Transcript GSM Spectrum Allocation

GSM Spectrum Allocation
 P-GSM Spectrum (Primary GSM)
 E-GSM Spectrum (Extended GSM)
 DCS-1800 Spectrum
 PCS-1900 Spectrum
1
P-GSM Spectrum (Primary GSM)
The initial allocation of spectrum for
GSM provided 124 carriers with
Frequency Division Duplex for uplink
and downlink:
 Duplex sub bands of width 25 MHz duplex spacing 45 MHz
 Uplink sub band: 890 MHz to 915
MHz
 Downlink sub band: 935 MHz to 960
MHz
 Frequency spacing between carriers
is 200 kHz (0.2 MHz)
 One carrier is used for guard bands.
 Total number of carriers (ARFCNs) =
(25 – 0.2) / 0.2 = 124
 Uplink frequencies: Fu(n) = 890 + 0.2 n
(1 ≤ 𝑛 ≤ 124)
 Downlink frequencies: Fd(n) = Fu(n) + 45
 Where n = ARFCN
(ARFCN – Absolute Radio Frequency Carrier
Number)
2
E-GSM Spectrum (Extended GSM)
 E-GSM allocated extra carriers at the low
end of the spectrum. The ARFCN
numbers of P-GSM were retained (with 0
now included) and new ARFCNs
introduced for the lower end, numbered
975 – 1023.
 Duplex sub bands of width 35 MHz duplex spacing 45 MHz (same as PGSM)
 Uplink sub band: 880 MHz to 915 MHz
 Downlink sub band: 925 MHz to 960
MHz
 Frequency spacing of 200 kHz
 One carrier used to provide guard bands
0
 Total number of carriers (ARFCNs) =
(35 – 0.2) / 0.2 = 174
 Uplink frequencies:
Fu(n) = 890 + 0.2n
(0 ≤ 𝑛 ≤ 124)
Fu(n) = 890 + 0.2 (n – 1024)
(975 ≤ 𝑛 ≤ 1023)
 Downlink frequencies: Fd(n) = Fu(n) + 45
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900 MHz Utilization in Jordan
880
885
890
Zain
925
902.5
Orange
930
935
915
MHz
960
MHz
Zain
947.5
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DCS-1800 Spectrum
 Digital Communication System – 1800
MHz introduced a further spectrum
range for GSM, typically used for smaller
microcells overlaid over existing
macrocells.
 Duplex sub bands of width 75 MHz duplex spacing 95 MHz
 Uplink sub band: 1710 MHz to 1785 MHz
 Downlink sub band: 1805 MHz to 1880
MHz
 Frequency spacing of 200 kHz
 Total number of carriers (ARFCNs) =
 One carrier used to provide guard bands
(75 – 0.2) / 0.2 = 374
 Uplink frequencies: Fu(n) =
1710.2 + 0.2 (n – 512)
(512 ≤ 𝑛 ≤ 885)
 Downlink frequencies: Fd(n) = Fu(n) + 95
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1800 MHz Utilization in Jordan
1710
1740
1755
1785
MHz
1880
MHz
Umniah
1805
1835
1850
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1800 MHz Utilization in UK
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PCS-1900 Spectrum
 Personal Communication System –
1900 MHz is used in USA and Central
America to provide a service similar to
GSM.
