GPRS AND EDGE

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30 Οκτ 2013 (πριν από 4 χρόνια και 11 μέρες)

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GPRS AND EDGE


-

GSM & TDMA are circuit switched 2G technologies that provide

a)

SMS

b)

Circuit switched data (9.6 kbps)

c)

FAX at 9.6 kbps


-

GPRS and EDGE are 2.5 G technologies


IS
-
136 (CDPD)


and provide upto 19.6
kps


-

UMTS is 3G technology.


-

In this module we sh
all study GPRS and EDGE technologies.


-

GPRS is a packet switched data Network that provides the speeds of 40 kbps to 53
kbps over the same basic air interface as GSM (200 kHz
, divided into 8 time slots).


-

Thus GPRS is built on the same infrastructure as GS
M and therefore requires no
special RF design or frequency planning.


-

However, with GPRS, the mobile station can have access to more than one TS and
the channel coding scheme is different.


-

The most common channel coding scheme used is CS
-
2 which gives app
roximately
10.4 kbps of data rate per time slot excluding OH (with OH it is about 13.4 kbps per
time slot).


-

Higher speed is achieved by allowing access of multiple time slots.


-

Since GPRS is a packet switched technology, the transmission occurs on a packe
t
-
by
-
packet basis. Thus the mobile uses the Air Interface (the radio resource) only when
sending and receiving data.


-

During the time that a user is not sending or receiving data (web browsing for
example), the TS on the Air Interface can be assigned to o
ther users.


-

Thus GPRS allows multiple users to share the air interface at the same time and this is
the biggest advantage of GPRS.



GPRS APPLICATIONS


a)

Email

b)

Web based application

c)

Fax service

d)

Text messages

e)

Multimedia

f)

File transfer

g)

Database access

h)

Telemetr
y

i)

Point of sale credit card transactions (especially in a flea market or taxi payment
where there is no modem).



GPRS ARCHITECTURE


-

GPRS is a solution for providing packed data service for mobile wireless networks.


-

The GPRS provides two independent solut
ions


one for the Air Interface and second
for the network.


-

Fig. below shows the Basic Architecture of GPRS.













Mobile

Laptop

B

S

C

GSM Circuit
Switched Core

Network

PSTN

GPRS Core
Network

IP

PDN, X.25,
ISDN,
ATM, FR

-

Fig. below shows the detailed architecture of the GPRS portion of the network.


Solid line represents the

data and signaling while dashed lines represent signaling only.




We shall revisit the fig. later.


DETAILS OF THE AIR INTERFACE


-

GPRS is a packet switching data service, overlaid on the GSM infrastructure.


-

GPRS service can b
e grouped into 3 classes:


a)

Class A: Simultaneous use of data and voice services. Thus class A
service allows a user to hold a conversation and transfer GPRS data at the
same time.


b)

Class B: Supports simultaneous GSM and GPRS attach but not
simultaneous
use of both services. Thus a class B user can be registered
on both services at the same time, but cannot use both services at the same
time.

M

O

B

I

L

E

BSC

PCU

MSC

/

VLR


SMSC

H

L

R


SGSN


SGSN


GGSN

Charging
G/W
Function
CGF

Billing
System

IP, PDN
OR
X.25,
ISDN,
FR,
ATM

G
b

G
s


c)

Class C: Can attach to only one service at a time.


-

As mentioned earlier, GPRS allows access to multiple slots.

It is also asymmetric in
the sense that downlink slots may be greater than uplink slots since higher data rates
are needed during download operation.
Table shows common multislot

class


Multislot Class

Downlad Slots

Uplink

2

2

1

4

3

1

8

4

1

12

4

2





GPRS AIR INTERFACE


-

GPRS uses the same Air Interface as GSM i.e. 200 kHz RF Carrier and 8 Timeslots,
however, at any given time, same of the slots may be carrying voice and same data.


-

This is achieved by using a different logical channel allocation an
d coding scheme.



GPRS LOGICAL CHANNELS


-

GPRS logical channels are called PACKET DATA CHANNELS (PDCH). They are
of 2 types

a)

PD Control Channel

b)

Packet Data Traffic Channel


-

When a TS is carrying GPRS data or signaling information, it is called PDCH.


FRAME

STRUCTURE


-

PDCH uses a 52
-
multiframe structure instead of 26 used for GSM as shown in the
fig. below.


