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CS6956: Wireless and Mobile Networks
Lecture Notes: 3/23/2015
GSM – Global System for Mobile Communications
(reference – From GSM to LET by Martin Sauter)
There were ~3 billion GSM users in 2010.
GSM Voice
Frequency Bands
Europe: 890 – 915 MHz (uplink)
935 – 960 MHz (downlink)
25 MHz split into 125 channels with a bandwidth of 200 KHz each
Additional range: 1170 – 1785 MHz (uplink)
1805 – 1880 MHz (downlink)
75 MHz split into 375 additional channels
North America – 1900 MHz and 850 MHz bands
Base Transceiver Station (BTS)
Vodafone has 20,000 GSM base stations in Germany
Theoretically, a base station can cover an area with radius 35 Km. This area is called a cell. Cells are
much smaller in practise, especially in dense urban environments. Typically, 3-4 Km down to 100m
radius (with minimal transmission power in areas like shopping centers and downtown streets). Rural
area radius – 15 Km, 1 to 2 W of power of mobile device is the limiting factor.
All neighboring cells have to send on different frequencies -> different BSs must not interfere with each
other.
Increase the capacity of a base station -> coverage area split into two or three sectors, each covered by
different frequencies by dedicated transmitters. Better reuse of frequencies. Each sector of the BS forms
its own independent cell.
GSM Air Interface
200 KHz -> eight subscribers can communicate with the BS simultaneously.
Let each base station have three sectors. Let each sector have two separate transmitters and receivers
(at different frequencies). Total time slots = 2 X 8 = 16 per sector. Out of these, 2 are used for signaling
and 4 for GPRS packet switching (to be covered later). Therefore, only 10 slots are available per sector.
Thus, we have 30 slots (or 30 simultaneous calls) per base station. A BS is able to provide service to 60
times the no. of maximum active subscribers. Therefore, BS with 30 slots is able to provide service to
1800 subscribers (statistical multiplexing – recall first homework).
557 microsec -> transmit 114 bits of data, ocassionally data bursts are used for signaling.
A GSM TDMA frame is shown below:
For transmission of user or signaling data, the timeslots are arranged into logical channels. Logical
channels can be visualized in the following figure. The first two slots are usually used for logical signaling
channels while remaining 6 are used for user data channels. Different types of signaling channels can
use the signaling channels (time slots).
TCH -> the traffic channel
FACCH -> urgent signaling messages (e.g., a handover command) in the TCH channels -> voice data
meant for the TCH channel is ignored. Loosing a slot is not that much of a problem for voice data.
SACCH -> Slow assisted control channel – assigned to a dedicated connection – used in the uplink
direction to report signal quality measurements of the serving cell and neighboring cells to the network.
In the downlink direction, the SACCH is used to send power control commands to the mobile device.
SDCCH -> The stand alone dedicated control channel is a pure signaling channel that is used during call
establishment when a subscriber has not yet been assigned a traffic channel. Furthermore, the channel
is used for signaling that is not related to call establishment, such as location update procedure or for
sending or receiving a text message.
TCH, FACCH, SACCH, SDCCH, are dedicated channels assigned to individual mobiles.
The figure below shows assignment of channels in two types of multiframes.
Common Channels
Broadcast Common Control Channel (BCCH) – identification of cell consistes of location area (LAC) and
the cell-ID, frequencies used by neighboring cells.
Paging Channel (PCH) – broadcast to a loation area contains IMSI of a device or a TMSI.
Random Access Channel (RACH) – only common channel in the uplink direction.
If the mobile device receives a message via a PCH that the network is requesting a connection
establishment or if the users wants to establish a call or send an SMS, the RACH is used for the initial
communication with the network. This is done by sending a channel request message. This message can
collide with another message – random backoff is used to minimize the possibility of collisions. The
figure below shows the establishment of a signaling connection.
AGCH: Access Grant Channel
The Base Station Controller:
While the BS is the interface element that connects the mobile device with the network, the BSC is
respobsible for the establishment, release, and maintenance of all connections of cells that are
connected to it.
BSC – check if an SDCCH is available. BSC is also responsible for establishing signaling channels for
incoming calls or SMS messages (paging message from the MSC contains the IMSI or the TMSI).
The establishment of a traffic channel for voice calls is always requested by the MSC for both mobile
originated and mobile terminated calls.
Once the mobile device sand the MSC have exchanged all necessary information for establishment of a
voice call via an SDCCH, the MSC sends an assignment request to the BSC.
BSC allocated a TCH if available.
Once the call is established the SDCCH is released.