The Network Switching Subsystem Computer Science Essay

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The global system for mobile communication (GSM) is a worldwide accepted standard for digital cellular systems today. It is based on the time division multiple access (TDMA) technology. It is a second generation (2G) cellular standard which was developed to offer voice and data services using digital modulation. GSM was first developed by the Group Spéciale Mobile under the regulation of CEPT (Conference of European Post and Telecommunication). The first commercial GSM system was installed in 1991 in Europe. The basic aim of the GSM technology was to replace the incompatible analog systems. Today there are hundreds of millions of customers using this technology in their everyday life. The GSM architecture is comprised of many different sub-systems and network elements. Though Radio network planning only involves the base station (BTS) and the mobile station (MS) as a part of network and the interface between them but it is necessary to get a brief overview of the whole GSM architecture and its different components in order to have a clear understanding

2.2.2 Network switching subsystem

The network switching subsystem (NSS) contains the network elements MSC, VLR, HLR, AC and EIR.

A network switching subsystem

The main functions of NSS are:

Call control: This identifies the subscriber, establishes a call, and clears the connection after conversation is over.

Charging: This collects the charging information about a call (the numbers of the caller and the called subscriber, the time and type of the transaction, etc.) and transfers it to the billing centre.

Mobility management: This maintains information about the subscriber's location.

Signaling: This applies to interfaces with the BSS and PSTN.

Subscriber data handling: This is the permanent data storage in the HLR and temporary storage of relevant data in the VLR.

2.2.3 Mobile switching Centre (MSC)

The MSC is responsible for controlling calls in the mobile network. It identifies the origin and destination of a call (mobile station or fixed telephone), as well as the type of a call. An MSC acting as a bridge between a mobile network and a fixed network is called a Gateway MSC. The MSC is responsible for several important tasks, such as the following.

Call control: MSC identifies the type of call, the destination, and the origin of a call. It also sets up, supervises, and clears connections.

Initiation of paging: Paging is the process of locating a particular mobile station in case of a mobile terminated call (a call to a mobile station).

Charging data collection: It is also a function of MSC.

2.2.4 Visitor Location register (VLR)

Visitor Location Register (VLR) is integrated with the MSC. VLR is a database which contains information about subscribers currently being in the service area of the MSC/VLR, such as:

• Identification numbers of the subscribers

• Security information for authentication of the SIM card and for ciphering

• Services that the subscriber can use

The VLR carries out location registrations and updates. It means that when a mobile station comes to a new MSC/VLR serving area, it must register itself in the VLR, in other words perform a location update. Please note that a mobile subscriber must always be registered in a VLR in order to use the services of the network. Also the mobile stations located in the own network is always registered in a VLR.

The VLR database is temporary, in the sense that the data is held as long as the subscriber is within its service area. It also contains the address to every subscriber's Home Location Register.

2.2.5 Home Location Register (HLR)

HLR maintains a permanent register of the subscribers, for instance subscriber identity numbers and the subscribed services. In addition to the fixed data, the HLR also keeps track of the current location of its customers. As you will see later, the MSC asks for routing information from the HLR if a call is to be set up to a mobile station (mobile terminated call). In the Nokia implementation, the two network elements, Authentication Centre (AC) and Equipment Identity Register (EIR), are located in the HLR.

2.2.6 Authentication Centre (AuC)

The Authentication Centre provides security information to the network, so that we can verify the SIM cards (authentication between the mobile station and the VLR, and cipher the information transmitted in the air interface (between the MS and the Base Transceiver Station). The Authentication Centre supports the VLR's work by issuing so-called authentication triplets upon request.

2.2.7 Equipment Identity Register (EIR):

As for AuC, the Equipment Identity Register is used for security reasons. But while the AuC provides information for verifying the SIM cards, the EIR is responsible for IMEI checking (checking the validity of the mobile equipment).

When performed, the mobile station is requested to provide the International Mobile Equipment Identity (IMEI) number. This number consists of type approval code, final assembly code and serial number of the mobile station.

The EIR contains three lists:

• Mobile equipment in the white list is allowed to operate normally.

• If we suspect that mobile equipment is faulty, we can monitor the use of it. It is then placed in the grey list.

• If the mobile equipment is reported stolen, or it is otherwise not allowed to operate in the network, it is placed in the black list.

