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• General
• Multiple telephone calls with one telephone line
• Access to data services
• Supplementary services
• Multiple Subscriber Number (MSN)
• Calling Line Identification Presentation (CLIP)
• Call forwarding (CF)
• Connecting line identification presentation (COLP)
• Call waiting (CW)
• Brokering
• Park call (PARK)
• Conference calling (3PTY)
• Completion of calls to busy subscriber (CCBS)
• User-to-User Signalling (UUS) and SMS
• Advice of charge (AOC)
• Links for more information
General
ISDN is the abbreviation for Integrated Services Digital Network. In comparison to the PSTN it stands for:
• excellent speech quality;
• multiple telephone calls with one telephone line;
• higher data rates (e.g. for access to Internet); and,
• a comprehensive set of Supplementary Services.
As indicated by the name, ISDN is a digital network contrary to the PSTN, which is an analog network. Digital signalling is what brings along all these nice new features, for example a more sophisticated handling of noise which gives the superior voice quality, or, multiplexing which brings the possibility of 2 independent connections over one copper pair plus a third signalling channel.
The traditional (PSTN) network layout has one sub-network for telephony, a second for telex, a third for packet-switched data traffic, and so on. In other words, basically one networks for each service category. The Integrated Services Digital Network (ISDN) came to existence aiming to solve this problem and provide highly advanced signalling to meet different service and bandwidth requirements in ONE network. The first ISDN recommendations were worked out from 1980 to 1984. The ISDN network uses the existing PSTN telephone network wiring hence no new wiring is required if to switch from PSTN to ISDN services. Although ISDN is basically a circuit-switched network, the first standard recommendations described the possibility of using the network for packet-switched traffic too. Today, recommendations also comprise one more transfer mode: frame relay.
There are different ISDN implementations, which differ from area to area - the one most commonly used in Europe is called Euro ISDN.
Finally, there are two general types of ISDN: a narrowband ISDN (N-ISDN) and a broadband ISDN (B-ISDN). The traditionally available, at least for the moment, ISDN service to home users is the N-ISDN. As one could guess from the name, the difference between the two is in the data rates provided: up to 128 kbit/s by the N-ISDN and up to 2048 kbit/s (2 Mbits/s) by the B-ISDN respectively.
Multiple telephone calls with one telephone line
One of the main characteristics that distinguish ISDN from PSTN network is that ISDN is a digital network, which allows time multiplexing to be applied. The consequence is that multiple, in parallel, channels can be provided over one and the same line. For the possibility to have multiple telephone numbers with one ISDN line (the so-called MSN) see the Supplementary Services section bellow.
Two types of channel are used in N-ISDN:
• The B-channel (64 kbit/s) is the regular traffic channel for transmission of subscriber information through the network.
• The D-channel (64 or 16 kbit/s) is mainly used for signalling but also for packet traffic.
Depending on the configuration of channels offered, two rate access models are defined:
• Basic rate access (BRA) is defined as "2B+D" (2x64 kbit/s + 16 kbit/s). "2B+D" requires 144 kbit/s, in addition 13 kbit/s is used for synchronisation, and 3 kbit/s for network management - all together 160 kbit/s. The BRA is the usual service offered to private customers and allows for 2 independent voice/data channels carried on by each of the B-channels and the additional D-channels which may be used for user related services as well.
• Primary rate access (PRA) is defined as "30B+D" (30•64 kbit/s + 64 kbit/s). It uses a standard 2,048 kbit/s pulse code modulation (PCM) system. 64 kbit/s is used for synchronisation and network management. The PRA is service usually offered to business customers.
The ISDN reference model is shown on the next figure:
Normally, in compare to the PSTN case, it is impossible that the user just plugs her ISDN phone into the phone-line wall socket even if, after contacting her/his local operator, she/he is connected to the ISDN network. Instead, a special device (NT) is required that separates the user terminals from the ISDN network. Such device may contain a number of digital (ISDN) outputs to which the user can connect her/his telephones, PCs, etc. To find out more about the extensive swissvoice ISDN product portfolio go to corded ISDN or cordless ISDN.
