ISDN: Theory & Practice Basic Concepts................................................................................................................................ 2 B & D channels............................................................................................................................... 5 ISDN Networks............................................................................................................................. 12 Bearer Capabilities ....................................
Basic Concepts In this section you will find out: • The definition of ISDN • Basic Rate and Primary Rate ISDN • What an ISDN line provides to the user • How ISDN provides services to the user • What the B and D channels are used for What is ISDN? Integrated Services Digital Network is a set of digital transmission protocols defined by the international standards body for telecommunications, the ITU-T (previously called the CCITT).
What does ISDN give you? - 1 There are two forms in which ISDN is supplied. Basic Rate • Access to the network is called Basic Rate Access (BRA). • It is provided through a Basic Rate Interface (BRI). • This kind of interface is also called an S0 Interface. • There are two channels that you can use. Primary Rate • Access to the network is called Primary Rate Access (PRA). • It is provided through a Primary Rate Interface (PRI). • This kind of interface is also called an S2 Interface.
• Network Services define how the user and the network interact with each other in order to manage calls. • The user can use Network Services to request the network to perform functions such as making and clearing calls, transferring calls to another user, and so on. • This activity is known as signalling. Bearer Services • Bearer services carry the call activity that the user is performing at any given moment. • This includes voice calls, fax and modem calls, and connections to the Internet.
B & D channels In this section you will learn • What the B and D channels do • How the B and D channels share the line • How the structures of BRI and PRI compare • What Bearer Capabilities are and on which channel they reside • How to vary the the amount of bandwidth you have available to you What does a B channel do? The B channel carries ISDN Bearer Services across the network and so carries the content of call (the voice, fax or data) between users.
B & D channel characteristics An ISDN channel has two and only two ends. B channels terminate at a user. A B channel can therefore connect two and only two users. A B channel cannot be Y-shaped. B channels are therefore described as end-to-end. In the case of the D channel one end is with the user. The other end is in the network. A D channel is not end-to-end. You cannot normally, therefore, use a D channel to carry data between two users.
Notice that there is one timeslot which has no channel assigned to it. This is reserved for your network supplier to use for diagnostic purposes. The D channel appears between B channel 15 and B channel 16. Fractional PRI ISDN service suppliers have the opportunity to supply interfaces where not all the channels are active. In most countries, when you take a Primary Rate Interface, you are charged a rental per channel.
You have a much greater choice of protocols for the B channel since the B channel is a neutral conduit for data of any type. You can use it to transmit any protocol you wish (eg. SNA or PPP). However, if the network doesn't understand the protocol it cannot give you any assistance if your call has to be delivered to a different type of network (eg. PSTN) where data conversion is required. B channel charateristics • It is important to remember that ISDN channels cannot be divided up into smaller units.
In the diagram below, the two users are connected using two B channels in parallel. The ISDN is able to route these B channels independently, because it takes no account of the fact the both channels connect the same pair of users. The speed of the two B channels is identical. The time it takes for data to travel from one end of the channel to the other is, however, different. One user transmits two items of data simultaneously.
arrived. It is his responsibility to understand this and to take appropriate action. Normally this will be done in software such as MLPPP. Summary • • • • • The D channel carries Network Services in the form of signalling. This is the way the user maintains his relationship with the network. Each user has one and only one D channel. The B channel carries Bearer Services which are the communication between two users. A single B channel cannot connect more than two users together.
7. If two ISDN channels are used together, each connecting the same pair of users, how many calls are being made? Two. Even though the calling party and the called party are the same, there are two channels being used and two calls are being made.
ISDN Networks In this section you will learn • How ISDNs are connected together • How users are connected to a public ISDN through an ISPBX • How configuration errors in the ISPBX can prevent successful connections ISDN and PBX A normal PABX (Private Automatic Branch Exchange) handles only speech calls, whereas an Integrated Services Private Branch Exchange (ISPBX) is connected to the ISDN and itself provides ISDN extension lines.
The real (physical) users connect with the network inside the ISPBX over their D channels. If they need to communicate with the public ISDN network (for example, to ring home), the "virtual" user inside the ISPBX sends the service request to the public network on their behalf. The services provided by the ISPBX are often different from those provided by the public network. This can cause problems! The network service request that a user sends on his D channel arrive in the ISPBX's internal ISDN.
As far as the interface to the public network is concerned, however, the network simply sees the ISPBX as single user with multiple channels available. 4. A user on an ISPBX can use different D-Channel protocols to those in use by the public ISDN network connected to the ISPBX - true or false? True. Since the ISPBX is effectively a private network to which all the users are connected, the ISPBX is in total control of the ISDN protocol it wishes to use.