 Duplex sub bands of width 60 MHz duplex spacing 80 MHz
 Uplink sub band: 1850 MHz to 1910
MHz
 Downlink sub band: 1930 MHz to 1990
MHz
 Frequency spacing of 200 kHz
 Total number of carriers (ARFCNs) =
 One carrier used to provide guard
bands
(60 – 0.2) / 0.2 = 299
 Uplink frequencies: Fu(n) =
1850.2 + 0.2 (n – 512)
(512 ≤ 𝑛 ≤ 810)
 Downlink frequencies: Fd(n) = Fu(n) + 80
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Multiple Access Techniques
 Purpose: to allow several users to share the
resources of the air interface in one cell
 Methods:
 FDMA - Frequency Division Multiple
Access
 TDMA - Time Division Multiple Access
 CDMA - Code Division Multiple Access
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FDMA - Frequency Division Multiple Access
 Divide available frequency spectrum into channels
each of the same bandwidth
Frequency
 Channel separation achieved by filters:
 Good selectivity
Channel BW
 Guard bands between channels
 Signalling channel required to allocate a traffic
channel to a user
 Only one user per frequency channel at any time
Time
 Used in analog systems, such as AMPS, TACS
 Limitations on:
 frequency re-use
 number of subscribers per area
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TDMA - Time Division Multiple Access
Access to available spectrum is limited to timeslots
 User is allocated the spectrum for the duration of one
timeslot
 Timeslots are repeated in frames
TS0
TS1
TS2
TS3
TS4
TS5
TS6
TS7
TS0
TS1
TS2
TS3
TS4
TS5
TS6
TS7
Frequency
Frame
Time slot
Time
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CDMA - Code Division Multiple Access
 Each user is assigned a unique digital
code (pseudo - random code sequence)
 Code is used at Mobile Station and Base
Station to distinguish different user’s
signals
 Many users’ communications can be
transmitted simultaneously over the same
frequency band
 Advantages:
 very efficient use of spectrum
 does not require frequency planning
 Used in IS - 95 (cdmaOne)
 Not used in GSM
 Wideband CDMA techniques used in
Frequency
Code
Time
UMTS
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GSM
TDMA/FDMA
Using FDMA and TDMA techniques, each carrier is divided into 8 Physical channels (timeslots)
935-960 MHz
124 channels (200 kHz)
downlink
890-915 MHz 124 channels (200 kHz) uplink
higher GSM frame structures
time
GSM TDMA frame
TS0
TS1
TS2
TS3
TS4
TS6
TS5
TS7
4.615 ms
guard
space
GSM time-slot (normal burst)
“Physical Channel”
tail
3 bits
user data
57 bits
S Training
S
user data
1 26 bits
1
57 bits
156.25 bit periods
guard
tail space
3
⇒ ≈ 270.8𝑘𝑏𝑝𝑠
546.5 µs
577 µs
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Uplink and Downlink Synchronization
 TDMA is used to provide a set of 8 physical channels (timeslots) on
each carrier
 One cycle of 8 timeslots forms the TDMA frame of 4.615 ms duration
 Each timeslot lasts for 0.577 ms (156.25 bit periods) and can contain
one of several types of data burst
 A mobile station cannot transmit and receive simultaneously.
 The MS transmit burst is delayed by 3 timeslots after the BTS burst.
 This delay allows the MS to compare signal quality from neighboring
cells
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GSM Channels
 A timeslot is the basic physical resource (channel) in GSM,
which is used to carry all forms of logical channel
information, both user speech/data and control signaling.
 Logical Channels - the various ways we use the resourceone physical channel may support many logical channels.
logical channels are piggybacked on the
physical channels
 Multiframe structures is used to provide all the logical
channels required.
 Different structures of data burst are used in the timeslot
for different purposes.
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Logical Channels
 GSM uses a set of logical channels to carry call traffic,
signaling, system information, synchronization etc.
