|
Newsletters
|
|
|
|
|
Mobile Wireless Communications Today (cont.)
by Puneet Gupta
Global System for Mobile Communication (GSM)
GSM's air interface is based on narrowband TDMA technology, where available frequency bands are divided into time slots,
with each user having access to one time slot at regular intervals. Narrow band TDMA allows eight simultaneous communications
on a single 200Khz carrier and is designed to support 16 half-rate channels. The fundamental unit of time in this TDMA scheme is
called a burst period and it lasts 15/26 ms (or approx. 0.577 ms). Eight burst periods are grouped into a TDMA frame
(120/26 ms, or approx. 4.615 ms), which forms the basic unit for the definition of logical channels. One physical channel
is one burst period per TDMA frame. A GSM mobile can seamlessly roam nationally and internationally, which requires that registration,
authentication, call routing and location updating functions exist and be standardized in GSM networks.
GSM offers a variety of data services. GSM users can send and receive data, at rates up to 9600 bps, to users on POTS
(Plain Old Telephone Service), ISDN, Packet Switched Public Data Networks, and Circuit Switched Public Data Networks using a
variety of access methods and protocols, such as X.25 or X.32. Other data services include Group 3 facsimile, as described in
ITU-T recommendation T.30, which is supported by use of an appropriate fax adapter. A unique feature of GSM, not found in older
analog systems, is the Short Message Service (SMS). SMS is a bi-directional service for short alphanumeric (up to 160 bytes) messages.
Messages are transported in a store-and-forward fashion. For point-to-point SMS, a message can be sent to another subscriber
to the service, and an acknowledgment of receipt is provided to the sender. SMS can also be used in a cell-broadcast mode,
for sending messages such as traffic updates or news updates. Messages can also be stored in the SIM card for later retrieval.
The European version of GSM operates at the 900 MHz frequency (and now at the newer 1800 MHz frequency). Since the North American
version of GSM operates at the 1900 MHz frequency, the phones are not interoperable, but the SIMs are. Dual-band 900 -1800
and 900 -1900 phones are already released and in production. Tri-band 900 -1800 -1900 GSM phone are expected to be manufactured
in the next few years, which will allow interoperability between Europe and North America
A GSM network consists of mobile stations talking to the base transceiver station, on the Um interface. Many BTS are connected to a BSC
via the Abis interface and the BSC connect to the MSC (The core switching network) via the A interface.
HLR and VLR provide customized subscriber services and allow seamless movement from one cell to another. The Authentication
register and the equipment register provide security and authentication. An OMC and a cell broadcast center allow configuration
of the network and provide the cell broadcast service in the GSM network (not shown in the diagram).The voice transmitted on
the air interface can be encrypted. The speech is coded at 13kbps over the air interface. Using EFR (Enhanced Fullrate Coding)
the voice quality approaches the land line quality. Recent developments like AMR (adaptive multi-rate coding) allow speech
coding and channel coding to be dynamically adjusted giving acceptable performance even in case of bad radio conditions.
The GSM network supports automatic handovers. Since the mobiles are not transmitting or receiving at all times battery
consumption can be conserved. Further using DTX and DRX (Discontinuous transmission and reception, mobile transmits or receives
only when there is a voice activity detection) batter power can be conserved even more - a highly desirable characteristic
of any mobile system. Also since the mobile is not transmitting or receiving at all times, this allows the mobile to listen
to control channels and to provide useful information about other channels back to the cell.
Recent developments and initiatives include:
- GSM Association together with the Universal Wireless Communications Consortium (UWCC), which represents the interests of the TDMA community, are working towards inter-standard roaming between GSM and TDMA (ANSI-136) networks.
- The majority of European GSM operators plan to implement general packet radio system (GPRS) technology as their network evolution path to third-generation
- MExE will allow operators to provide customized, user-friendly interfaces to a host of services from GSM, through GPRS and eventually UMTS. The first implementations of MExE are expected to support the wireless application protocol (WAP) and Java applications. MExE can extend the capabilities that currently exist within WAP by enabling a more flexible user- interface, more powerful features and security.
- GSM cordless telephony system to provide a small home base station to work with a standard GSM mobile phone in similar mode to a cordless phone. The base station would be connected to the PSTN.
- Number portability will allow customers to retain their mobile numbers when they change operators or service providers
- Location services to standardize the methods for determining a GSM subscriber's physical location
- Tandem free operation where the compressed speech is passed unchanged over the 64 kbps links between the transcoders, hence improving the voice quality.
Next: Time Division Multiple Access (TDMA) IS-136
|
|
|