In order to connect some of these devices to the NT an additional device may be required - it is called Terminal Adapter (TA). A TA may provide for example access to the ISDN network for ordinary PSTN terminals, e.g. telephones, fax, analogue PC modems, etc., or a direct access to the ISDN network to a PC. In some cases this terminal adapter may be combined with the NT - the "NT+2ab" on the figure above is such a device where a TA for analog terminals is incorporated into the NT, in another it may be a stand alone device, e.g. an ISDN card for a PC, or, part from a telephone as the one on the figure above -- see for example swissvoice Eurit 535 USB.
Access to DATA services
For data transmission each B-channel may provide up to 64 kbit/s speed for access and utilisation of DATA services, e.g. access to Internet via your PC or via your WAP enabled terminal -- see for example swissvoice Eurit 4000. This scenario leaves one B-channel free for possible second connection to the ISDN network that may be used for a voice call or a second, independent Internet session. A second scenario allows for the 2 B-channels to be used together, called channel-bundling, and provides for data rate of up to 128 kbit/s. Of course in this case in-parallel second connection that needs a B-channel, e.g. a voice call cannot be provided. If, for example, an incoming speech call arrives during the utilisation of such scenario normally the call will be signalled with ringing provided over the free at this moment D-channel, but unless one B-channel is made free the call cannot be handled.
In addition to the B-channel data services, some ISDN network implementations provide possibility for provision and access to data services via the D-channel - messaging is an example of such services.
Few more words on the data speeds. You have analog modem that claims 56 and even 112 Kbps data rate, so why bother with ISDN when it comes down to internet access. Well first of all you should be aware how analog modems work - it is not the analog line that provides these sort of speeds - the maximum that the analog line provides is about 30,000 bps the rest is achieved by compression. Though on theory compression should increase up to 4 times the speed of the analog line, in real life, compression depends on the data and only in rare cases reaches even 50 percent increase, and then only with plain-text data. The effect of the noise on an analog line should not be forgotten either which normally leads to repetition of data transmission regardless of how speedy you analog modem might be. Because ISDN is purely digital, data is more easily delivered intact from end to end. Other consideration in favour of ISDN might be the connection time. Because the 64 Kbps channel is essentially a pure "bit pipe," with no rate negotiation or handshaking involved, there are no modem speed or protocol differences to cause conflicts. In fact, because the negotiation phase with ISDN is so simple, ISDN takes only a second or two to dial and establish a connection (modems may take as long as a minute to accomplish the same thing).
Supplementary Services
Simply put, Supplementary Services (SS) are those nice adds-on to the speech sometime ago considered as the one and only telecom service. There are hundreds of SS that have been standardise for ISDN - which of them you may have at your home depends on your ISDN service/network provider, here we will mention just few. For the SS supported by the extensive swissvoice ISDN product portfolio go to corded ISDN or cordless ISDN.
Supplementary Services are most often offered to the user per subscription - some of them may be included in the user basic subscription, for other the user may be required to request explicit enabling and to pay an extra fee. Even without a fee some SS may need a special user intervention before they are to become active.
Multiple Subscriber Number (MSN)
The MSN service allows for the user to posses a number of ISDN telephone numbers. Numbers are allocated by the operator/service provider and may vary throughout regions and type of subscriptions. These telephone numbers can be freely allocated to various users of the line, e.g. in a family household, where there could be one user "father", another "mother", and yet another "daughter/son", different MSN number can be allocated to each of these users, allowing for each user to possess individual number. Modern cordless ISDN phones, e.g. swissvoice DECT phones, allow for these numbers to be allocated to different handsets, therefore allowing for a call to a particular number to be signalled on a particular handset only providing a higher degree of privacy.
Calling Line Identification Presentation (CLIP)
This is one of the most commonly used SS. In some countries, especially outside of Europe, this service is known as Caller ID as well. If activated, the service allows for the telephone number of the caller of one incoming call to be displayed on the display of the call-receiving terminal during the ringing phase. Seeing the number of the caller, the user may decide to answer the call or not.
Even the service is activated it there is no guarantee that the calling number will be displayed. Why? In order to provide you with the number your ISDN network needs first to have it and second to be allowed to send it to you. If the caller comes from an external to you network it is possible that his number will not be provided to your network, hence for your network the calling user/number will be unknown. Secondly, an ISDN user may decide to impose restriction to the network and forbid submission of her telephone number to the called terminal - this is another SS service called Calling Line Identification Restriction (CLIR). If you do not want to receive calls that do not show the calling party number there is a SS offered for such cases too - it is called Anonymous Call Rejection (ACR), which you need to activate.