 The logical channels are divided into traffic channels and
control channels
 They can then be further divided as shown:
TCH Traffic Channels
TCH/F Traffic Channel (full rate) (U/D)
TCH/H Traffic Channel (half rate) (U/D)
BCH Broadcast Channels
FCCH Frequency Correction Channel (D)
SCH Synchronization Channel (D)
BCCH Broadcast Control Channel (D)
CCCH Common Control Channels
PCH Paging Channel (D)
RACH Random Access Channel (U)
AGCH Access Grant Channel (D)
CBCH Cell Broadcast Channel (D)
NCH Notification Channel (D)
DCCH Dedicated Control Channels
SDCCH Stand alone Dedicated Control Channel (U/D)
SACCH Slow Associated Control Channel (U/D)
FACCH Fast Associated Control Channel (U/D)
U = Uplink D = Downlink
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Traffic Channels (TCH)
 TCH carries payload data - speech, fax, data- normally time
slots 1 - 7 if TS0 is used for control signaling
 Connection may be:
 Circuit Switched - voice or data or
 Packet Switched – data
 TCH may be:
o Full Rate (TCH/F)


one channel per user
13 kbps voice, 9.6 kbps data
 or
o Half Rate (TCH/H)
 one channel shared between two users (alternatively from frame to frame)
 6.5 kbps voice, 4.8 kbps data
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Broadcast Channels (BCH)
BCH channels are all downlink and are allocated to timeslot zero some times called BCCH. The
RF carrier used to transmit the BCCH is referred to as the BCCH carrier.
BCH Channels are:
o FCCH: Frequency correction channel sends the mobile a burst of all ‘0’ bits which allows it
to fine tune to the downlink frequency
o SCH: Synchronization channel, the SCH carries the information to enable the MS to
synchronize to the TDMA frame structure and know the timing of the individual timeslots,
it sends the absolute value of the frame number (FN), which is the internal clock of the BTS,
together with the Base Station Identity Code (BSIC).
o BCCH: Broadcast Control Channel sends radio resource management and control messages:
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Location Area Identity (LAI).
List of neighboring cells that should be monitored by the MS.
List of frequencies used in the cell.
Cell identity.
Power control indicator.
DTX permitted.
Access control (i.e., emergency calls, call barring ... etc.).
CBCH description.
Some messages go to all mobiles, others just to those that are in the idle state.
 As the name suggests, the broadcast channels send information out to all mobiles in a cell.
These channels are also important for mobiles in neighboring cells which need to monitor
power levels and identify the base stations.
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Common Control Channels (CCCH)
CCCH contains all point to multi-point downlink channels (BTS to several
MSs) and the uplink Random Access Channel:
o CBCH: Cell Broadcast Channel is an optional channel for general
information such as road traffic reports sent in the form of SMS.
o PCH: Paging Channel sends paging signal to inform mobile of a call,
(paging can be performed by an IMSI, TMSI or IMEI).
o RACH: Random Access Channel is sent by the MS to request a channel
from the BTS or accept a handover to another BTS. A channel request is
sent in response to a PCH message.
o AGCH: Access Grant Channel allocates a dedicated channel (SDCCH) to
the mobile.
o NCH: Notification Channel informs MS about incoming group or broadcast
calls.
 The main use of common control channels is to carry the information
needed to set up a dedicated channel. Once a dedicated channel (SDCCH) is
established, there is a point to point link between the base station and
mobile. Associated control channels carry additional signalling to support
dedicated channels. SACCH is associated with either SDCCH or TCH.
FACCH is only associated with TCH.
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Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink / downlink)
point to point control channels:
o SDCCH: Standalone Dedicated Control Channel is used for call set
up, Authentication, location updating and also point to point SMS.
o ACCH: Associated Control 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.
o SACCH: Slow Associated Control Channel 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 during a call or operations associated with SDCCH.
o FACCH: Fast Associated Control Channel is used (when needed) for
signalling during a call, mainly for delivering handover messages and for
acknowledgement when a TCH is assigned.
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Multiframes
 To provide all the logical channel operations with the physical resources (timeslots)
available, an additional time frame structure is required in which the logical
channels are multiplexed onto the timeslots. This is the concept of multiframes.
 Multiframes provide a way of mapping the logical channels on to the physical
channels (timeslots).
 A multiframe is a series of consecutive instances of a particular timeslot.
 GSM uses multiframes of 26 and 51 timeslots.
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Traffic Channel Multiframe
 The TCH multiframe consists of 26 timeslots.
 This multiframe maps the following logical channels:
 TCH
 SACCH
 FACCH
 TCH Multiframe structure:
Frame #
T = TCH, S = SACCH, I = Idle
FACCH is not allocated slots in the multiframe. It steals TCH slots when required - indicated by the stealing flags in
the normal burst.