52 TDMA frames

Radio
Block
0

Radio
Block
1

Radio
Block
2

T

Radio
Block
3

Radio
Block
4

Radio
Block
5

X

Radio
Block
6

Radio
Block
7

Radio
Block
8

T

Radio

Block
9

Radio
Block
10

Radio
Block
11

X


X = IDLE Frame

T = Packet Timing Control (PTCH)


-

As shown in the fig., out of 52 frames in a multiframe, 12 radio blocks carry user data
and signaling, 2 idle frames and 2 PTCCH frames.


-

With each radio block carr
ying 4 TDMA frames,

(4 x 12 = 48) + 2 IDLE + 2 PTCCH = 52 frames


-

The X frames are used by the mobile for signal measurements and PTCCH is used for
timing control.


GPRS AIR INTERFACE


-

GPRS uses the same frequency allocation as GSM, 200 kHz wide, except s
ame TS
may be carrying voice and some GPRS data.


-

GPRS uses a 52 frames multiframe structure instead of 26.


-

Each frame contains 8 timeslots. The uplink frame is offset by 3 timeslots from the
downlink as in GSM.


-

Fig. below shows this scheme.


-

52 frames are transmitted in 12 Radio Blcoks of 4 frames each + 2 Idle frames + 2
Timing control frames as shown below


0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 7

200
kHz

Frame 0

Frame 1

Frame 2







Ch 1

Ch

3

Ch

2


X = Idle frame, mobile can use it to transmit power management

T = is the timing control
frame used for timing control and synchronization



GPRS CHANNELS


PHYSICAL CHANNEL
S
:
-


-

A physical channel is a stream of all time slots in a particular position of each TDMA
frame as shown below.











-

In GPRS, physical channels are called P
acket Data Channels (PDCH).


-

The GPRS dynamically allocates the number of PDCH depending upon the traffic
requirements in the cell.





B0

B1

B2

T

B3

B4

B5

X

B6

B7

B8

T

B9

B1
0

B1
1

X

Frame

Frame

Frame

Frame

0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 7

0 1 2 3 4

5 6 7

0 1 2 3 4 5 6 7

One Mobile is using TS 3 of RADIO Block 4

0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 7

A

physical channel using TS 4

for a particular function

LOGICAL CHANNELS


-

A logical channel refers to the flow of information between two entities for a
particular purpose. Lo
gical Channels are carried on physical channels


-

GPRS has 4 kinds of logical channels (notice that in general all 4 will be called
PDCH in GPRS).


a)

Broadcast Control Channel (PBCCH):
-



BCCH is always on TS0


Used to broadcast system information to all mobi
les in a cell. The mobile uses
this information to access the Network in a packet switched mode.


b)

PCCCH:
-

The common control channels are used to transmit common control
signaling information for packet data. The information needed to use the CCCH
is bro
adcast on PBCCH. There are 3 kinds of CCCH



PPCH (Paging Channel) (Downlink)

-

Used to page the mobile for an incoming call.


-

This channel can be used for both packet data as well as circuit
switched data.




PRACH (Random Access Channel) (Uplink)

-

Used by the
mobile to request PDTCH.




AGCH (Access Grant Channel)

-

Used by the base station to assign a radio channel to the mobile
during call setup.


c)

Packet Data Traffic Channels (PDTCH):
-



-

Use
d to transmit user data traffic (uplink or downlink).


-

Temporarily dedica
ted to a user.


-

When the application demands higher data rate, multiple slots may be
assigned.


d)

Packet Dedicated Control Channels:
-



-

Used to transfer signaling and other information to a specific mobile.


-

There are 2 types of kinds:




Packet Associated Con
trol Channel (PACH)

-

Bi
-
directional channel used to pass signaling and other information
between the mobile and the Network during packet transfer. It is
associated with a given PDT
CH.

-

It is called packet associated because while mobile is transferring
dat
a in the uplink on PDTCH, it is listening to the same channel in
the downlink for acknowledgement from the network.




Packet Timing Advance Control Channel (PTCCH)

-

Used for timing control. Downlink only.



GPRS NETWORK ARCHITECTURE


PLMN
:
-

A Public Land Mo
bile Network. It is a cellular wireless network superimposed.


-

GPRS is effectively a packet data network superimposed upon a GSM network.


