<浼婅惤涓?gt; 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銆and interference. Each burst belonging to a particular physical channel will be transmitted銆on a different carrier frequency in each TDAM frame. Thus the hopping rate is equal to the銆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"
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<浼婅惤涓?gt; illidan.modeler [at] gmail.com
Northern Capital, Republic of Pandaren
System Architecture
UTRAN
UTRAN, short for UMTS Terrestrial Radio Access Network, is a collective term for the Node B's and Radio Network Controllers which make up the UMTS radio access network.
The UTRAN allows connectivity between the UE (user equipment) and the core network. The UTRAN contains the base stations, which are called Node Bs, and Radio Network Controllers (RNC). The RNC provides control functionalities for one or more Node Bs. A Node B and an RNC can be the same device, although typical implementations have a separate RNC located in a central office serving multiple Node Bs.
The RNC and its corresponding Node Bs are called the Radio Network Subsystem (RNS). There can be more than one RNS present in an UTRAN.
Radio Resource Control
RRC handles the control plane signalling of Layer 3 between the UEs and UTRAN.
The major part of the control signalling between UE and UTRAN is RRC messages. RRC messages carry all parameters required to set up, modify and release layer 2 and layer 1 protocol entities. RRC messages also carry in their payload all higher layer signalling (mobility management (MM), connection management (CM), session management (SM), etc.). The mobility of user equipment in the connected mode is controlled by RRC signalling (measurements, handovers, cell updates, etc.).
Literature
1. [web] "UMTS Terrestrial Radio Access Network." Wikipedia, The Free Encyclopedia. 20 Apr 2009, 09:51 UTC. 21 Apr 2009 <http://en.wikipedia.org/w/index.php?title=UMTS_Terrestrial_Radio_Access_Network&oldid=284982903>.
2. [book] [Holma 2007] Section 7.8 "The Radio Resource Control Protocol"
3. [web] "Radio Resource Control." Wikipedia, The Free Encyclopedia. 12 Feb 2009, 04:30 UTC. 5 May 2009 <http://en.wikipedia.org/w/index.php?title=Radio_Resource_Control&oldid=270157640>.
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Northern Capital, Republic of Pandaren
Of the Net, by the Net, for the Net
General packet radio service (GPRS) is a packet oriented mobile data service available to users of the 2G cellular communication systems global system for mobile communications (GSM), as well as in the 3G systems. In the 2G systems, GPRS provides data rates of 56-114 kbit/s.2G cellular systems combined with GPRS are often described as 2.5G. It provides moderate speed data transfer, by using unused time division multiple access (TDMA) channels.
GPRS data transfer is typically charged per megabyte of traffic transferred, while data communication via traditional circuit switching is billed per minute of connection time, independent of whether the user actually is using the capacity or is in an idle state. GPRS is a best-effort packet switched service, as opposed to circuit switching, where a certain quality of service (QoS) is guaranteed during the connection for non-mobile users.
Originally there was some thought to extend GPRS to cover other standards, but instead those networks are being converted to use the GSM standard, so that GSM is the only kind of network where GPRS is in use. GPRS is integrated into GSM Release 97 and newer releases. It was originally standardized by European Telecommunications Standards Institute (ETSI), but now by the 3rd Generation Partnership Project (3GPP).
GPRS was developed as a GSM response to the earlier CDPD and i-mode packet switched cellular technologies.
The diagram that depicts the interfaces and key components is shown below;
Interfaces in the GPRS network
GbInterface between the base station subsystem and the SGSN the transmission protocol could be Frame Relay or IP.
Gn
IP Based interface between SGSN and other SGSNs and (internal) GGSNs. DNS also shares this interface. Uses the GTP Protocol.
Gp
IP based interface between internal SGSN and external GGSNs. Between the SGSN and the external GGSN, there is the border gateway (which is essentially a firewall). Also uses the GTP Protocol.
Ga
The interface servers the CDRs (accounting records) which are written in the GSN and sent to the charging gateway (CG). This interface uses a GTP-based protocol, with modifications that supports CDRs (Called GTP' or GTP prime).
Gr
Interface between the SGSN and the HLR. Messages going through this interface uses the MAP3 protocol.
Gd
Interface between the SGSN and the SMS Gateway. Can use MAP1, MAP2 or MAP3.