•
•
•
TCH is always allocated on the 26 frame multiframe structure shown above.
During a call the mobile is continually monitoring power levels from neighboring base stations.
It does this in the times between its allocated timeslot. Once each traffic channel multiframe
there is a SACCH burst which is used to send a report on these measurements to the current
serving base station.
The downlink uses this SACCH burst to send power control and other signals to the mobile.
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Control Channel Multiframe
 The control channel multiframe is formed of 51 timeslots.
 CCH multiframe maps the following logical channels:
Downlink
Uplink
FCCH
SCH
RACH
BCCH
CCCH (combination of PCH and AGCH)
 A basic BCCH multiframe is shown below which use TS0. The main
reason for other structures is the allocation of SDCCH/SACCH.
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Different Control Channel structures
TS0
TS1
While TS0 as in the
previous slide
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GSM hierarchy of frames
hyperframe
0
1
...
2
2045 2046 2047 3 h 28 min 53.76 s
superframe
0
1
0
...
...
2
1
48
49
24
50
25
traffic
6.12 s
control
multiframe
0
...
1
0
1
24
...
2
traffic
25
48
49
50
120 ms
control 235.4 ms
frame
0
1
...
slot
burst
577 µs
6
7
4.615 ms
The timing of the hyperframe relates to the cycle of frame numbers transmitted on
the synchronization channel (SCH). After 26 x 51 x 2048 = 2715648 frames, the
frame number (which consists of 22 bits) resets to zero.
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Types of Data Burst
 The 156.25 bit periods of a timeslot can hold different
types of data burst:
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Timing Advance
Timing Advance is needed to compensate for different time delays
in the transmission of radio signals from different mobiles.
 Signal from MS1 takes longer to arrive at BTS than that from MS2
 Timeslots overlap - collision
 Timing Advance signal causes mobiles further from base station
to transmit earlier - this compensates for extra propagation delay
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TA Cont.
 The maximum value of Timing Advance sets a limit on the size of the
cell.
 Timing Advance is calculated from delay of data bits in the RACH burst
received by the base station – long guard period allows space for this
delay
 It is adjusted during the call in response to subsequent normal burst
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positions.
TA signal is transmitted on SACCH as a number between 0 and 63 in
units of bit periods
TA value allows for ‘round trip’ from MS to BTS and back to MS
Each step in TA value corresponds to a MS to BTS distance of 550
metres
Maximum MS to BTS distance allowed by TA is 35 km
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TA Cont.
 Timing Advance value reduces the 3 timeslot offset
between downlink and uplink
TA
Uplink
Actual delay
 The Timing Advance technique is known as adaptive
frame alignment
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GSM Modulation Technique
Gaussian Minimum Shift Keying (GMSK):
 Frequencies are arranged so there is no phase discontinuity
at the change of bit period.
 Data pulses are shaped using a Gaussian filter:
 Smoothes phase transitions
 Gives a constant envelope
 QPSK is used in IS-95 (CDMA).
 Comparison of GMSK and QPSK:
 GMSK requires greater bandwidth
 QPSK reduces interference with adjacent carrier frequencies
 GMSK is more power efficient - less battery drain from MS on
uplink
 GMSK has greater immunity to signal fluctuations
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Speech over the Radio Interface
31
Speech Coding
 GSM transmits using digital modulation - speech must
be converted to binary digits
 Coder and decoder must work to the same standard
 Simplest coding scheme is Pulse Code Modulation
(PCM):
 Sampling every 1/(2*4k)=125 μs
 Assume each sample is mapped to an 8 bit codeword
(256 levels of an equalizer) then this requires data rate
of 8k*8=64 kbps
 This is too high for the bandwidth available on the
radio channels
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Advanced Speech Coding
 Several approaches to modeling human speech which requires
less data than PCM have been attempted.