2.2.8 Base station subsystem (BSS)

The Base Station Subsystem is responsible for managing the radio network, and it is controlled by an MSC. Typically, one MSC contains several BSSs. A BSS itself may cover a considerably large geographical area consisting of many cells (a cell refers to an area covered by one or more frequency resources). The BSS consists of the following elements:

• BSC (Base Station Controller)

• BTS (Base Transceiver Station)

• TRAU (Transcoder and Rate Adaption Unit)

The Base Station Subsystem (BSS)

Some of the most important BSS tasks are listed in the following:

Radio path control: In the GSM network, the Base Station Subsystem (BSS) is the part of the network taking care of radio resources, that is, radio channel allocation and quality of the radio connection.

Synchronization: The BSS uses hierarchical synchronization, which means that the MSC synchronizes the BSC, and the BSC further synchronizes the BTSs associated with that particular BSC. Inside the BSS, synchronization is controlled by the BSC. Synchronization is a critical issue in the GSM network due to the nature of the information transferred. If the synchronization chain is not working correctly, calls may be cut or the call quality may not be the best possible. Ultimately, it may even be impossible to establish a call.

Air- and A-interface signaling: In order to establish a call, the MS must have a connection through these interfaces to the BSS.

Connection establishment between the MS and the NSS: The BSS is located between two interfaces, the air- and the A-interface. The MS must have a connection through these two interfaces before a call can be established. Generally speaking, this connection may be either a signaling connection or a traffic (speech, data) connection.

Mobility management and speech transcoding: BSS mobility management mainly covers the different cases of handovers. These handovers and speech transcoding are explained in later sections.

2.2.9 Base Station Controller (BSC)

The BSC is the central network element of the BSS and it controls the radio network. It has several important tasks, some of which are presented in the following

Connection establishment between the MS and the NSS: All calls to and from the MS are connected through the group switch of the BSC (GSWB).

Mobility management: The BSC is responsible for initiating the vast majority of all handovers, and it makes the handover decision based on, among others, measurement reports sent by the MS during a call.

Statistical raw data collection: Information from the Base Transceiver Stations, Transcoders, and BSC are collected in the BSC and forwarded via the DCN (Data Communications Network) to the NMS (Network Management Subsystem), where they are post-processed into statistical views, from which the network quality and status is obtained.

Air- and A-interface signaling support: In the A-interface, SS#7 (Common Channel Signaling System No. 7) is used as the signaling language, while the environment in the air interface allows the usage of a protocol adapted from ISDN standards, namely LAP-Dm (Link Access Protocol on the ISDN D Channel, modified version). Between the Base Transceiver Station and the BSC (Abis interface), a more standardized LAP-D protocol is used. The BCSU (Base Station Controller Signaling Unit) in the BSC will therefore need to convert from LAP-D to SS#7 and vice versa in the uplink/downlink directions. The BSC also enables the virtual signaling connection needed between the MSC/VLR and the MS.

2.2.10 Base transceiver station

The BTS is the network element responsible for maintaining the air interface and minimizing the transmission problems (the air interface is very sensitive for disturbances. The BTS parameters handle the following major items: what kind of handovers (when and why), paging organization, radio power level control, and BTS identification.

The BTS has several very important tasks, some of which are presented in the following:

Air interface signaling: A lot of both call and non-call related signaling must be performed in order for the system to work. One example is that when the MS is switched on for the very first time, it needs to send and receive a lot of information with the network (more precisely with the VLR) before we can start to receive and make phone calls. Another example is the signaling required for setting up both mobile originated and mobile terminated calls. A third very important signaling in mobile networks is the need to inform the MS when a handover is to be performed and later when the MS sends a message in the uplink direction telling the network that the handover is completed.

BTS and TC control: Inside the BSS, all the BTSs and TCs are connected to the BSC(s). The BSC maintains the BTSs. In other words, the BSC is capable of separating (barring) a BTS from the network and collecting alarm information. Transcoders are also maintained by the BSC, that is, the BSC collects alarms related to the transcoders.

Ciphering: Both the BTS and the MS must be able to cipher and decipher information in order to protect the transmitted speech and data in the air interface.

Speech processing: Speech processing refers to all the functions the BTS performs in order to guarantee an error-free connection between the MS and the BTS. This includes tasks like speech coding (digital to analogue in the downlink direction and vice versa), channel coding (for error protection), interleaving (to enable a secure transmission), and burst formatting (adding information to the coded speech / data in order to achieve a well-organized and safe transmission).

