GSM standard designed with time division and multistation access (TDMA).
The periodic sequence in TDMA-structure called of the hyper-still.
He shares on 2048 super-stills. super-still will consist of the multi-stills who
will consist actually of the stills. The TDMA-still shares on eight time
positions, called time-slots, or windows. What physical sense of the
time-slot? It anything other as time during which carrier frequency is modulated
by the digital information stream corresponding to the transmitted information
- voice or data.|
In classical GSM the mobile terminal has the right to occupy one time-slot in the TDMA-still. Therefore the link speed of the data in networks GSM is limited.
Otherwise at use of GPRS technology. The channel capacity given on demand at which physical channels of cell are appointed dynamically for use in switching of channels and packet switching mode.
Here, depending on a situation, for data transfer the base station allocates to the mobile terminal in the staff any number of time-slots: from 0 up to 8. And asymmetric appointment of resources is allowable. That is during batch transfer resources of the communication line <upwards> and <downwards> are set independently. Here <upwards> - from the mobile terminal to base station, <downwards> - from base station to the mobile terminal. For example, <upwards> - three time-slots, <downwards> - two.
Naturally, any link works in nonideal conditions, therefore GSM use superfluous coding. Moreover, in GPRS technology 4 schenas of data coding are possible: from CS1 (for work in conditions of strong noise) up to CS4 (for work at the minimal noise). Thus, at constant speed in the channel 22,8 Kb/sec the quantity of the transmitted helpful information depends on operating conditions MT and BS. It is obvious, that the worse conditions of noise, the less helpful information can pass in unit of time. Major importances for schemas of coding are resulted in table 1.
As, more than one channel are used for download, mobile phones with GPRS have the big link speeds of the data.
There are some types of phones concerning number of channels which they use for data transfer
Type 2+1 - two downlink channel and one uplink channel in the data link
Type 3+1 - three downlink channel and one uplink channel in the data link
Type 4+1 - four downlink channel and one uplink channel in the data link, etc
GPRS-enabled mobile phones can be classified on three classes in an opportunity of simultaneous queries (through GSM) and data transfer (through GPRS).
Class A - the terminal allows to carry out simultaneously transfer of voice and data in GPRS-mode. Terminals simultaneously carry out the signaling and management for GSM and GPRS. In case of incoming data and by an incoming call of phone of class A will support simultaneous job with a voice and the data.
Class B - the terminal supports voice connection and data transfer in a batch mode (GPRS), but these modes are used not simultaneously (during data transfer through GPRS the subscriber cannot make and accept voice calls and vice versa). During each moment of time traffic GSM or GPRS can be passed only. If during loading the information from the Internet (GPRS-mode) phone accepts a call for voice connection, and interrupt data transfer. At end of voice connection loading of the data renews, as logic connection between GPRS-network and phone is saved. The choice of a mode GPRS/speech is carried out automatically, with a priority of the last.
Class C - the terminal provides only data transfer in a batch mode or a voice. The choice of type (GPRS/speech) is carried out manually. For example, if MT serves data transfer, voice connection is impossible.
Speed of data transfer depends on featuring of the mobile terminal, such as quantity of the supporting channels of data transfer. Thus one channel supports transfer of the information with the maximal speed 13.4 Kb/sec.
GPRS networks support four schemas of coding|
The choice of schema of coding depends on a status of the channel provided with a network (quality of communication between phone and station). If in the channel it is a lot of noises, or he is overrided, the network can use CS-1 to guarantee higher reliability in this case data transfer - only 9.05 Kb/sec in the time-slot. If the channel provides a good status, the network could use CS-3 or CS-4 to receive optimum speed, and will then have up to 21.4 Kb/sec in the time-slot.
|GPRS link speed dependence on signal/noise ratio|
Work with schemas CS-4/CS-3 demands upgrade of the equipment and software
of BS while for CS-2/CS-1 it is enough to modernize only BS software. For example,
in Russia operators use CS-2 schema. Real maximal link speed of data transfer.
It makes 107.2 Kb/sec (8*13,4 Kb/sec).|
Depending on intensity of the voice traffic and resources of base station (its bandwidth) operators allocate under data transfer 1 - 2 time-slots in <rush hours>, and 2 - 4 time-slots in the rest of the time. Therefore today speed 52 Kb/sec is considered limiting for GPRS-technology. Otherwise operators are compelled to increase resources base stations and infrastructures, reserving a part of time-slots under data transfer.
The operator can assign to voice connection parameter <utmost> (best effort), providing automatic liberation of the time intervals preserved for GPRS, - as soon as these resources are required to subscribers for voice connection. It is obvious, that thus there will be a delay in data transfer.
The mobile phones issued today, basically support modes CS-2/CS-1.
The time-slots number selected under transfer of the information, in the TDMA-still can change from 1 up to 8. And value has number of time-slots both for transfer <upwards<, and for transfer <downwards>. Therefore in GPRS-specifications the second class determining has appeared, how much times - slots in the staff are accessible to transfer <upwards>, and how much <downwards> (table 2). In characteristics of phones manufacturers specify either a class, or concrete values, for example 4/1.
The maximal number of selected time-slots not always corresponds
to the arithmetic sum <upwards> + <downwards>. For example,
the class 6 is a maximum 4 time-slots under batch data transfer.|
Technology GPRS while remains far from high-grade realization. Today operators suggest so-called truncated GPRS - that the question is only phone of a class B, and with an opportunity work in a mode 4/1 or 4/2.
High-grade GPRS from 8 time-slots for information interchange in the near future it will be inaccessible.
Source - Amobile.ru