 Estimates are that speech only contains 50 bits per second of
information
 Compare time to speak a word or sentence with time to transmit
corresponding text
 Attempts to encode speech more efficiently:
 speech consists of periodic waveforms -so just send the frequency
and amplitude
 model the vocal tract - phonemes, voiced and unvoiced speech
 Vocoder - synthetic speech quality
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ASC Cont.
 Speech obviously contains far more information than the
simple text transcription of what is being said. We can
identify the person speaking, and be aware of much
unspoken information from the tone of voice and so on.
 Early vocoders which reduced the voice to just simple
waveform information lacked the human qualities which
we need to hold a meaningful communication.
 Hybrid encoders give greater emphasis to these qualities
by using regular pulse excitation which encodes the
overall tone of the voice in great detail.
34
GSM Voice Coder
 Hybrid model using multi-pulse excitation linear predictive coding
 Regular Pulse Excitation - Long Term Prediction (RPE-LTP)
 Divides speech into three parts:
 Short term prediction
 Long term prediction
Sent as frequency and amplitude
 Residual periodic pulse - sent as coded waveform like PCM - requires more
bits than the other two parts , this is to ensure that the characteristic tone of the
voice is reproduced well.
 Speech is divided into blocks of 20 ms
 Each block of 20 ms is represented by a pattern of 260 bits:
 260 bits every 20 ms, gives an output rate of : 260 / 20 𝑥10−3 = 13 kbps
35
Error Correction Coding
 To reproduce speech, decoder needs bit error rate no
more than 0.1%
 Radio channel typically gives error rate of 1% - need
error correction
 Two approaches to error correction:
 Backward error correction: Automatic Repeat Request
(ARQ)
 Forward error correction
36
ARQ
 In backward error correction, we assume that if the known
check bits have been transmitted correctly, the rest of the
data is correct. If the check bits do not match what is
expected, the system asks for re-transmission.
 Not suitable for speech as the timing could become
unintelligible if several repeats were necessary. However, in
normal conversation, we naturally apply backward error
correction by asking the person to repeat something we
have not understood.
37
FEC
 Coding is added to the information bits which enable the
original to be reconstructed even if there are errors redundancy
 Repeat transmission is not required - suitable for speech
 Two types of FEC:
 Block codes
 Convolutional codes
 GSM uses a combination of both code types
38
GSM Error Correction Scheme
 The GSM coding scheme is described as ‘concatenated’. It divides
the data into three prioritized sections and applies different
levels of coding to each, the resultant code is then put together
(concatenated) for transmission.
 260 bits from voice coder are divided into 3 classes, according to
their importance for speech reproduction:
 Rate of coding describes the amount of redundancy in the coded
data:
 1/2 rate code transmits twice as many bits as actual data
 Data rate is halved
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Interleaving
 The algorithms used to recover the data are based on an assumption that errors
will be randomly distributed.
 In practice errors tend to clump together as the mobile passes in and out of fade
regions.
 To overcome this, the data bursts are not sent in their natural order, but are
interleaved according to a pseudo-random pattern among a set of timeslots
within the multiframe.
 Interleaving is applied after error coding and removed at the receiver before the
decoding. Thus the coding algorithm has a more random distribution of errors
to deal with.
40
Protocol Stack
 A protocol is a set of rules, agreed by both sides, to allow meaningful
communication to take place
 Protocols are needed whenever systems need to pass information
from one to another
 ISO 7-Layer OSI Reference Model:
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Vertical vs. Horizontal Communications
Horizontal (Peer-to-Peer) Communication
Vertical (Entity-to-Entity) Communication
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Vertical (Entity-to-Entity) Communication
 Each layer requests a service from the layer below
 The layer below responds by providing a service to the layer
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above
Each layer can provide one or more services to the layer above
Each service provided is known as a service ‘Entity’
Each Entity is accessed via a Service Access Point (SAP) or a
‘gate’.