Bearer Capabilities In this section you will learn: • Bearer Capabilities in depth • The significance of Bearer Capabilities in diagnosing problems • A practical example of using Bearer Capabilities to resolve a problem Bearer Capabilities - What are they? Although the ISDN doesn't always need to know what protocol is in use in the B channel, there are circumstances where this information is useful.
• Do all the ISDN networks traversed by the call support the requested Bearer Capabilities? • Does the PBX equipment at each end support these Bearer Capabilities? • Has it been enabled? • Has it been enabled for the port in question? Bearer Capabilities - An example – 1 Consider this example of the subtleties of Bearer Capability requirements. A user of an ISDN PBX (ISPBX) may have no problem making analogue calls, yet experience difficulties sending faxes.
PBX manufacturers often extend or amend the rules for internal ISDN communications, so the cause code returned may not be exactly the same as the international standard. The point to note in the example given, is that neither the ISDN equipment, nor the public network, nor the intended recipient, is the cause of the failed communication. Sending the same fax call via a direct ISDN line would prove this.
Voice, Fax, and Modems In this section you will learn about • How ISDN and analogue networks interact • Digital and analogue conversions in the network • Where and how these conversions take place • MODEMs and CODECs • How they impact making fax and modem calls • ISDN equipment types and their fax and modem capabilities Analogue Calls and ISDN The key characteristic of ISDN is that it is a digital network.
The analogue signal originating in the microphone of the telephone handset is sampled and transformed into a stream of bits (64 000 of them every second) that is placed on the B channel. Similarly, the incoming bit stream from the B channel is converted back into an analogue signal and sent to the ear-piece of the handset. A B channel is full duplex, which means that it can carry data in both directions at once.
Analogue fax and modem over ISDN Another important idea is introduced here; this is the Terminal Adapter. A Terminal Adapter (TA) is always necessary to connect non-ISDN devices (such as a serial port of a PC) to the ISDN. However, a TA can also contain a CODEC if it is intended to support analogue phones, fax machines and modems. In diagram below, the modem at the top left can plug in to the POTS ports on the TA.
This would appear to present no problem for the ISDN which is digital and can therefore carry the data passed into the modem (which is digital as well). The next step in evolving this configuration is to use an internal modem in the PC. Logically this is no different from the previous scenario. We could replace the internal modem with an internal ISDN adapter and a driver which appears to the application software to be a modem. In reality, this driver combines the functions of both the modem and the CODEC.
Soft-fax and Soft-modem This means that an ISDN adapter with an on-board DSP can also communicate with any device that contains a modem. In the diagram below, you can see that we've added a fax machine. To perform the job of a modem and a CODEC at the same requires a large amount of processing power. DSPs are very powerful processors. Nevertheless, you need one DSP for each B channel for which you want to use this technique.
• • • • • Modems and CODECs always work in pairs - they can be nested together but each must have a partner. A Terminal Adapter is an interface between the ISDN and any non-ISDN device, such as a computer or an analogue phone If analogue devices need to be connected to the ISDN, then the Terminal Adapter will need to perform the function of a CODEC, and have analogue (POTS) ports available for modems and fax machines to plug into.
6. Does a Terminal Adapter always contain a Codec or Modem? A Terminal Adapter does not necessarily need to have a CODEC inside it. For example, a basic Terminal Adapter could connect a PC via its serial port to an ISDN network without any requirement for a CODEC. The bit stream from the serial port is digital, as is the bit stream that goes out to the ISDN. Although a protocol conversion does indeed occur, it is not an analogue to digital conversion, so no CODEC is required.
Terminology In this section you will learn: • The naming conventions used within ISDN • The essential difference between the key items • The significance of the various reference points • The practical implications of international variations Where the network terminates: The NT1 Most ISDN services are provided through a device known as an NT1. This stands for Network Termination type 1. The device is provided by your network operator and is part of the ISDN.
The NT1 Plus or Super NT1 The 'NT1 Plus' or 'Super NT1' provides additional services to the user by incorporating one or more Terminal Adapters (TAs) in the same enclosure as the NT1. Examples: • A TA built in to the NT1 housing may provide one or two analogue phone ports so that you can plug in an analogue telephone, modem, or fax machine. • A serial or USB port provided by another type of TA allows you to connect your PC via a serial cable.
Reference Point T is on the user's side of the network termination and allows a single TE or TA to be connected. Reference Point S allows more than one TE or TA to be connected. It only exists for Basic Rate ISDN. In this configuration, Reference Point S and Reference Point T are effectively in the same place. The only distinction is the number of devices attached and so this reference point is often referred to as S/T. The S-bus The passive bus illustrated below is an example of Reference Point S.
It is therefore necessary for the user's own NT1 to be connected to reference point U. This will be necessary if the user wishes to attach more than one device. He would have to use equipment designed to operate on the S or T Reference Points. Reference Point U is explicitly not defined in EuroISDN, because the NT1 is always provided and permanently connected to the wires by the service provider.