Call forwarding (CF)
CF is a service that allows for an ISDN subscribers to be reached on other than her dedicated phone, i.e. the user may decide incoming calls to be transferred to any external telephone number, for example to her office number during working hours or a mobile telephone during holiday for example. There are different types of CF to choose from and the transfer of the call can be individually programmed for each multiple subscriber number. The following types of call transfer can be programmed:
• Call Forwarding Unconditional (CFU) - All incoming calls are transferred immediately, regardless of whether there may be someone available to answer the call.
• Call Forwarding Busy (CFB) - Incoming calls are transferred only when the user line is busy, i.e. both B-channels are occupied.
• Call Forwarding No Reply (CFNR) - Incoming calls are transferred only if the call is not answered within several seconds when the user still has the opportunity to accept the call.
Normally the different types of CF are to be activated by the user and the user should not forget to disactivate them when not needed. Calls that arrive during the user is already engaged in a call could be forwarded with a touch of a single button to a predefined number - the service is called Call Deflection.
Connecting line identification presentation (COLP)
If a number you are calling has been forwarded by its owner and you would like to see the number to which you are forwarded, the COLP service is for this. By activating the "Connected line identification presentation" on your ISDN telephone you can avoid having an expensive call if the called fixed line telephone was forwarded to a mobile telephone for example.
Call waiting (CW)
CW is used to show to a user already engaged in a call, that a new incoming call is present. The new presence of call may be announced by a special ringing tone and if CLIP service is possible, the telephone number of the person calling will be shown on the display. Upon indication for CW, the user may decide to reject, accept or transfer the incoming call to another phone or a telephone answering machine for example.
Brokering
An ISDN user can maintain 2 calls in parallel, this service is called Call Hold (HOLD). Brokering is an SS that allows the user to switch back and forth between two calls when using HOLD. One of the calls is active at a time and the calling parties that are not actually being spoken to, cannot listen in to the conversation being active. Brokering and HOLD are normally used in combination with the CW service.
Park call (PARK)
Consider the user does not have a cordless ISDN phone but a corded one only and is engaged in conversation through his telephone in one room being connected to the wall socket in that room. Let us assume now that the user wants to move during the call into another room to verify something and this room has as well a telephone wall socket. The PARK is SS handling this scenario. The user can park the existing connection for few minutes, unplug the telephone from the current room, move to the desirable room, plug in the telephone and continue with the conversation after un-parking the call.
Conference calling (3PTY)
A call is usually communication between two sides, if one would like to talk to 2 other sides, i.e. 3 users talking/listening one with/to each other, the 3PTY (which stands for 3 party) service is to be used. 3PTY allows for the user, after one call has been activated, to set up a second call and subsequently activate the "Conference call" service. In this way all three parties can talk to / listen each other at the same time (unlike the case of the HOLD when one of the parties does not here the other two when they speak one to another).
Completion of calls to busy subscriber (CCBS)
Remember the frustrating experience when you are trying to call a number that is busy? The CCBS is for this. If the dialled number is busy, the automatic "Completion of calls to busy subscriber" can be activated. With the automatic completion of calls activated the user does not need to try calling again and finding that the called number is still busy - instead, as soon as busy number finishes the call and goes on-hook, the network will automatically call it and indicate this to you by a special ringing tone.
User-to-User Signalling (UUS) and SMS
If the telephone network supports it, ISDN offers the possibility to exchange short written messages between ISDN telephones. A prerequisite for sending and receiving these mails is that both parties have ISDN telephones that support this service.
The UUS service is normally the basis for the new Short Message Service (SMS) over the ISDN network service. The "SMS over fixed network" service from content point of vies is the same service you know from the cellular (e.g. GSM) world.
Advice of charge (AOC)
AOC provides the user with the charging information being displayed during a call, hence you can control your expenses and complete a call when it is becoming too long and too expensive.
Links for more information
Dan Kegel's ISDN Page http://www.alumni.caltech.edu/~dank/isdn/
Global ISDN Forum http://www.giif.com/
To find out more about the extensive swissvoice ISDN product portfolio go to corded ISDN or cordless ISDN. .