ISDN & Mobile Solutions

ISDN

The Integrated Services Digital Network (ISDN) uses telephone cables wired directly to you from a local switching centre. These cables are dedicated to data transmission and provide a rate of 64 or 128 kbit/s. Although not much faster than a 56 kbit/s modem, ISDN always runs at these speeds and doesn’t suffer from a system that’s designed for speech signals.

• ISDN isn’t Internet-based. This restricts its use to organisations who need peer-to-peer communication with others, such as publishing companies, that already employ ISDN.
• In the UK, an existing twisted-pair telephone line can be converted to ISDN, but only if you’re within 3000 metres of a local exchange. Otherwise, a dedicated circuit is required, although even this can’t exceed 4000 metres in length.

ISDN Connections

Each ISDN cable conveys up to two Bearer channels (B-channels) running at 64 kbit/s, (56 kbit/s in the USA), which can be combined into a single channel at 128 kbit/s. Protocols for B-channels include PPP, X.25, X.75 (ISDN LAP-B), X.100, T70, BTX, HDLC Transparent, V.110, V.120, channel bundling and bit-rate adaption. Each cable also has a Data channel (D-channel) running at 16 bit/s, which is used for data control purposes and is inaccessible to the user.

For higher data rates you can use more cables, giving correspondingly more channels. The technique of using several channels to effectively create one faster circuit is known as channel bundling. For example, if you have four lines there are 8 channels, giving a total rate of 8 times 64 kbit/s or 256 kbit/s. This allows around 1 MB of data to be transferred in one minute. Up to 30 channels can be bundled, giving a maximum speed over the bearer channels of 1,920 kbit/s.

A coaxial cable or fibre-optic version of ISDN is available, running at 2.048 Mbit/s and giving up to 30 B-channels, plus the extra D-channels. Other variants of ISDN include 1TR6, ESS1, NJ-1, AT&T, TS.013, INSNet64 and Swissnet-2.

Primary Rate ISDN

In the UK the basic form of ISDN, consisting of a single twisted-pair cable, is known as primary rate ISDN or ISDN2e, where the number ‘2’ indicates the number of B-channels and ‘e’ represents Euro ISDN. When used for phone or fax operation one or both of the B-channels are used as a voice channel. As an option, you can expand primary ISDN to 8, 16 or 30 channels.

A primary ISDN connection box usually has four sockets for using two devices at once, such as:-

• Phone and Phone
• Phone and Fax
• Phone and Data at 64 kbit/s
• Data 1 at 64 kbit/s and Data 2 at 64 kbit/s (or 128 kbit/s combined)

The ISDN data circuit on the connection box usually appears as two RJ45 sockets wired in parallel. These can accept one ISDN terminal adaptor (ISDN TA), preferably with the ability to work in multilink mode, or two individual TAs. In most instances, each TA can be connected to a computer via a suitable modem cable or USB cable. Alternatively, if you have a network, you can avoid using terminal adaptors, instead wiring the connection box directly to an ISDN-capable router.

• The RJ45 connectors on an ISDN connection box are the same as those used for Ethernet ports. Don’t plug Ethernet connectors into the ISDN sockets, as the voltages used for ISDN can damage Ethernet equipment.
• If you require a digital route when calling out from the United Kingdom via ISDN you should use 000 as the international dialling prefix instead of 00.

ISDN Operation

To use a single channel of data you only need standard PPP software, as built into the Mac OS. But to use both channels at once requires Multilink PPP (ML-PPP) software, which must also be installed at the other end of the ISDN link.

Unfortunately ML-PP normally ties up both of your channels, preventing you from receiving any normal voice calls. To get around this problem you can employ dynamic bandwidth allocation, a mechanism that works alongside ML-PPP. This lets a single incoming call ‘borrow’ one channel without upsetting the transfer of data, apart from slowing it down. When the voice call is complete the channel is returned to its original use and the data transfer returns to its original speed.

Another option is bandwidth on demand, which normally gives you one data channel. However, when the ‘traffic’ on this channel exceeds a specified level, the link also takes over the second channel until the demand falls.

The two phone numbers given to the ISDN line are usually ‘mapped’ to the sockets fitted on the ISDN box but can be reassigned to the PSTN sockets on the back of a TA by using multiple subscriber numbering (MSN), a mechanism that also lets you use your ISDN connection to run a small telephone system, commonly known as a private branch exchange (PBX).

Mobile Solutions

A mobile telephone, also known as a cellular telephone, is a common accessory for people on the move. When linked to a computer, it can also be used for transferring data. Although several different systems are already in use, other radio-based data transfer systems, such as the Local Multipoint Distribution Service (LMDS) are also being developed.

• Cellular phones shouldn’t be confused with cordless telephones that operate over a local area, usually within the confines of a house and linked to a base station and a standard telephone line. Such devices, which usually employ Digitally Enhanced Cordless Telephony (DECT), don’t usually support Internet access.
• Every mobile phone has a unique 15-digit International Mobile Equipment Identity (IMEI) serial number programmed into its hardware. To ‘block’ the use of a phone, usually in the event of it being stolen, you must give your telephone service provider this number, which can be obtained by entering #06# before the phone is stolen.
• The account details for a mobile phone, including your phone number, are held in a Subscriber Identification Module (SIM) card that can easily be replaced.
• The small screens on mobile phones require a modified form of the HTML Web language. This Wireless Markup Language (WML) operates in conjunction with the Wireless Application Protocol (WAP).

The standard cellular systems are known as GSM, GPRS, UMTS and HSCSD.

GSM

The Global System for Mobile Communications (GSM) is widely used. For example, the GSM 1800 standard, using a frequency of 1800 MHz, operates in over 23 countries, GSM 900 is used in 49 countries and GSM 1900 is employed in the USA and elsewhere. Unfortunately, all forms of GSM are limited to a maximum data rate of 9600 bit/s.

Some GSM phones have an infrared link that can communicate with the matching port on some types of portable computer. Fortunately, IrDAprotocol is normally employed, ensuring compatibility with almost any suitable device. Alternatively, you can use a wired connection via a PC card slot or via a USB cable adaptor, together with the appropriate software.

• To use a GSM phone in the Classic Mac OS you’ll need to place a suitable modem script in the Modem Scripts folder, inside the Extensionsfolder in your System Folder. If you create your own script you don’t need to buy any software, even with a phone not designed for the Mac OS.

GPRS (2G)

The speed of GSM is totally inadequate. Fortunately, the later second generation (2G) cellular technology, more accurately known as the General Packet Radio Service (GPRS), provides a permanent connection at 56 or 110 kbit/s. Fortunately, GPRS phones also work with GSM signals, although the service is limited to the abilities of the actual network in use.

• You can get GPRS access for your laptop computer by installing a PCMCIA card.

UMTS (3G)

The third generation (3G) standard, also known as the Universal Mobile Telecommunications System (UMTS), gives higher rates of up to 2 Mbit/s. UMTS phones can also connect to GPRS and GSM networks, although the service may be limited.

Other Systems

There are several other wireless networks not discussed here, including CDPD, CDMA, PDC and Mobitex, as well as the High Speed Circuit Switched Data (HSCSD) system, which gives a transfer rate of 28.6 kbit/s.