Each SAP has a unique SAP Identifier (SAPI)
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GSM Protocols
 In the OSI Reference Model, the
logical channels of the air interface
are at the Service Access Point (SAP)
of the Physical Layer (Layer 1)
 ISDN Reference Model divides the
protocol plane into a Control Plane
and a User Plane
 corresponds to the control and traffic
channels of the logical channels
 some user data (notably SMS text
messages) is carried by the control
plane
44
Protocols on the GSM Air Interface
45
User Plane - Speech Transmission
 Speech is encoded at the MS by the GSM Speech Codec (GSC) using
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hybrid encoders to give a data rate of 13 kbps. Then Forward Error
Correction (FEC) is applied
At the BSS the FEC and any encryption is decoded by the TRX and the
data is converted to the ISDN format (ITU-T A-law) by a Transcoding
and Rate Adaption Unit (TRAU).
The A-law format carries data at 64 kbps across the fixed network.
The TRAU may be part of the BTS or part of the BSC.
If the TRAU is located at the BSC, then up to 4 speech channels may be
multiplexed at the BTS (MPX in the diagram) onto an ISDN B channel
which reduces the bandwidth required across the Abis interface.
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Control Plane-GSM Signalling Protocols
CM: Connection Management
MM: Mobility Management
RR: Radio Resources Management
LAPD: Link Access Procedure D
LAPDm: Link protocol adapted for air interface (Um)
BTSM: Base Transceiver Station Management
BSSMAP: Base Station System Management
Application Part
DTAP: Direct Transfer Application Part
SCCP: Signalling Connection Control Part
TCAP: Transaction Capabilities Application Part
MTP: Message Transfer Part
MAP: Mobile Application Part
UP: User Part
ITU-T G.703, G705, G.732: Protocols for digital
transfer of signalling messages on the Abis and A
interfaces at 2048 kb/s or 64 kb/
47
Protocols Functionality
 Layer 1 – Physical Layer
 On the air interface, the physical layer uses FDMA/TDMA,
multiframe structure, channel coding etc. to implement the
logical control channels.
 Services provided by layer 1 are:
 Access capabilities – multiplexing logical onto physical channels
 Error protection – error detection / correction coding mechanisms
 Encryption
 Layer 2 – LAPDm – Link Access Procedure on Dm channels
 Data link protocol responsible for protected transfer of signalling
messages between MS and BTS.
 LAPDm supports the transport of messages between protocol
entities on Layer 3, in particular: BCCH, PCH, AGCH and SDCCH
signalling.
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Cont.
 Layer 3 - Network

Sub-layers:
 Radio Resource Management (RR)
 Mobility Management (MM)
 Connection Management – 3 entities:
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RR is responsible for:
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Call Control (CC)
Supplementary Services (SS)
Short Message Service (SMS)
Monitoring BCCH and PCH
Administering RACH
Requests for and assignments of data and signalling
channels
Measurements of channel quality
MS power control and synchronization
Handover
Synchronization of data channel encryption and decryption
Within Connection Management, Call Control
(CC) is responsible for:
 Set up of normal calls (MS originated, MS
terminated)
 Set up of emergency calls (MS originated
only)
 Terminating calls
 DTMF signalling
 Call related supplementary services
 Service modification during a call (e.g.
speech/data, speech/fax)
MM is responsible for:
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TMSI assignment
Location updating
Identification of MS (IMSI, IMEI)
Authentication of MS
IMSI attach and detach
Confidentiality of subscriber identity
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Enhancing GSM
 AMR (Adaptive multi-rate) speech coder
 Trade off speech and error correction bits
 Fewer dropped calls
 DTX — discontinuous transmission
 Less interference (approach 0 bps during silences)
 More calls per cell
 Frequency hopping
 Overcome fading
 Synchronization between cells
 DFCA: dynamic frequency and channel assignment

Allocate radio resources to minimize interference
 Also used to determine mobile’s location
 TFO — Tandem Free Operation
Tandem Free Operation (TFO) Concepts
 Enchance GSM operation through Improve voice
quality by disabling unneeded transcoders during
mobile-to-mobile calls
 Operate with existing networks (BSCs, MSCs)
 New TRAU negotiates TFO in-band after call setup
 TFO frames use LSBits of 64 Kbps circuit to carry
compressed speech frames and TFO signaling
 MSBits still carry normal G.711 (PCM)speech samples
 Limitations
 Same speech codec in each handset
 Digital transparency in core network (EC off!)