4. What is the name of the reference point that is located on the user side of an NT1? The reference point on the user side of the NT1 is called Reference Point S/T 5. Does the network provider supply the NT1 in North America? No, in North America the user is required to supply the NT1. 6. In order to be able to plug analogue devices into sockets on an NT1, what extra item of equipment would need to be included within the the NT1? The NT1 would need to incorporate a Terminal Adapter. 7.
Device Addressing In this section you will learn: • Why a device needs a unique address • What the address is used for • Forms of address management • The difference in address management between PRI and BRI lines • Practical implications of mixing address types Why does ISDN need addresses? A Basic Rate Interface (BRI) line may have as many as eight devices attached. All these devices may use the common D channel at the same time.
In this case, the device which currently has no address is able to request one from the network. The network responds with an address for the device which it will use for all future communications. TEI assignment - potential problems Problems will arise if the line is configured to expect a fixed address device, and the device connected is expecting to be allocated an address. In this example, the network will attempt to activate device 0, which doesn't exist.
However, before a device without a TEI can answer a broadcast (to answer an incoming call, for instance), it must request and receive an address in the normal way.
Call Setup In this section you will: • See the step-by-step process for establishing and disconnecting an ISDN call • See the exchange of messages between Terminal Equipment and the Network on both sides of a call. • Understand how several devices attempting to answer a call are managed Network Services The interactions between the user and the network are all carried by messages in the D channel.
It is mandatory to specify the Bearer Capabilities in a SETUP. Although an initial SETUP would normally specify the destination number for the call, this is not mandatory. A SETUP is quite valid even if there is no called party number specified. This is how the Eicon DIVA LineCheck utility works. Where the destination phone number is available before the call is placed, for example, with Dial-Up networking, it is normal that the SETUP would contain this information.
Frequently, delivery of the calling (origin) number is an extra service from your service provider that you have to pay for. This service is often called CLIP (Caller Line Identification Presentation) or sometimes just CLI. See the section Managing Inbound Calls for more information on this subject. At the destination, an ISDN device capable of receiving the incoming call responds with an ALERT message. An ALERT is sent to the originator of the call.
Finishing a call Call disconnection also occurs through messages sent on the D-channel. The message which conveys this information is the DISC, which also carries a cause code. A DISC may occur because one of the users terminates the call (by hanging up the phone, for instance). The cause code will be 'normal call clearing'. A disconnection also may happen before the call has been completed, for instance with cause code 'destination busy', or for some other reason.
When there is more than one device capable of receiving the incoming call at the destination, a mechanism exists to ensure that only one device can answer. The SETUP is broadcast to all devices at the destination, and they in turn respond with an ALERT. At this point, both devices are "ringing". In this example, the upper device answers the call - a user picks up the telephone. This stops the bell ringing. What about the lower telephone? Its bell is still ringing.
Connection collision What happens if more than one device attempts to answer the call at the same moment? The CONN messages collide in the D channel. If you're wondering why we're not worried about the ALERT messages colliding (which could happen), this is because these messages are not critical to what happens next. Don't forget that there is only one D channel and all devices that want to transmit have to contend for it. This is where the NT1 helps.
2. What channel does the end-to-end connection use? End-to-end communications are carried on the B Channel. 3. What is the initial call request message called? The initial call request message is a SETUP. 4. What is the call termination message called? The call termination message is called a DISC (from DISConnect). 5. What information also accompanies this message? The reason for the call termination accompanies the DISC message. 6.
Managing inbound calls In this section you will learn: • How to manage more than one device on a single ISDN line • How to ensure that the appropriate call types are directed to the correct type of device. • What call filters are and how to set them Handling incoming calls Consider the example in the diagram. An ISDN line has a telephone, a fax machine, and a PC attached to it. The objective is to achieve the following: • When someone rings the phone, only the phone rings.
• PC doesn’t answer because the are no programs running that are listening for incoming calls However, there is a potential pitfall in this solution. This is because of the restriction that only one filter can be applied to each setup message. A fax device, for example, may be configured to accept calls whose bearer capability is specifically Fax Group 3. However, fax messages which originate within an analogue network do not come with this very precise call type information.
In this case, the destination fax machine will not answer the call as it is configured to accept only analogue calls. In practice, a device can only apply one filter to each parameter of the incoming SETUP. A filter such as "CallType=G3Fax OR CallType=Analogue" is not normally possible. For these reasons, this solution cannot be recommended. It would work very reliably if all networks were ISDN; and all Fax machines placed G3 Fax type calls and they were configured to answer such calls.
1. What must each SETUP message contain? Every SETUP message must also carry a Call Type. This provides information about the contents - such as 'Group 3 Fax' or 'Generic Analogue'. 2. How many filters can be applied to a SETUP message? Only ONE filter can be applied per SETUP message. 3. What call type is given to a fax call coming from an analogue network? All calls originating on the analogue PSTN network will be automatically allocated the call type 'Generic Analogue'.