2.2.11 TRAU (Transcoder and Rate ADAPTION UNIT)

In the air interface (between MS and BTS), the media carrying the traffic is a radio frequency. To enable an efficient transmission of digital speech information over the air interface, the digital speech signal is compressed. We must however also be able to communicate with and through the fixed network, where the speech compression format is different. Somewhere between the BTS and the fixed network, we therefore have to convert from one speech compression format to another, and this is where the transcoder comes in.

For transmission over the air interface, the speech signal is compressed by the mobile station to 13 Kbits/s (Full Rate and Enhanced Full Rate) or 5.6 Kbits/s (Half Rate). The compression algorithm for Full Rate is known as "Regular Pulse Excitation with Long Term Prediction" (RPE-LTP). For Enhanced Full Rate, speech coding is based on the algorithm "Algebraic Code Excited Linear Prediction" (ACELP). "Vector Sum Excited Linear Prediction" (VSELP) is used in the case of Half Rate. However, the standard bit rate for speech in the PSTN is 64 Kbits/s. The modulation technique is called "Pulse Code Modulation" (PCM).

Location of TRAU

2.2.12 Operation and Management system

The Operation Management Subsystem (OMS) is the third subsystem of the GSM network in addition to the Network Switching Subsystem (NSS) and Base Station Subsystem (BSS). The purpose of the OMS is to monitor various functions and elements of the network. It consists of a number of workstations, servers, and a router, which connects to a Data Communications Network (DCN).

The OMS and the GSM network

GSM CHANNEL STRUCTURE

2.3.1 TYPES OF GSM CHANNELS

The GSM channels are divided in physical and logical channels.

2.3.1.1 Physical Channels

They are physical resource available for use. A physical channel designates a particular RFC (Radio Frequency Channel) and timeslot. There are eight physical channels per RFC. Using FDMA and TDMA techniques, each carrier is divided into 8 time slots:

One burst of data (0.577 msec or 156.25 bit Period) is a physical channel. This is used via multi-frame structures to provide all the logical channels required.

2.3.1.2 Logical Channels

They are the various ways we use the resource. One physical channel may support many logical channels. There are two main groups of logical channels, traffic channels and GSM control channels.

2.3.1.3 Traffic Channels (TCH)

The traffic channel carries speech or data information. The different types of traffic channel are listed below:

Full rate:

TCH/FS: Speech (13 Kbit/s net, 22.8 Kbit/s gross)

TCH/EFR: Speech (12.2 Kbit/s net, 22.8 Kbit/s gross)

TCH/F9.6: 9.6 Kbit/s - data

TCH/F4.8: 4.8 Kbit/s - data

TCH/F2.4 2.4 Kbit/s - data

Half rate:

TCH/HS: Speech (6.5 Kbit/s net, 11.4 Kbit/s gross)

TCH/H4.8 4.8 Kbit/s - data

TCH/H2.4 2.4 Kbit/s - data

Acronyms:

TCH Traffic Channel

TCH/FS Full rate speech channel

TCH/EFR Enhanced full rate speech

TCH/HS Half rate speech channel

TCH/9.6 Data channel 9.6 Kbit/s

TCH/4.8 Data channel 4.8 Kbit/s

TCH/2.4 Data channel 2.4 Kbit/s

GSM Traffic channels

Speech channels: Speech channels are supported by two different methods of coding known as Full Rate (FR) and Enhanced Full Rate (EFR). Enhanced Full Rate coding provides a speech service that has improved voice quality from the original Full Rate speech coding, whilst using the same air interface bandwidth. EFR employs a new speech coding algorithm and additions to the full rate channel coding algorithm to accomplish improved speech service; however, it will only be supported by Phase 2+ (mobile network integration with intelligent network (IN) mobiles onwards. It produces 12.2 Kbit/s from each 64 Kbit/s PCM channel.

2.3.1.4 GSM Control Channels

These include:

• Broadcast Control Channel (BCCH)

• Common Control Channel (CCCH)

• Dedicated Control Channel (DCCH).

BCCH Group: The Broadcast Control Channels are downlink only (BSS to MS) and comprise the following:

• BCCH carries information about the network, a MS's present cell and the surrounding cells. It is transmitted continuously as its signal strength is measured by all MS's on surrounding cells.

• The Synchronizing Channel (SCH) carries information for frame synchronization.

• The Frequency Control Channel (FCCH) provides information for carrier synchronization.

