GSM概述
<伊落丹> illidan.modeler [at] gmail.com
Northern Capital, Republic of Pandaren
Of the Net, by the Net, for the Net
History
In 1982, the European Conference of Postal and Telecommunications Administrations (CEPT) created the Groupe Spécial Mobile (GSM) to develop a standard for a mobile telephone system that could be used across Europe.
In 1987, a memorandum of understanding was signed by 13 countries to develop a common cellular telephone system across Europe. Finally the system created by SINTEF lead by Torleiv Maseng was selected.
In 1989, GSM responsibility was transferred to the European Telecommunications Standards Institute (ETSI) and phase I of the GSM specifications were published in 1990. The first GSM network was launched in 1991 by Radiolinja in Finland with joint technical infrastructure maintenance from Ericsson.
By the end of 1993, over a million subscribers were using GSM phone networks being operated by 70 carriers across 48 countries.
Network structure
The network behind the GSM seen by the customer is large and complicated in order to provide all of the services which are required. It is divided into a number of sections and these are each covered in separate articles.
- the Base Station Subsystem (the base stations and their controllers).
- the Network and Switching Subsystem (the part of the network most similar to a fixed network). This is sometimes also just called the core network.
- the GPRS Core Network (the optional part which allows packet based Internet connections).
- all of the elements in the system combine to produce many GSM services such as voice calls and SMS.
Packet control unit
The packet control unit (PCU) is a late addition to the GSM standard. It performs some of the processing tasks of the BSC, but for packet data. The allocation of channels between voice and data is controlled by the base station, but once a channel is allocated to the PCU, the PCU takes full control over that channel.
The PCU can be built into the base station, built into the BSC or even, in some proposed architectures, it can be at the SGSN site. In most of the cases, the PCU is a separate node communicating extensively with the BSC on the radio side and the SGSN on the Gb side.
Physical and Logical Channels
Traffic Channels (TCHs)
Full-Rate TCH
Full-Rate Speech Channel (TCH/FS) : Carries speech digitized at a raw data rate of 13 kbps, sent at 22.8 Kbps.
Full-Rate Data Channel for 9600 bps (TCH/F9.6) : Carries data sent at 9.6 Kbps. With FEC code, the data is sent at 22.8 Kbps.
Full-Rate Data Channel for 4800 bps (TCH/F4.8) : Carries data sent at 4.8 Kbps. With FEC code, the data is sent at 22.8 Kbps.
Full-Rate Data Channel for 2400 bps (TCH/F2.4) : Carries data sent at 2.4 Kbps. With FEC code, the data is sent at 22.8 Kbps.
Half-Rate TCH
Half-Rate Speech Channel (TCH/HS) : Carries speech digitized at 6.5 Kbps, sent at 11.4 Kbps.
Half-Rate Data Channel for 4800 bps (TCH/H4.8) : Carries data sent at 4.8 Kbps. With FEC code, the data is sent at 11.4 Kbps.
Full-Rate Data Channel for 2400 bps (TCH/H2.4) : Carries data sent at 2.4 Kbps. With FEC code, the data is sent at 11.4 Kbps.
(For more details about FEC channel coding, turn to [7].)
Control Channels (CCHs)
Broadcast Channels (BCHs)
Broadcast Control Channel (BCCH) - DOWNLINK -
Frequency Correction Channel (FCCH) - DOWNLINK -
Synchronization Channel (SCH) - DOWNLINK -
Common Control Channels (CCCHs)
Paging Channel (PCH) - DOWNLINK -
Random Access Channel (RACH) - UPLINK -
Access Grant Channel (AGCH) - DOWNLINK -
Dedicated Control Channels (DCCHs)
Stand-alone Dedicated Controls (SDCCHs) - UPLINK/DOWNLINK -
Slow Associated Control Channel (SACCH) - UPLINK/DOWNLINK -
Fast Associated Control Channel (FACCHs) - UPLINK/DOWNLINK -
BCHs
- BCCH: This channel contains system parameters needed to identify the network and gain access. These paramters include the Location Area Code (LAC), the Mobile Network Code (MNC), the frequencies of neighboring cells, and access parameters.