Rate Adaption In this section you will: • Learn the difference between asynchronous and synchronous data streams • Understand how they can be connected • Examine the technologies available to achieve this • Extend your understanding of the functions of an ISDN TA (Terminal Adapter) Note: Rate adaptation is sometimes called 'rate adaption' in North America and some other countries.
Bit stuffing The bits required to fill up the 64kbps channel must be added in a systematic way so that the receiving device can remove the bits using exactly the same methodology to retain the information being transmitted. There are two protocols commonly used that define systems to insert and remove extra bits: • V.120 • V.110 You will use either one or the other, depending on the protocol required by the service you are calling. Rate Adaptation is usually implemented in a Terminal Adapter.
Many Terminal Adapters support protocol conversion between synchronous PPP and asynchronous PPP. These protocols are not identical and conversion is required. The serial port of the PC is asynchronous so this determines the type of PPP used between PC and Terminal Adapter. The ISP, however, in natively supporting ISDN (rather than using Rate Adaptation), requires synchronous PPP.
Cabling In this section you will learn about • Wiring requirements for BRI and PRI • Maximum cable lengths • How to connect several devices to a BRI • When you need to use termination • The power sources available through the interface What you get with ISDN EuroISDN is now the standard in most parts of the world, except North America and certain other countries. The EuroISDN network is supplied to the user through a box mounted on the all. This box is called a Network Termination Type 1 or NT1.
If you are connecting only one device to your BRI NT1, the cable can be as long as 1 km if you are using Category 5 twisted pair. In practice, if you really need to run a cable for this distance it would probably be safer to re-site your NT1 closer to the equipment to be connected. All NT1s have two sockets for devices, so you could add another device to the other socket at a similar distance. This form of wiring is known as point-to-point.
Another form of wiring is known as the extended passive bus. In this case the total maximum distance allowed is greater, up to about 500 metres, but there is a restriction on where the devices may be located on the main cable. The maximum length of the drop cable is the same as before (ten metres). The restriction is that the device nearest the NT1 must be no more than either 25 or 50 metres from the end of the main cable furthest from the NT1.
the user's local power supply has failed. Contrast this with PS2 where the NT1 provides this power from its own mains plug. If none of the equipment that you have connected to your NT1 requires a power supply through the interface, you do not have to plug the NT1 into the mains. The NT1, which is not a passive device, normally powers itself from PS1. Only if the NT1 contains a TA (so it is a 'NT1-plus' or 'Super-NT1') or if it can supply power to attached devices will it need to have a mains power supply.
This also shows how you arrange the wiring for the short and long passive bus configuration. If you also need the power supplies PS2 and PS3, you should include conductors 1, 2, 7 and 8 as well. Summary In this section you have learned that: • The NT1 is an integral part of the network in EuroISDN. • The user provides the NT1 in North America. • You can use most types of twisted pair cabling for a BRI.
More Termins http://www.freesoft.org/CIE/Topics/index.htm Analogue What does "Analogue" mean? Often this refers to a quantity that is can vary continuously. That doesn't mean that it's always changing. It means that it can have any value. One example of this is temperature. The temperature outside today might be 25°C or it might be 35°C. It could also be any other temperature, perhaps 33.5°C or 36.1274°C*. If you were a farmer, you might have six cows or seven cows, or 200. You wouldn't have 11.
Protocol A protocol describes a procedure. When you declare your taxes, you complete a form. There will be boxes on this form where you state your name, your income, deductions, etc. This makes it easier for the person (or machine) that has to process the form; they will know exactly where to find each item of data. There is also a set of rules which will describe what happens to the form inside the tax office.
Remember that MLPPP is not the same two separate PPP links. MLPPP is a single connection which uses more than one data link. Two PPP connections are two separate connections to two independent destinations. PBX or PABX PBX stands for Private Branch Exchange. PABX means Private Automatic Branch Exchange. Nowadays, these both mean the same thing.
The bearer capabilities are communicated to the destination, so that he may act upon them. For example, an analogue call could be presented to a modem, but a transparent data call would be presented to an ISDN adapter. The ISDN might also use the bearer capabilities for its own purpose. For example, in North America, some network operators make different charges for voice and for data calls. Some networks are able to optimise their internal performance by using the bearer capabilities.
Synchronous and Asynchronous PPP Synchronous and asynchronous PPP are basically the same, but they are not compatible with each other. Both can be used to send binary data. One of the chief difference is that asynchronous PPP can be sent across an asynchronous interface. Such interfaces have a flow control mechanism used to regulate the flow of data so that no component (eg. buffer, modem) is overwhelmed. There is often a choice of flow control: hardware or software.