CCCH Group: The Common Control Channel Group works in both uplink and downlink directions. It includes:

• Random Access Channel (RACH) is used by MSs to gain access to the system.

• Paging Channel (PCH) and Access Granted Channel (AGCH) operate in the "downlink" direction. The AGCH is used to assign resources to the MS, such as a Stand-alone Dedicated Control Channel (SDCCH). The PCH is used by the system to call a MS. The PCH and AGCH are never used at the same time.

• Cell Broadcast Channel (CBCH) is used to transmit messages to be broadcast to all MSs within a cell, for example, road traffic information, sporting results.

DCCH Group: Dedicated Control Channels are assigned to a single MS for call setup and subscriber validation. DCCH comprises:

• Stand-alone Dedicated Control Channel (SDCCH), which supports the transfer of data to and from the MS during call setup and validation.

• Associated Control Channel consists of Slow ACCH which is used for radio link measurement and power control messages. Fast ACCH is used to pass "event" type messages, for example, handover messages. Both FACCH and SACCH operate in uplink and downlink directions.

GSM Control channels

2.3.1.5 Broadcast control channel (BCCH)

The Broadcast Control Channel is transmitted by the BTS at all times. The RF carrier used to transmit the BCCH is referred to as the BCCH carrier. The information carried on the BCCH is monitored by the MS periodically (at least every 30 seconds), when it is switched on and not in a call.

Broadcast Control Channel (BCCH) carries the following information:

• Location Area Identity (LAI).

• List of neighboring cells which should be monitored by the MS.

• List of frequencies used in the cell.

• Cell identity.

• Power control indicator.

• DTX (Discontinuous Transmission) permitted.

• Access control (for example, emergency calls, call barring).

• CBCH description.

The BCCH is transmitted at constant power at all times, and its signal strength is measured by all MS which may seek to use it. "Dummy" bursts are transmitted to ensure continuity when there is no BCCH carrier traffic.

Frequency Correction Channel (FCCH): This is transmitted frequently on the BCCH timeslot and allows the mobile to synchronize its own frequency to that of the transmitting base site. The FCCH may only be sent during timeslot 0 on the BCCH carrier frequency and therefore it acts as a flag to the mobile to identify Timeslot 0.

Synchronization Channel (SCH): The SCH carries the information to enable the MS to synchronize to the TDMA frame structure and know the timing of the individual timeslots. The following parameters are sent:

• Frame number.

• Base Site Identity Code (BSIC).

The MS will monitor BCCH information from surrounding cells and store the information from the best six cells. The SCH information on these cells is also stored so that the MS may quickly resynchronize when it enters a new cell.

Broadcast Control Channel (BCCH)

2.3.1.6 Common Control Channels (CCCH)

The Common Control Channel (CCCH) is responsible for transferring control information between all mobiles and the BTS. This is necessary for the implementation of "call origination" and "call paging" functions. It consists of the following:

Random Access Channel (RACH): Used by the mobile when it requires to gain access to the system. This occurs when the mobile initiates a call or responds to a page.

Paging Channel (PCH): Used by the BTS to page MS, (paging can be performed by an IMSI, TMSI or IMEI).

Access Grant Control Channel (AGCH): Used by the BTS to assign a dedicated control channel to a MS in response to an access message received on the Random Access Channel. The MS will move to the dedicated channel in order to proceed with either a call setup, response to a paging message, Location Area Update or Short Message Service.

Cell Broadcast Channel (CBCH): This channel is used to transmit messages to be broadcasted; to all MSs within a cell. The CBCH uses a dedicated control channel to send its messages, however it is considered a common channel because the messages can be received by all mobiles in the cell.

Active MSs must frequently monitor both BCCH and CCCH. The CCCH will be transmitted on the RF carrier with the BCCH.

Common Controls Channel (CCCH)

2.3.1.7 Dedicated Control Channels (DCCH)

The DCCH is a single timeslot on an RF carrier which is used to convey eight, Stand-alone Dedicated Control Channels (SDCCH). A SDCCH is used by a single MS for call setup, authentication, location updating and SMS point to point.

2.3.1.8 Associated Control Channels (ACCH):

These channels can be associated with either an SDCCH or a TCH. They are used for carrying information associated with the process being carried out on either the SDCCH or the TCH.

Slow Associated Control Channel (SACCH): Conveys power control and timing information in the downlink direction (towards the MS) and Receive Signal Strength Indicator (RSSI), and link quality reports in the uplink direction.

