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Agenda

Topic 2: Case Study: The GSM Network

1 GSM System General Architecture

2 GSM Access network.

3 Traffic Models for the Air interface

4 Models for the BSS design.

5

UMTS and the path towards 4G6 UMTS cell design


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Study Case for GSM

A very important point in the study of a GSM system isthe the network planning which is divided into RadioNetwork Planning,Access Network Planning and Core

Network planning.

However there is another important point which is toknow about how the network works.

We will focus on:

InterfacesTraffic CasesCore Network Configurations


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GSM Case StudyUM Interface: Radio

The Um interface is the interface between the MS and theBTS. Here the communication is carried out using radiowaves.

The frequencies band for GSM are shown in the figure.


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As you remember in the GSM system the BW is 200 KHzthen we have:

124 pairs of carriers in the GSM 900 band374 pairs of carriers in the GSM 1800 bandpairs of carriers in the GSM 1900 band

Each carrier is divided into eight Times Slots (TS).

A TS has a duration of 3/5200 seconds (577 ms).

Eight TSs form a TDMA frame, with approximately 4.62

ms duration.

Note: We are in a synchronous system, so the frames hasto be aligned in the uplink and in the downlink;


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The following figure shows the Um frame.

This structure, and the synchronous feature of the GSMsystem makes that if the mobile wants to use the sametime slot for the uplink and the downlink it has totransmit and to receive simultaneously.

To avoid this, the uplink frame is delayed three time slots(TS)


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GSM Case StudyUm Interface: Radio


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GSM Case StudyFrames Structure

I think that in this point is important to make a break tostudy the GSM Frame Structure

As you can suppose the synchronous feature of the systemdoes not end in the basic frame.

At higher levels the frames are grouped into SignallingMultiframes (MF51) or Traffic Multiframes (MF26)depending on the content of the time slots.

26 MF51 or 51 MF26 compound a Superframe.

2048 Superframes form a Hiperframe which is the

maximum period.

Note that a Hiperframe has 20485126 = 2715648frames, which means 1566 s.

This structure is performed in this way to handle with the

different speed of the channels.


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GSM Case StudyUm Interface: Radio


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GSM Case StudyUm Interface: Logical Channels

The Radio Interface between the MS and the BTS isstructured using logical channels.

The figure shows the division of the channels.


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GSM Case StudyUm Interface: Logical Channels

Logical channels can be separated into two categories:Traffic and signaling/control channels.

There are two forms of Traffic CHannels (TCH):1 Full rate TCH (TCH/F).2 Half rate TCH (TCH/H)).

Signaling channels are subdivided into three categories:

1 Broadcast CHannels, BCH

2 Common Control CHannels, CCCH3 Dedicated Control CHannels, DCCH


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GSM Case StudyUm Interface: Broadcast Channels

Frequency Correction CHannel (FCCH): Allow the MS tosynchronize to the frequency. Only downlink on carrier 0in TS 0.

Synchronization CHannel (SCH)The MS needs tosynchronize to the time-structure within this particularcell. Listening to the SCH, the MS receives informationabout the frame number of the chosen BTS. Onlydownlink on carrier 0 TS 0.

Broadcast Control CHannel (BCCH)The MS must receivesome general information concerning the cell in order tostart roaming, waiting for calls to arrive or making calls.Only downlink on carrier 0 in TS 0.


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GSM Case StudyUm Interface: Dedicated Control Channels

Stand alone Dedicated Control CHannel (SDCCH) The call set-upprocedure is performed on this channel as well as all the signallingover the connection done by the MS and the BTS SDCCH istransmitted both uplink and downlink.

Slow Associated Control CHannel (SACCH) The SACCH isassociated with SDCCH or TCH (i.e. sent on the same physicalchannel). On the uplink, the MS sends averaged measurements on itsown BTS (signal strength and quality) and neighboring BTSs (signalstrength). On the downlink, the MS receives information concerningthe transmitting power to use and instructions on the timing

advance. SACCH is transmitted both uplink and downlink.Fast Associated Control CHannel (FACCH) If a handover is requiredthe FACCH is used. FACCH works in stealing mode meaning thatone 20 ms segment of speech is exchanged for signaling informationnecessary for the handover.


