Wireless networks - Lecture 18: GPRS: General packet radio service (Part 2)

Wireless Networks

Lecture 18
GPRS: General Packet Radio Service (Part II)
Dr. Ghalib A. Shah

1


Outlines
 GPRS Protocol Architecture
►
►
►
►







MS – BSS
BSS – SGSN
SGSN – GGSN
GGSN – PDN

GPRS Air Interface
Data Routing and Mobility
Uplink Data Transfer
Downlink Data Transfer

QoS in GPRS
2


Last Lecture






Introduction to GPRS
GPRS Architecture
Registration and Session Management
Routing Scenario in GPRS
Channels Classification

3


Logical Channels
 Mapped by the MAC to physical channels
 Control channels for control, synchronization
and signaling
• Common
• Broadcast
• Dedicated

 Packet Traffic channels
► Encoded speech

► Encoded data

4


Control Channels
 Packet Common Control Channel (PCCCH)
► When allocated in a cell, GPRS related mobiles
camp on it
► Divded into
• Random Access (PRACH): MS initiate packet transfer or
respond to paging messages
• Paging (PPCH): to page an MS prior to packet transfer
• Access Grant (PAGCH): send resource assignment to MS
prior to packet transfer
• Packet Notification (PNCH): used to send a PTM-Multicast
notification to group of MS

5


Control Channels
 Packet Dedicated Control Channel (PDCCH)
• Slow Associated Control Channel (SACCH)
–
–

Radio measurements, power control and data
SMS transfer during calls

• Fast Associated Control Channel (FACCH)
–
–

For one Traffic Channel (TCH)
Carry Ack

• Stand-alone Dedicated Control Channel (SDCCH)
–

is used in the GSM system to provide a reliable connection for
signalling and Short Message Service.

6


Control Channels
 Packet Broadcast Control Channel (PBCCH)
► Frequency correction channels
• Allows the MS to synchronize their Local Oscillator (LO) to
the Base Station LO, using frequency offset estimation and
correction.

► Synchronization channel (MS freq. vs. BS)
► Broadcast control channel for general information on
the base station

7

GPRS Architecture
Other GPRS
PLMN
GGSN

Gp

BSC Gb

SGSN
Gf
Gs

BTS

MS

Gn

BTS

EIR

Gr

D

Gc GGSN Gi
HLR

MSC/VLR

8

PDN


Protocol Architecture
Trans mis s io n Plane
► The protocols provide transmission of user data and
its associated signaling 
 S ig naling  Plane
► Comprises protocols for the control and support of
functions of the transmission plane


9


Transmission Plane


GPRS  Bac kbo ne :S GS N GGS N
►



S ub­ne two rk de pe nde nt c o nve rg e nc e  pro to c o l
►



It is used to transfer packets between SGSN and MS

Data link laye r
►
►



GTP tunnels the user packets and related signaling information
between the GPRS support nodes.

LLC(MS-SGSN)
RLC/MAC(MS-BSS)

Phys ic al laye r
►
►

PLL:channel coding,detection of errors, forward error correction,
interleaving, detection of physical link congestion
RFL:modulation and demodulation

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    MS

BSS

Application
Network Layer
SNDCP
LLC
RLC

Relay
RLC              BSSGP

MAC

MAC        Network
                 Service

PLL
RFL

PLL                      
                     PHY 
RFL             Layer

     Um
  
SNDCP : Sub­network dependent convergence protocol
LLC     : Logical link control
RLC     : Radio link control
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Radio Link Control
 Can provide reliability for MAC transmissions
 Transparent mode
► No functionality

 Acknowledged mode
► Selective Repeat ARQ
► Sender: Window
► Receiver: Uplink ACK/NACK or Downlink ACK/NACK

 Unacknowledged mode
► Controlled by numbering within TBF
► No retransmissions
► Replaces missing packets with dummy information bits

12


Media Access Control (MAC)
 Performs contention resolution between
channel access attempts
 Connection oriented
 Connections are called Temporary Block Flows
(TBF)
► Logical unidirectional connection between two MAC
entities
► Allocated resources on PDCH(s)
► Temporary Flow Identity (TFI) is unique among
concurrent TBFs in the same direction
13

MAC: Channel Access & Resource Allocation
 Slotted Aloha
► Used in PRACH
• MSs send packets in uplink direction at the beginning of a
slot
• Collision: Back off ->timer (arbitrary) ->re-transmit