Fast Associated Control Channel (FACCH): The FACCH is transmitted instead of a TCH. The FACCH ''steals" the TCH burst and inserts its own information. The FACCH is used to carry out user authentication, handovers and immediate assignment.

All of the control channels are required for system operation, however, in the same way that we allow different users to share the radio channel by using different timeslots to carry the conversation data, the control channels share timeslots on the radio channel at different times. This allows efficient passing of control information without wasting capacity which could be used for call traffic. To do this we must organize the timeslots between those which will be used for traffic and those which will carry control signaling.

Dedicated Control Channels (DCCH)

2.2.1.9 Channel Combinations

The different logical channel types mentioned are grouped into what are called channel combinations. The four most common channel combinations are listed below:

• Full Rate Traffic Channel Combination - TCH8/FACCH + SACCH

• Broadcast Channel Combination - BCCH + CCCH

• Dedicated Channel Combination - SDCCH8 + SACCH8

• Combined Channel Combination - BCCH+CCCH+SDCCH4+SACCH4

The Half Rate Channel Combination is very similar to the Full Rate Traffic Combination.

• Half Rate Traffic Channel Combination - TCH16/FACCH + SACCH

2.2.1.10 Channel Combinations and Timeslots

The channel combinations we have identified are sent over the air interface in a selected timeslot. Some channel combinations may be sent on any timeslot, but others must be sent on specific timeslots. Below is a table mapping the channels combinations to their respective time-slots:

If broadcast is assigned to timeslots 2, 4 or 6 then FCCH and SCH will be replaced with dummy bursts since these control channels may only occur on timeslot 0.

Note that only one BCCH/CCCH timeslot is required per cell (not RF carrier).

2.4 GSM Multiple Access Scheme

The GSM Multiple Access Scheme defines how the GSM radio frequency can be shared for different simultaneous communications between different mobile stations located in different cells. GSM uses a mix of Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) combined with frequency hopping for its Multiple Access Scheme. Each user is given a pair of frequencies (one for uplink and one for downlink) and a time slot during a time frame. The time frame provides the basic unit of logical channels. The unit of time in TDMA is called a burst. Each user is assigned its own burst, within a collection of bursts, called a frame.

TDMA with use of frequency hopping technique

2.4.1 GSM Bursts

The diagram below illustrates a GSM burst:

GSM Burst and TDMA Frame

It consists of several different elements. These elements are described below:

Info: This is the area in which the speech, data or control information is held.

Guard Period: The BTS and MS can only receive the burst and decode it, if it is received within the timeslot designated for it. The timing, therefore, must be extremely accurate, but the structure does allow for a small margin of error by incorporating a 'guard period' as shown in the diagram. To be precise, the timeslot is 0.577 ms long, whereas the burst is only 0.546 ms long therefore there is a time difference of 0.031 ms to enable the burst to hit the timeslot.

Stealing Flags: These two bits are set when a traffic channel burst has been ''stolen" by a FACCH (The Fast Associated Control Channel). One bit set indicates that half of the block has been stolen.

Training Sequence: This is used by the receiver's equalizer as it estimates the transfer characteristic of the physical path between the BTS and the MS. The training sequence is 26 bits long.

Tail Bits: These are used to indicate the beginning and end of the burst.

2.4.1.1 GSM Burst types

The burst is the sequence of bits transmitted by the BTS or the MS. The timeslot is the discrete period of real time within which it must arrive in order to be correctly decoded by the receiver.

The diagram below shows the five types of burst employed in the GSM air interface. All bursts, of whatever type, have to be timed so that they are received within the appropriate timeslot of the TDMA frame.

GSM Burst types

Normal Burst: The normal burst carries traffic channels and all types of control.

Frequency Correction Burst: This burst carries FCCH downlink to correct the frequency of the MS's local oscillator, effectively locking it to that of the BTS.

Synchronization Burst: It is so called because its function is to carry SCH downlink, synchronizing the timing of the MS to that of the BTS.

Dummy Burst: Used when there is no information to be carried on the unused timeslots of the BCCH Carrier (downlink only).

Access Burst: This burst is of much shorter duration than the other types. The increased guard period is necessary because the timing of its transmission is unknown. When this burst is transmitted, the BTS does not know the location of the MS and therefore the timing of the message from the MS cannot be accurately accounted for. (The Access Burst is uplink only.)

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