 TFO disabled upon cell handover, call transfer, in-band
DTMF, announcements or conferencing
TFO – Tandem Free Operation
 No TFO : 2 unneeded transcoders in path
A
D
GSM Coding
Abis
G.711 / 64 kb
D
A
Ater
A
PSTN*
TRAU
MS BTS
BSC
A
D
MSC
GSM Coding
D
A
TRAU
BSC
MSC
BTS MS
 With TFO (established) : no in-path transcoder
A
D
GSM Coding
Abis
[GSM Coding + TFO Sig] (2bits) + G.711 (6bits**) / 64 Kb
T
F
O
Ater
A
PSTN*
TRAU
MS BTS
BSC
MSC
(*) or TDM-based core network
(**) or 7 bits if Half-Rate coder is used
T
F
O
GSM Coding
TRAU
MSC
BSC
BTS MS
D
A
GSM Evolution
A lot of developments within GSM leads towards 3G
technology and the high data rates which this is intended
to offer. These technologies are collectively known as 2.5
or B2G Generation GSM technologies and include:
 High Speed Circuit-Switched Data (HSCSD)
 General Packet Radio Service (GPRS)
 Enhanced Data for GSM Evolution (EDGE)
 CAMEL (Customized Application for Mobile Enhanced
Logic)
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2.5 G
 In GSM data transmission standardized with maximum
9.6 kbit/s
 advanced coding allows 14,4 kbit/s
 not enough for Internet and multimedia applications
 Main requirement is for increased data rates
Mobile access to:
 Internet
 E-mail
 Corporate networks
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GSM Evolution for Data Access
2 Mbps
UMTS
384 kbps
115 kbps
EDGE
GPRS
9.6 kbps
GSM
1997
2000
GSM evolution
2003
2003+
3G
HSCSD (High-Speed Circuit Switched Data)
 Increases bit rate for GSM by a mainly software upgrade
 Uses multiple GSM channel coding schemes to give 4.8 kb/s, 9.6 kb/s or 14.4 kb/s
per timeslot
 Multiple timeslots for a connection e.g. using two timeslots gives data rates up to
28.8 kb/s
 Timeslots may be symmetrical or asymmetrical, e.g. two downlink, one uplink,
giving 28.8 kb/s downloads but 14.4 kb/s uploads
 HSCSD handsets are typically limited to 4 timeslots, allowing:
 2 up / 2 down (28.8 kb/s in both directions)
 3 down and 1 up (43.2 kb/s down 14.4 kb/s up)
 This limitation arises because the handset operates in half duplex and needs time to
change between transmit and receive modes
 Advantage: ready to use, constant quality, simple
 Disadvantage: channels blocked for voice transmission
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GPRS (General Packet Radio Service)
 Packet switching:
 Data divided into packets
 Packets travel through network individually
 Connection only exists while packet is transferred from one
node to next
 When packet has passed a node, the network resources
become available for another packet
 User sees an ‘always on’ virtual connection through the




network
Using free slots only if data packets ready to send
(e.g., 115 kbit/s using 8 slots temporarily)
Standardization 1998, introduction 2000.
Advantage: one step towards UMTS, more flexible
Disadvantage: more investment needed
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GPRS Network Elements
GPRS network elements
 GSN (GPRS Support Nodes): GGSN and SGSN
 GGSN (Gateway GSN)
 interworking unit between GPRS and PDN (Packet Data Network)
 acts as an interface and a router to external networks. The GGSN
contains routing information for GPRS mobiles, which is used to
tunnel packets through the IP based internal backbone to the correct
Serving GPRS Support Node. The GGSN also collects charging
information connected to the use of the external data networks and
can act as a packet filter for incoming traffic.
