Layer 2 Status:
    TEI = 64, Ces = 1, SAPI = 0, State = MULTIPLE_FRAME_ESTABLISHED
  Layer 3 Status:
    0 Active Layer 3 Call(s)
  Activated dsl 0 CCBs = 0

Resolving Issues with ISDN BRI

Just as with any technology, there are many things that can go wrong on an ISDN circuit, particularly a BRI circuit that is weighed down with services. This section is intended to serve as a set of pointers for troubleshooting BRI circuits and not just Cisco equipment.

First, make sure that your SPIDs and your switch type are correct. Even some vendor equipment that employs auto-SPID download can malfunction and cause a SPID mismatch. You also want to make sure that the SPIDs given to you by your service provider are actually what the switch programmer put on your circuit. They are as human as you are and can provision wrong SPIDs from time to time.

In the United States, basic-ni gets you by 99 percent of the time and is typically safe as a selection. European and some Oceania countries can usually use the basic-net3 switch type as a similar encompassing switch type. If you know that you are on a Nortel DMS-100, you configured it as such, and your device is still not working, you can try the basic-ni switch type. Sometimes that is the best switch type to use even when yours is known. It might just be a quirk between your vendor equipment and the switch.

Another problem that you can encounter is a cable throw. Cable throws occur when a line technician, needing a pair of wires, takes your set and then reroutes you through a longer pair of cables. For instance, there is a line technician who is looking for a pair of wires for analog service. To find an available pair, he uses his test set to check which of the cables have a dial tone. When he finds a pair with no dial tone, he assumes that the pair is not currently being used and procures them for his analog application.

Remember, ISDN does not provide dial tone, so this can and does happen quite often. When the pair is redirected, your circuit runs the risk of going over the prescribed 18 kft or –dB loss. If the pair is thrown over or close to the loop length requirements, you can have intermittent or complete service loss. This is no easy problem to track down, but there are things that you can do to help figure it out.

Cable length is a funny thing. You would be surprised to find how many problems arise from it. There have been issues with time of day operation, and no, this isn't a new feature. The setting is usually summertime, and the circuit does not function during the day. However, through the midnight hours it appears to work properly.

In cases such as this also try to check the loop length of the circuit. In many cases, the circuit does not operate during the day because it is close to the allowed loop length. When the sun comes out and heats up the cabling, the cable pair expands, which throws the circuit past operational lengths. When the sun goes down the cables cool off, retract, and the circuit can function properly.

As was stated before, –dB can also have an effect on the signal. As the signal travels down the circuit, the signal attenuates. If the signal attenuates too much, the terminal equipment is unable to synch to the digital signal. That being said, somewhere between the range of –38 dB and –42 dB the service provider should be adding a repeater to the circuit. If you are having intermittent problems and all line hardware is testing fine, ask what the –dB on the circuit is. Even if they can't test for you while you wait, they can send a technician to do it manually.

If you are set to have an ISDN BRI circuit installed and you're not sure if it is turned on yet, you can plug an analog phone into the ISDN service jack (only if you are plugging into a U interface). If you listen closely, you can hear a slight clicking noise. That is the heartbeat of the ISDN service, which means that you at least have a signal. The circuit can still have a hardware problem, and you can take it further by calling your local service provider for loop testing. The closer you are to the CO the faster the click is generally.

If you can't get the click in your house, try it again at the NID outside your premises. If you can get a click there, your ISW appears to have a problem. Also, try not to use a cordless phone, as they do not work sometimes even when a signal is present.

When wiring your premises (if you are doing your own wiring), make sure that the jacks that are used for ISDN have what is called a home run. A home run is a length of cable that runs from the NID directly to the jack without any splices. Standard analog lines in houses typically have several splices, but each splice can cause line errors and degraded throughput on an ISDN circuit. For the same reason, ISDN circuits must be devoid of bridge taps on the carrier side. Messy analog ISW can be a headache if upgrading to ISDN.

Also, be aware of how the cable is installed at your premises if you are hiring someone to do it for you. Many analog install technicians will install an analog circuit, and then just coil up any excess wiring before connecting it. These coils can cause what are known as magnetic loops on an ISDN circuit. Magnetic loops can cause performance below even basic analog dialup rates. Any coils should be removed and connected properly.

If there are several locations within your premises that you want to have ISDN at, you can't have an ISDN TA at each location. The problem is that with BRI you can only terminate the circuit once, so additional NT1s cause none of them to work. One thing you can do is purchase a basic NT1, terminate the ISDN line, and then connect your equipment to the S/T bus (they have to be TE1 devices).

This was touched on before, but make sure that you are aware that you are limited to the number of B channels that you have. Eight devices cannot all work at the same time on the ISDN circuit. One data and one voice, two data, two voice, and so on.

Millions of people get their bill every day for ISDN service. A few of them are unpleasantly surprised to find that their bill is about 10 times over what they expected. Most bills come in an envelope, but some come in a box. This is typically indicative of a couple things:

  • The subscriber is on a timed package with some sort of pay as you play pricing rate.

  • Their Internet service provider (ISP) might not support two B-channel connections.

You might be wondering why this is a problem, but this causes what is affectionately termed thrashing. If your device connects to your ISP on the first channel, but the second is not allowed, the secondary channel is dropped after it connects momentarily. Most equipment attempts to reacquire that connection, and if it isn't configured properly, it continues to attempt to acquire that channel.

This is bad for you if you have a low minute package rate or a strictly pay as you play rate because you end up going way over your allotted time limit in a month. Because most service providers only require about 1/10 of a second to bill you for a call, you can easily rack up charges.

To avoid this problem, be sure that your ISP allows both B channels and that your equipment is configured properly. Most modern equipment can be set to only dial with one B channel or at the very least to discontinue reconnect attempts for the second channel after a couple of tries.

By using a serial device you lose a portion of the BRI bandwidth. A typical 16550 Universal Asynchronous Receiver Transmitter (UART), found in personal computer serial ports, only allows a maximum of 115,200 bps instead of the full 128,000. You can get around this problem by using an Ethernet connection to an ISDN router because you then have at least 10 Mbps.

Last but not least, if your circuit completely quits you can ask to verify that the office equipment (OE) is still assigned to your circuit. The OE is the hardware that is required for ISDN service to operate at the CO and from time to time it can be inadvertently commandeered for other applications, thereby destroying your service.

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