- FCCH: This channel is used by the MS as a frequency reference. This channel contains frequency correction bursts.
- SCH: This channel is used by the MS to learn the Base Station Information Code (BSIC) as well as the TDMA frame number (FN). This lets the MS know what TDMA frame they are on within the hyperframe.
CCCHs
- PCH: This channel is used to inform the MS that it has incoming traffic. The traffic could be a voice call, SMS, or some other form of traffic.
- RACH: This channel is used by a MS to request an initial dedicated channel from the BTS. This would be the first transmission made by a MS to access the network and request radio resources. The MS sends an Access Burst on this channel in order to request access
- AGCH: This channel is used by a BTS to notify the MS of the assignement of an initial SDCCH for initial signaling.
DCCHs
- SDCCH: This channel is used for signalling and call setup between the MS and the BTS.
- SACCH: This channel is a continuous stream channel that is used for control and supervisory signals associated with the traffic channels.
- FACCH: This channel is used for control requirements such as handoffs. There is no TS and frame allocation dedicated to a FAACH. The FAACH is a burst-stealing channel, it steals a Timeslot from a TCH.
Frame Structure
For a frame for traffic channe, a super frame consists of 51 multiframe that is made of 26 TDMA frames. For a frame for control channel, a super frame consists of 26 multiframe that contains 51 TDMA frames. Each TDMA frame spans 4.615 ms, consisting of 8 time slots, during each of which a user sends data called "burst". Of a normal burst, the payload (information-bearing part) occupies two 57 bit blocks.
Data Rates
The gross data rate is 32500bits/120ms = 270.83 kbit/s, resulting in 270.83/8 = 33.854 kbit/s per user. User data is actually sent at 24.7 kbit/s (57 bits * 2 / 4.615ms), excluding the overhead in the burst.
Slow Frequency Hopping
GSM employs slow frequency hopping (SFH) to mitigate the effects of multipath fading and interference. Each burst belonging to a particular physical channel will be transmitted on a different carrier frequency in each TDAM frame. Thus the hopping rate is equal to the frame rate (i.e.' 217 frames/s). The only physical channels that are not allowed to hop are the broadcast and common control channels (i.e. the FCH, SCH, BCCH, PCH and AGCH).The effect of frequency hopping on interference
In a non-frequency hopping GSM system, an MS will tend to experience interference from the same set of MSs in neighbouring co-channel cells. In a frequency hopped system, the hopping patterns (i.e. the sequence of transmission frequencies) are different in co-channel cells and the MS will experience interference from a different set of MSs on each burst. This effectively randomises the interference and each MS will experience an average level of interference.
Literature
1. [web] "GSM." Wikipedia, The Free Encyclopedia. 16 Apr 2009, 16:20 UTC. 18 Apr 2009 < http://en.wikipedia.org/w/index.php?title=GSM&oldid=284232567>
2. [web] "Base Station subsystem." Wikipedia, The Free Encyclopedia. 30 Mar 2009, 19:49 UTC. 20 Apr 2009 < http://en.wikipedia.org/w/index.php?title=Base_Station_subsystem&oldid=280712891>
3. [book] [Rappaport 2001] Section 11.3 "Global System for Mobile"
4. [book] [Tanenbaum 2004] Sec. 2.6.2 "Second-Generation Mobile Phones: Digital Voice"
5. [web] "GSM Network Architecture". 29 Apr 2009. < http://www.gsmfordummies.com/architecture/arch.shtml >
6. [web] "Logical Channels". 29 Apr 2009. < http://www.gsmfordummies.com/tdma/logical.shtml >
7. [book] [Steele 2001] Sec. 2.3.9 "Speech transmission"