-

It provides packet data channel on the air interface as well as a packet data
switching network that is separated fr
om the GSM switch and transport network.


-

Fig. shows the GPRS network Architecture.


-

Notice the following new Network Elements in the GPRS architecture

a)

Packet Control Unit (PCU)

b)

Serving GPRS Support Node (SGSN)

c)

Gateway GPRS Support Node (GGSN)

d)

Charging Gat
eway Function(CGF)


SGSN (Serving Gateway Support Node)


-

SGSN provides the same functions for a GPRS network what MSC/VLR provides
for GSM network.


-

Mobile Stations access the GPRS Network through SGSNs. Thus, SGSN provide
a point of attachment for the GP
RS mobiles through the BSS.


-

The SGSN is responsible for the transport and delivery fo packets to and from the
mobiles.


-

It therefore keeps track of the current location of the mobiles attached to it.


-

Like a MSC in GSM, it perform following function.



Mobi
lity Management



Security, Authentication and Ciphering
and access validation



Co
-
ordinates paging, location update and SMS requests with MSC/VLR.


-

It interfaces with one or more BSS via Frame Relay interface (interface G
b
).


-

It also interfaces with the HLR
via G
R

(SS #7)



GGSN (Gateway GPRS Support Node)


-

Serves as the Gateway between the GPRS Network and other packet switched
data networks such as IP, ISDN, FR, ATM, etc.


-

It also performs routing of data with mobile stations at their current point of
attac
hments to SGSN.


-

It receives the location updates for SGSN and performs routing from mobile to
PDN (Packet Data Network) and vice versa.


-

Once the mobile activates
its Packet Data Address, the mobile is registered with
the corresponding GGSN.


-

The GGSN mai
ntains a routing table associating the GPRS mobiles active in the
system with a particular SGSN.


-

There may be several GGSNs within a PLMN with each of these GGSN may
have multiple SGSNs connected to it. Also, one GGSN may connect to several
different ext
ernal PDNs.


-

The GGSN connects to SGSN via IP Backbone GPRS Tunneling Protocol (GTP).
It tunnels packets through an IP backbone between SGSN and GGSN.


-

The GGSN performs Mobile Station Address Management and Activation. It
assigns an address to the mobil
e if required and also activates this address.


PCU


-

PCU is typically a part of BSC. It performs the following functions.



Air Interface Access Control



Packet Scheduling on the Air Interface



Packet Assembly and Re
-
assembly


GPRS ATTACH & DETACH PROCE
DURE


1.

Carrier Select
:
-

When a GPRS mobile is turned ON, it looks for a radio carrier that is
carrying either PBCCH or BCCH (depending upon what kind of station it is).


2.

PLMN Selection
:
-

It sends the information for the PBCCH/BCCH and selects the
most sui
table PLMN.


3.

Cell Selection:
-

It then selects the strongest cell (closest cell).


4.

GPRS Attach:
-

It then attaches to the most suitable SGSN.


5.

Camp ON:
-

It then listens to the Control channels of the selected cell. This is called
“CAMP ON”.


6.

PDP Control
Activation:
-

The mobile then performs the PDP Control Activation to
activate an address for the mobile. It uses this address to exchange traffic.


7.

Cell Reselection (Hand off):
-

The process of moving to a new cell


8.

GPRS Detach:
-

The mobile may de
-
regist
er itself anytime to detach itself from the
network by
turning the power off.


See fig. for PDP Control Activation.









Mobile

BSS

SGSN

GGSN

HLR/AUC

Channel Request

Packet Update Assignment

Activate PDP Control Activation Request

Authentication

Create PDP Control

BSS Packet Flow

Activate PDP Control Accept

1

2

3

4

DATA TRANSFER IN GPRS VIA TUNNELING


-

Consider the fig shown in which a mobile is communicating with a co
mputer
connected to the Internet (Fig 7.5 Mullet, Page 308).


-

Notice that tunneling is used in both directions.


-

A packet containing the PDP ADDRESS of the mobile arrives at GGSN from the
external computer using regular IP Protocol.


-

GGSN encapsulates it i
n another header (similar to packing a letter in an envelop)
and forwards it to SGSN to which the mobile is attached currently.


-

The SGSN removes the envelop and forwards the packet to the mobile.