 SGSN (Serving GSN)
 responsible for authentication of GPRS mobiles, registration of
mobiles in the network, mobility management, and collecting
information for charging for the use of the air interface.
 GR (GPRS Register)

user addresses
58
GPRS architecture and interfaces
SGSN
Gn
BSS
MS
Um
SGSN
Gb
Gn
Gi
HLR/
GR
MSC
VLR
PDN
GGSN
EIR
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GPRS modifications on GSM network
GSM Network Element
Modification or Upgrade Required for GPRS
Mobile Station (MS)
New Mobile Station is required to access GPRS services. These new
terminals will be backward compatible with GSM for voice calls.
BTS
A software upgrade is required in the existing base transceiver site.
BSC
The base station controller (BSC) requires a software upgrade and the
installation of new hardware called the packet control unit (PCU). The
PCU directs the data traffic to the GPRS network and can be a separate
hardware element associated with the BSC.
GPRS Support Nodes (GSNs)
The deployment of GPRS requires the installation of new core network
elements called the serving GPRS support node (SGSN) and gateway GPRS
support node (GGSN).
Databases (HLR, VLR, etc.)
All the databases involved in the network will require software upgrades
to handle the new call models and functions introduced by GPRS.
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GPRS Circuit/Packet Data Separation
61
2.5G Architectural Detail
2G MS (voice only)
NSS
BSS
E
Abis
PSTN
A
PSTN
B
BSC
MS
C
MSC
BTS
Gs
VLR
D
GMSC
SS7
H
Gb
2G+ MS (voice & data)
Gr
HLR
AuC
Gc
Gn
SGSN
BSS — Base Station System
BTS — Base Transceiver Station
BSC — Base Station Controller
NSS — Network Sub-System
MSC — Mobile-service Switching Controller
VLR — Visitor Location Register
HLR — Home Location Register
AuC — Authentication Server
GMSC — Gateway MSC
Gi
IP
PSDN
GGSN
SGSN — Serving GPRS Support Node
GGSN — Gateway GPRS Support Node
GPRS — General Packet Radio Service
62
GPRS protocol architecture
MS
BSS
Um
SGSN
Gb
Gn GGSN
Gi
apps.
IP/X.25
IP/X.25
SNDCP
LLC
RLC
MAC
RLC
MAC
BSSGP
FR
radio
GTP
LLC
GTP
UDP/TCP
UDP/TCP
BSSGP
IP
IP
FR
L1/L2
L1/L2
SNDCP
radio
63
GPRS Air Interface
 New ‘Packet’ logical channels defined - PBCCH, PDTCH




etc.
New multiframe structure based on ‘radio blocks’ of 4
timeslots
Allows up to 8 mobiles to share a timeslot
For high data rates, several physical channels may be
allocated to one user
4 levels of channel coding schemes (CS-1 to CS-4):
 Decreasing level of error checking
 Greater data throughput rates
 Scheme selected according to interference level (C/I)
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Enhanced Data rates for GSM Evolution (EDGE)
 Use 8 Phase-Shift Keying (8PSK) modulation -




3 bits per symbol
Improved link control allows the system to
adapt to variable channel quality - leads to
slightly reduced coverage area
Applied to GSM, EDGE allows a maximum data
rate of 48 kb/s per timeslot, giving the quoted
𝑠4
figure of 384 kb/s per carrier (8 timeslots)
EDGE can be applied to HSCSD (ECSD) and
GPRS (EGPRS)
𝑠5
EDGE will be expensive for operators to
implement:
 Each base station will require a new EDGE
transceiver
 Abis interface between BTS and BSC must be
upgraded
𝜓2 (𝑡)
𝑠3
𝑠2
𝑠1
𝜓1 (𝑡)
𝑠6
𝑠8
𝑠7
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