Gs
Interface between the SGSN and the MSC (VLR). Uses the BSSAP+ protocol. This interface allows paging and station availability when it performs data transfer. When the station is attached to the GPRS network, the SGSN keeps track of which routing area (RA) the station is attached to. An RA is a part of a larger location area (LA). When a station is paged this information is used to conserve network resources. When the station performs a PDP context, the SGSN has the exact BTS the station is using.
Gi
IP based interface between the GGSN and a public data network (PDN) either directly to the Internet or through a WAP gateway.
Ge
The interface between the SGSN and the service control point (SCP); uses the CAP protocol.
Gx
The on-line policy interface between the GGSN and the charging rules function (CRF). It is used for provisioning service data flow based charging rules. Uses the diameter protocol.
Gy
The on-line charging interface between the GGSN and the online charging system (OCS). Uses the diameter protocol (DCCA application).
Gz
The off-line (CDR-based) charging interface between the GSN and the CG. Uses GTP'.
Gmb
The interface between the GGSN and the broadcast-multicast service center (BM-SC), used for controlling MBMS bearers..
Enhanced Data Rates for GSM Evolution (EDGE)
Transmission techniques
In addition to Gaussian minimum-shift keying (GMSK), EDGE uses higher-order PSK/8 phase shift keying (8PSK) for the upper five of its nine modulation and coding schemes. EDGE produces a 3-bit word for every change in carrier phase. This effectively triples the gross data rate offered by GSM. EDGE, like GPRS, uses a rate adaptation algorithm that adapts the modulation and coding scheme (MCS) according to the quality of the radio channel, and thus the bit rate and robustness of data transmission. It introduces a new technology not found in GPRS, Incremental Redundancy, which, instead of retransmitting disturbed packets, sends more redundancy information to be combined in the receiver. This increases the probability of correct decoding.
EDGE can carry data speeds up to 236.8 kbit/s (with end-to-end latency of less than 150 ms) for 4 timeslots (theoretical maximum is 473.6 kbit/s for 8 timeslots) in packet mode. This means it can handle four times as much traffic as standard GPRS. EDGE meets the International Telecommunications Union's requirement for a 3G network, and has been accepted by the ITU as part of the IMT-2000 family of 3G standards. It also enhances the circuit data mode called HSCSD, increasing the data rate of this service.
1. [Web] "GPRS Core Network." Wikipedia, The Free Encyclopedia. 25 Apr 2009, 11:47 UTC. 28 Apr 2009 <http://en.wikipedia.org/w/index.php?title=GPRS_Core_Network&oldid=286027496>.
2. [Web] "Enhanced Data Rates for GSM Evolution." Wikipedia, The Free Encyclopedia. 16 Apr 2009, 22:13 UTC. 17 Apr 2009 <http://en.wikipedia.org/w/index.php?title=Enhanced_Data_Rates_for_GSM_Evolution&oldid=284298553>.
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<浼婅惤涓?gt; illidan.modeler [at] gmail.com
Northern Capital, Republic of Pandaren
Of the Net, by the Net, for the Net
What
Fading is the distortion that a carrier-modulated telecommunication signal experiences over certain propagation media.
Propagation models that characterize the rapid fluctions of the received signal strength over very short travel distances (a few wavelengths) or short time duration (on the order of seconds) are called small-scale or fading models. (Page 106 of [3])
Why
In wireless systems, fading is due to multipath propagation and is sometimes referred to as multipath induced fading. [1]
How
The effects of fading can be combated by using diversity to transmit the signal over multiple channels that experience independent fading and coherently combining them at the receiver.
Miscellaneous
The coherence time is a measure of the minimum time required for the magnitude change of the channel to become decorrelated from its previous value.
There's a good summary of large-scale and small-scall fadings on Page 40 and 41 of [5].
References
- Web http://en.wikipedia.org/wiki/Fading
- Web http://www.ylesstech.com/terminology.php?letter=all&id=15
- [Rappaport 2001]
- [Goldsmith 2005]
- [Tse 2005]
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Functions:
Encoding/decoding of signals (PSK, QAM, etc)
Preamble generation/removal (for synchronization)
Bit transimission/reception
Sublayers:
Pysical Layer Convergence Procedure (PLCP)
Physical Medium Dependent (PMD) sublayer
PLCP
map the MAC frame onto the medium, straddle between MAC and PHY, add a number of fields to the frame as it's transmitted into the air
PMD
transimit the frame
[References]
[Gast 2005]
[Stalling 2004]
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