GSM 2.5G Migration

Course 335

GSM 2.5G Migration:
GSM 2.5G Migration:
General Packet Radio Service GPRS
General Packet Radio Service GPRS

10-2001 GSM 2.5G Migration: GPRS v1.28 (c)2001 Scott Baxter 5-1

What’s GPRS All About? How Does It Fit In?

s GSM: Global System for Mobile Communication
• The world’s most widely used wireless phone technology
– Over 500,000,000 users worldwide!
– TDMA-based radio interface, 200 kHz.-wide signals
• But very limited data capability
– 9,600 or 14,400 bps maximum in circuit-switched mode
s WCDMA / UMTS: The Long-Term 3G Data Solution
• Uses spread-spectrum CDMA techniques, 4-MHz.-wide signals
• Provides both voice and high speed packet data access
• But not widely deployed and available until 2003 or later
s GPRS: General Packet Radio Service
• A packet-switched IP-capable way of using GSM radio infrastructure
• Defined in 1996, wide deployment beginning in 2001
• Provides both interim pre-WCDMA and long-term packet access


Communications Technology Family History
A Story of Births, Weddings and Funerals

s Commercial telegraphy gave birth to telephony, then died
s Telephony and Land Mobile Radio married, giving IMTS & Cellular
s IP networks developed, their usage and bandwidth are increasing
s The wedding of IP and Wireless is happening now in 3G!

Land Mobile Radio Extinction?
HF, VHF, UHF, Trunked

IP Networks
The Internet Voice over IP

Wireless Voice and IP Data
IMTS-Cellular-GSM-GPRS-WCDMA

Commercial Switched Telephony Extinction?
Digital Switching

Commercial Telegraphy Extinction!

50 60 70 80 90 10 20 30 40 50 60 70 80 90 10 20 30 40 50
1800s 1900s 2000s

GSM and GPRS

Background: GSM Technology
Background: GSM Technology
The Foundation of GPRS
The Foundation of GPRS


The Beginnings of GSM

s 1980’s: Europe used variety of first generation analog cellular
systems: TACS, ETACS, NMT450, NMT900, Netz, etc.
• Operation was limited to various national boundaries
• Poor roaming capabilities, poor economies of scale in mfg.
s In 1982, CEPT the Conference of European Posts and Telegraphs
created a group to study and define a 2G Pan-European system
• Group Spécial Mobile (GSM)
• In 1989, administration of GSM was transferred to the
European Telecommunications Standards Institute (ETSI)
• In 1990, the GSM specification, Phase I, was published
s GSM has become very popular due to many positive factors
• Non-proprietary: anyone can manufacture networks/handsets
• Thorough/integrated standard: well-defined RF air interface,
network architecture, call delivery and roaming features


GSM World Acceptance

s GSM commercial deployment began
in 1991
s By 1993, there were 36 GSM
networks in 22 countries
s In 2000, there were over 200 GSM
networks in over 110 countries
around the world
• Operation in 900 MHz., 1800
MHz., and 1900 MHz. bands
s The wide acceptance of GSM has provided tremendous
economies of scale in network, handset, and test equipment
manufacturing and distribution
s Worldwide in 2001, GSM users have passed the 500 million mark
• One in 12 human beings uses a GSM phone!
s The global dominance of GSM provides a large market for the
2.5G and 3G enhancements GPRS and UMTS WCDMA

GSM vs. North American Standards

s Two different approaches to wireless technology development!
• Americans: Invent cool new stuff driven by market forces, write
standards if it works and the market accepts it
• Europeans: Study, Plan, build Standards, build Consensus, Plan,
Review, build more Consensus, finally Deploy
s The differences are visible in the resulting standards
• American: multiple interim standards necessary to define functionality
• Europeans: single integrated standard covers all functionality

North American CDMA GSM

Other Features IS-637 IS-683 IS-707 Etc.
SMS OTA Data
Intersystem Roaming, The GSM Standard
IS-41C, D, P
Call Delivery, Handoff One coordinated, uniformly
structured family of documents
Network Architecture IS-634 A-interface

Air Interface
IS-95/J-Std 008 CDMA
RF Architecture

GSM Terminology

s Some terms have different
meanings when used in GSM
Sector or North American practice! Cell
α α

CELL Cell BTS Cell
Sector Sector
γ β γ β

It’s a Sector! It’s a Cell!