 Time Division Multiple Access (TDMA)
► Predefined slots allocated by BSS
► Contention-free channel access

14


Network Layer
(IP or X.25)
Relay
SNDCP                  GTP      

GTP

      
LLC              TCP/UDP
      

TCP/UDP

BSSGP               IP      

IP

MAC             Network 
                     Service

Network       Data Link 
Service            Service  

Data Link 
Layer

PLL                              
RFL            Phy Layer

Phy Layer      Phy Layer

Phy layer

Relay
RLC                    BSSGP

Gm
Gb
BSS
SGSN
GGSN
RLC :Radio link control                 BSSGP:BSS GPRS Application protocol
PLL  :Physical link layer                GTP     :GPRS tunneling protocol
RFL :Physical RF layer                  TCP      :Transmission control protocol

MAC:Medium access control         UDP     :user datagram protocol
                                                         IP         :Internet Protocol

Transmission Plane

15

Gi


GPRS Air Interface
Time Slot Number
0

1

2

3

4

5

6

7

0

1

2

3

4

F1

Uplink

F2
F3

  Carrier
Frequency

F4
0

1

2

3

4

5

6

7

0

1

2

3

F
1
F
2
F
3
F
4

4

Downlink

Voice User1

GPRS User1

Voice User2

GPRS User2

GPRS User3

16


GPRS Air Interface
 Master slave concept
•
•
•

One PDCH acts as Master
Master holds all PCCCH channels
The rest of channels act as Slaves

 Capacity on demand
•

PDCH(s) are increased or decreased according to
demand
• Load supervision is done in MAC Layer

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Uplink Data Transfer

MS
PRACH or RACH
PAGCH or AGCH
PACCH
PACCH

PDTCH
PACCH
PDTCH
PACCH

BSS
Packet channel Request
Packet Immediate assignment
Packet resource Request
Packet resource assignment
Random Access
Transmission

Frame Transmission
Negative Acknowledgement
Retransmission of blocks in error
Acknowledgement
18


Downlink Data Transfer
MS
PPCH or PCH
PRACH or RACH

PAGCH or AGCH
PACCH
PACCH or PAGCH 

PDTCH
PACCH
PDTCH
PACCH

BSS

Packet paging request
Packet channel Request
Packet Immediate assignment
Packet paging response
Packet resource assignment

Paging
Transmission

Frame Transmission
Negative Acknowledgement
Retransmission of blocks in error
Acknowledgement

19


Mobility
 A mobile station has three states in GPRS

system:
► Idle
► Standby
► Active

 The operation of GPRS is partly independent of
the GSM network. However, some procedures
share the network elements with current GSM
functions.
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 Data is transmitted between a mobile station and the
GPRS network only when the mobile station is in the
active state.
 In the active state, the SGSN knows the cell location of
the mobile station.
 In the standby state, the location of the station is known
only as to which routing area it is in.
 In the idle state, the mobile station does not have a
logical GPRS context activated or any Packet-Switched
Public Data Network (PSPDZ) addresses allocated,
The MS can receive only those multicast messages
that can be received by any GPRS mobile station.
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QoS Support
 Assumes that IP multimedia applications are able to
► Define their requirements

► Negotiate their capabilities
► Identify and select available media components

 GPRS specifies signaling that enable support for
various traffic streams
► Constant/variable bit rate
► Connection oriented/connection less
► Etc.

22


QoS Profile for GPRS Bearers
 4 parameters:
► Service precedence
•

3 classes

► Reliability parameter
•

3 classes

► Delay parameters
•

4 classes

► Throughput parameter

•

Maximum and mean bit rates

23


QoS Profile for GPRS Bearers
 QoS profile is included in Packet Data Protocol
(PDP) context
 Negotiation managed through PDP procedures
(activation, modification and deactivation)

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Conclusions
 S ame  GMS K mo dulatio n as  GS M
 4 c hanne l c o ding  mo de s
 Pac ke t­mo de  s uppo rting  up to  abo ut 144 
kbps
 Fle xible  time  s lo t allo c atio n (1­8)
 Radio  re s o urc e s  s hare d dynamic ally 
be twe e n s pe e c h and data s e rvic e s
 Inde pe nde nt uplink and do wnlink re s o urc e  
allo c atio n
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