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GSM Case StudyUm Interface: Channel Combinations

Only certain combinations of logical channels arepermitted according to the GSM recommendations.

1 TCH/F+ FACCH/F+ SACCH/TF

2 TCH/H(0.1) + FACCH/H(0.1) + SACCH/TH(0.1)3 TCH/H(0)+FACCH/H(0)+SACCH/TH(0)+TCH/H(1)4 FCCH+ SCH+ BCCH+ CCCH5 FCCH+ SCH+ BCCH+ CCCH+ SDCCH/4(0...3) +SACCH/C4(0...3)

6 BCCH

+CCCH

7 SDCCH/8(0...7) + SACCH/C8(0...7)

item Where CCCH= PCH+ AGCH+ RACH

GS C S


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GSM Case StudyUm Interface: Channel Combinations, FCCH+ SCH+ BCCH+ CCCH

GSM C S d


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GSM Case StudyUm Interface: Channel Combinations, SDCCH+ SACCH

GSM C S d


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GSM Case StudyUm Interface: Channel Combinations, SDCCH+ SACCH

GSM C St d


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GSM Case StudyUm Interface: Protocol Stack

Our next step is to analyze the protocol stack in theinterfaces of the Access Radio Network

Note that as it is shown in the figure, each interfaces has

two faces, and the functions may not be the same in bothsides.

GSM C St d


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GSM Case StudyUm Interface: Um Air Interface Now from the side of the Mobile Station

GSM C s St d


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GSM Case StudyUm Interface: Um Air Interface Now from the side of the Base Station

GSM Case Study


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GSM Case StudyAbis Interface: Introduction

It is the interface between the BTS and the BSC.

The physical level is implemented by 2Mbps links following

the recommendation G.703 of the GSM.The links are composed by 32 channels of 64 Kbps.

There are only three feasible configurations.

1 BTS with a single TRX.2 BTS with several TRX and a single link to the BSC3 BTS with several TRX each one with a particular link to

the BSC.

GSM Case Study


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GSM Case StudyAbis Interface: Channels

There are two types of channels in the Abis Interface.

Communication Channels

1 Traffic Channels: With speeds of 8, 16 or 64 Kbps, they

carry voice or data of the corresponding traffic channel.Considering that the TCH has a maximum speed of 13Kbps, with four channels, there is a remaining capacity inthe digital circuit of 12 Kbps. This remaining capacity isused for synchronization between the BTS and a network

element named TRAU.2 Signalling Channels: With Speeds of 16, 32 or 64 Kps, they

carry user signalling information of MS, BTS and BSC.

GSM Case Study


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GSM Case StudyAbis Interface: Channels

For the signalling in the Abis interface the protocol oflayer 2 , LAPD.

In each LAPD message, in the addressing field there is theBTS the message refers to.

The specification defines the following logical links.

1 Radio Signalling Link (RSL): Used for message interchangeabout the traffic management procedures between the MSand the network. There is a RSL per TRX of the BTS.

2 Operation and Management Link (OML): Used formanagement information exchange. There is a OML perTRX and BCF.

3 Layer 2 Management Link: It is the management link oflayer 2. There is a OML per TRX and BCF.

GSM Case Study


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GSM Case StudyAbis Interface: Abis from the BTS

GSM Case Study


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GSM Case StudyAbis Interface: Abis from the BSC

GSM Case Study


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GSM Case StudyConclusion

At this point we have ended a very hard part that deals

with the inner and deeper part of the GSM accessnetwork, protocols, channels and frame structure

Our next step is dedicated to try to understand how thenetwork works in a limited set of important scenarios.

gsm access network

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