Sector Sector Cell Cell
γ β γ β

That was a Handoff! That was a Handover!
The frequencies used The frequencies used
by each sector are by each cell are
its channel set. its allocation.


Structure of a GSM Signal

s GSM carriers are spaced 200 kHz.
apart
s In the BTS downlink signal, different 8 Slots
Required
1
2
C/I ≅ 9-12 dB
timeslots belong to different users - 4
3
a mobile listens only to its recurring
timeslots 200 kHz
Typical Frequency Reuse N=4
• During unused timeslots, a
mobile can measure the signal
strength of surrounding BTSs to
guide the handover process
s The mobile on its uplink transmits
only during its assigned timeslots
• Mobiles transmit only during BTS
their own timeslots
• Mobile transmit timeslots occur
three timeslots after the
corresponding BTS transmit
timeslot
– This avoids simultaneous
mobile TX/RX and the need
for duplexer at the mobile

The Frequencies Used by GSM
Europe and International
GSM Uplink GSM Downlink
124 ch. 124 ch.
890 915 MHz. 935 960

North American PCS Licensed Blocks
A D B E F C1 C2 C3 A D B E F C1 C2 C3
75 ch. 25 75 ch. 25 25 25 25 25 75 ch. 25 75 ch. 25 25 25 25 25
1850 1865 1885 1900 1910 1930 1945 1965 1975 1990
MHz.

s GSM operates in a variety of frequency bands worldwide
s GSM carrier frequencies are normally assigned in 200 KHz.
Increments within the operator’s licensed block of spectrum
s Spectrum is provided in “blocks”
• Base stations transmit in the upper block
• Mobiles transmit in the lower block
s Each cell uses a certain number of carriers, called its “allocation”

10-2001 GSM 2.5G Migration: GPRS v1.28 (c)2001 Scott Baxter 5 - 10

Multiple Carriers in a GSM Cell
Time
Frequency 6 TS1 TS2 TS3 TS4 TS5 TS6 TS7 TS8
1 timeslot 577 µs
1 frame 4.515 ms
s A GSM base station transceiver makes a signal ~240 kHz. wide
s The signal is time-divided into a repeating pattern of frames
• Each frame is 60/13 = 4.515 ms long, there are ~221.5 frames per second
s Each frame is further subdivided into 8 timeslots, each 15/26 ms = 577 µs long
• A timeslot can hold the bits of a channel of information
– One user’s voice signal, or a signaling/administrative channel
s One GSM base station can have several transceivers, each one producing a
GSM signal on a different frequency - six carriers in the example above
• Various repeating patterns of information can use the timeslots to carry
channels of information

Channels in GSM: Repeating Patterns

s Channels of information in GSM occupy physical timeslots of the GSM
signal in repeating patterns
• Similar to the way that classes and activities of a university occupy the
physical classrooms on a defined schedule
– Some classes meet daily, some only three days a week
– Some labs once or twice a week
– Meals daily in the cafeteria, movies on Friday nights
– Graduation ceremonies each semester
s Dedicated channels (carrying traffic or control information for individual
users) occur in a repeating 26-multiframe pattern 120 ms long
• 24 frames are used for traffic, one for SACCH, one is unused
• Full-rate TCHs occur in each traffic frame
• Half-rate TCHs (if used) occur in alternating traffic frames
• 1/8 rate dedicated channels are defined for special purposes and are
called SDCCHs (Stand-Alone Dedicated Control Channels)


GSM Traffic Channels:
Hyperframes, Superframes, Multiframes, Frames, and Bursts
One Hyperframe
2048 superframes 3h 28m 53.760s
0 1 2 3 4 5 2044 2045 2046 2047
51 multiframes of 26 frames each 6.120 s
One
Superframe 0 1 2 3 4 5 6 47 48 49 50

UNUSED
TCHs SACCH TCHs
One 26 Used for traffic channels and
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
0 1 2 3 4 5 6 7 8 9
Multiframe associated signaling only
26 frames 120 ms
One
BP 0 BP 1 BP 2 BP 3 BP 4 BP 5 BP 6 BP 7
Frame
1 frame 60/13 ms ~4.615 ms