-

Rules are reversed for traffic flowing from mobile to the
computer. SGSN
encloses the packet unit on envelop and forwards it to GGSN. GGSN removes
the envelop and forwards the packet to the Internet based on the regular IP
Address.



GPRS PROTOCOL REFERENCE MODEL

Fig. 7.6, Page 309 Mullet


-

Fig. shows GPRS Proto
col reference Model.


-

Notice the use of

a)

ISDN Signaling over the air interface between the MS and BS.

b)

The FR, a wireline protocol, to provide packet data communication
between the BSS and SGSN.

c)

The GPRS Tunneling Protocol (GTP) between the GPRS support nod
e
(GSN).

d)

TCP/UDP to carry GTP packets between the GSNs.

e)

BSS GPRS (BSSGP) Protocol to provide routing between the BSS and
SGSN.


-

GPRS system supports IP and X.25 delivery from end to end.


-

Also notice that both the BSS and SGSN have the protocol stacks to d
eal with the
completely different media and transport technologies used for the air interface
U
m
, between the mobile and the BSS, the wireline interface G
b
, using FR between
the BSS and the SGSN, and the G
n

interface that consists of an IP backbone
between

the SGSN and GGSN.





MS PROTOCOL STACK


It consists of

a)

RF Interface (Radio Channel)

b)

MAC (Media Access Control)

c)

RLC (Radio Link Control)

d)

LLC (Logical Link Control) performs framing between the MS and SGSN,
detection and recover of lost or corrupted frame
s, flow control , ciphering
(encryption).

e)

SNDCP (Sub Network Dependent Convergence Protocol)



Enables support for Multiple network protocols



Data Compression



Multiplexes several packet streams into a single logical link


BSS


-

FR is used to relay frames to M
S from G
b

interface to U
m

interface and also from
G
b

to G
n

interface



GPRS SIGNALING PROTOCOL





GMM/SM

LLC






RLC


MAC


RF


GMM/SM

LLC






BSSGP

NS


Layer 1


RLC

MAC

RF


BSSGP

NS

Layer 1

Relay

MS

U
m

BSS

G
b

SGSN

-

Upto LLC it is the same as the protocol reference model.


-

Top layer is GSM Mobility Management and
Session Management (GSM/SM)
pro
tocol instead of SNDCP.


-

GSM/SM is used for routing area updates, security , authentication, Packet
session establishment, activation and deactivation.




EDGE (Enhanced Data Rate for Global Evolution)


-

EDGE is an Enhancement to GSM/GPRS that allows GSM se
rvice providers to
use their present equipment yet offer higher data rates.


-

The Network Architecture for EDGE is the same as that of GSM/GPRS



Same Network Elements



Same Network Interfaces



Same 200 kHz Channel Structure



Same Radio frequency bands



Same FDMA
/TDMA structure with 8 time slots/radio channel



Same Protocol reference models


-

The only difference is that EDGE uses a combination of GMSK + 8 PSK or
8
-

PSK modulation scheme over the air interface to provide higher data rates.


-

In GSM the symbol ra
te is 270.833 kbps with each symbol represents 1 bit.


-

EDGE uses 8
-
PSK coding scheme with each group of 3 bits represents one
symbol as shown in the fig.


-

The symbol rate is still 270.833 kbps, but with each symbol now representing 3
bits, the data rate is

effectively tripled to 812.5 kbps.


-

The table shows the
net GPRS and EDGE user data notion/time slot (table 7.1,
Mullet). Notice that there are net user data rates of time slot excluding all
overheads.


-

With MSC


9, EDGE can theoretically support upto 4
73.6 kbps net user data rate
with 8 time slots.


-

The actual data rates depends upon the number of timeslots allotted and the level
of MSC employed.


-

The 8
-
PSK modulation is sensitive to noise and prone to errors.


-

The level of MSC used is determined by the

radio channel condition and is
automatically adjusted by the system in response to measured transmission bit
error rates.


-

Mobiles closer to BS experience less interference and closer to boundaries will
experience the largest interference.


-

Thus the syste
m automatically adjusts the MSC level as the mobile moves away
from the BS and gets close to the boundaries.


-

The MSC
-
level 9 will be used closer to BS and MSC
-
1 closer to boundaries.



8


PSK Relative Phase Positions

0,1,0

0,1,1

1,1,1

1
,1,0

1,0
,0

1,0
,
1

0,
0
,
1

0,0
,0