Stealing Stealing
One Burst (156.25 bits)
Bit Bit
Tail Bits

Tail Bits
Training Guard
Data Bits Data Bits Bits
Sequence
3 57 bits 1 26 bits 1 57 bits 3 8.25 bits
15/26 ms
Gross Rate 270.833 kbps ~0.577 ms


GSM Control Channels:
Hyperframes, Superframes, Multiframes, Frames, and Bursts
One Hyperframe
2048 superframes 3h 28m 53.760s
0 1 2 3 4 5 2044 2045 2046 2047
26 multiframes of 51frames each 6.120 s
One
Superframe 0 1 2 3 24 25

not used
BCCH 1
BCCH 2
BCCH 3
BCCH 4

CCCH0 or
FCCH

FCCH

FCCH

FCCH

FCCH
SYS_INFO CCCH3 or CCCH4 or CCCH5 or CCCH5 or CCCH6 or CCCH7 or
SCH

SCH

SCH

SCH

SCH
7&8 CCCH 1 CCCH 2 SDCCH SDCCH SDCCH SDCCH SACCH SACCH
One 51 0
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
1 2 3 4 5 6 7 8 9
Multiframe
51 frames 235.38 ms
Used for control channels only
One BP 0 BP 1 BP 2 BP 3 BP 4 BP 5 BP 6 BP 7
Frame 1 frame 60/13 ms ~4.615 ms

Stealing Stealing
One Burst (156.25 bits)
Bit Bit
Tail Bits

Tail Bits
Training Guard
Data Bits Data Bits Bits
Sequence
15/26 ms
Gross Rate 270.833 kbps ~0.577 ms


Typical Timeslot Allocation in Multiframe Patterns
on One GSM RF Carrier
TIME
S S
TimeSlot T T T T T T T T T T T T A T T T T T T T T T T T T T T T T T T T T T T T T A T T T T T T T T T T T T

IDLE

IDLE
C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C
7 H H H H H H H H H H H H C H H H H H H H H H H H H H H H H H H H H H H H H C H H H H H H H H H H H H
H H
S S

IDLE

IDLE
H H
S S

IDLE

IDLE
H H
S S

IDLE

IDLE
H H
S S

IDLE

IDLE
H H
S S

IDLE

IDLE
H H
Frame 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 2 3 4 5 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
1
Number

26 Multiframe Pattern for Traffic Channels 26 Multiframe Pattern for Traffic Channels
S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S
D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D D A A A A A A A A A A A A A A A A
TimeSlot C

IDLE
IDLE
IDLE
C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C
C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C
1 H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H
0 0 0 0 1 1 1 1 2 2 2 2 3 3 3 3 4 4 4 4 5 5 5 5 6 6 6 6 7 7 7 7 0 0 0 0 1 1 1 1 2 2 2 2 3 3 3 3
A A A A A A A A A A A A S S S S S S S S S S S S S S S S S S S S
B B B B G G G G G G G G G G G G
TimeSlot F C C C C C C C C F C C C C C C C C F D D D D D D D D F C C C C D D D D F A A A A A A A A
C S C S S C C C C C C C C S C C C C S C C C C C C C C

IDLE
C C C C H H H H C C H H H H H H H H C C C C C C C C C C C C B B B B C C C C C C C C C C C C C C
0 C H H H H / / / / C / / / / / / / / C C C C C C C
H H H H H H H H H H H H H H H H H H H H H H H H H H
P P P P H P P P P P P P P H H H H H H H
1 2 3 4 C C C C C C C C C C C C 0 0 0 0 1 1 1 1 3 3 3 3 0 0 0 0 1 1 1 1
H H H H H H H H H H H H

Frame 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 3 3 3 3 3 3 3 3 4 4 4 4 4 4 4 4 4 4 5
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0
Number

51 Multiframe Pattern for Control Channels

A GSM Uplink Normal Burst

s GSM is a TDMA system and a mobile’s transmission bursts are carefully
constructed not to overlap with bursts from other mobiles
s Different propagation delays of mobiles near and far mobiles the BTS are
compensated by automatically advancing mobile transmit timing
s Special training sequences are included in each uplink burst and downlink
timeslot to facilitate demodulation
s During unused timeslots, a mobile measures the strength of surrounding
base stations to guide the handover process (this is called MAHO, Mobile
Assisted Hand Over)


GSM Bursts on the Uplink: 4 Types

Frequency Correction Burst or Dummy Burst
Guard
Tail

Tail
Fixed ‘0’ or Fill-in Bits
Bits
3 142 bits 3 8.25 bits

Synchronization Burst
Guard
Tail

Tail
Data Bits Training Bits Data Bits Bits
3 39 bits 64 bits 39 bits 3 8.25 bits

Access Burst

Tail
Tail Guard
Bits
Training Bits Data Bits Bits
8 41 bits 36 bits 3 68.25 bits
Stealing Stealing
Normal Burst Bit Bit
Guard
Tail

Tail
Data Bits Training Bits Data Bits Bits

GSM Channels
DOWNLINK CHANNELS UPLINK CHANNELS
BTS identity, channel allocation,
BCCH frequency hopping sequences
Slotted aloha channel used to
FCCH Provides frequency reference
request network access RACH
Defines burst period boundaries
SCH and time slot numbering
Carries pages to mobiles,
PCH alerting of incoming calls
Stand Alone Dedicated
AGCH
Allocates SDCCH to mobile to
Control Channel SDCCH
obtain dedicated channel after
a request on the RACH
Traffic Channel TCH
Fast Associated Control
BTS Channel FACCH
Slow Associated Control
Channel SACCH

0 to many F-TRAFFIC
Stand Alone Dedicated
SDCCH Control Channel

Traffic Channel
TCH
Fast Associated Control Channel
FACCH
Slow Associated Control Channel
SACCH


The GPRS Timeslot Allocation
s In conventional GSM, a channel is permanently allocated for a particular
user during the entire call period (whether speaking or silent, whether
transmitting data or not)
• In GPRS, the channels are only allocated when data packets are
transmitted or received, and they are released after the transmission
• For bursty traffic this results in much more efficient use of the scarce
radio resources
• Multiple users can share one channel
s GPRS allows a single mobile to
transmit and/or receive on multiple
timeslots of the same frame (this is
called multislot operation)
• This provides “bandwidth on BTS
demand” in a very flexible
scheme
• One to eight timeslots per frame
can be allocated to a mobile
• Uplink and downlink allocations •This GPRS mobile is in “3+1” timeslot mode
•3 timeslots assigned on downlink
can be allocated separately, •1 timeslot assigned on uplink
which efficiently supports
asymmetric data traffic (suitable
for web browsing, for example)

Allocation of GPRS Channels

s A cell supporting GPRS may allocate physical channels for GPRS traffic
s Such a physical channel is denoted a Packet Data Channel (PDCH)
• The PDCHs are taken from the common pool of all channels available
in the cell
• The radio resources of a cell are shared by all GPRS and all non-
GPRS mobiles in the cell
• The mapping of physical channels to either GPRS or GSM usage can
be performed dynamically, based on:
– Capacity on demand principle
– Depending on the current traffic load, priority of service, and the
multislot class
s A load supervision procedure monitors the PDCHs in the cell
s The number of channels allocated to GPRS can be changed according to
current demand
• Physical channels not currently in use by conventional GSM can be
allocated as PDCHs to increase the GPRS quality of service
• When there is a resource demand for services with higher priority,
PDCHs can be de-allocated


GSM, GPRS and WCDMA / UMTS

GSM, GPRS, WCDMA
GSM, GPRS, WCDMA
Coordinated Network Architecture
Coordinated Network Architecture


3 Steps to 3G: The GSM Transition
GSM TODAY
PLMN Core Network SIM
PSTN MSC BSC BTS Mobile
VLR Base Base
Gateway Mobile Station
ISDN Station Transceiver
Switching Controller Stations
MSC Mobile
HLR Center
Equipment
Internet
2.5G: GSM + GPRS
Core Network
VLR MSC
PLMN Mobile SIM
PSTN Gateway Switching Mobile
MSC Center
BSC BTS
HLR Base Base
Station
ISDN Gateway Serving Station Transceiver Mobile
GPRS GPRS PCU Controller Stations Equipment
Support Support
Internet node node

3G: UMTS, UTRA
Core Network MSC UTRAN
RNC Node B UMTS
PLMN VLR Mobile
PSTN Gateway Switching Radio SIM
Network
MSC Center Controller Node B User
ISDN HLR Equipment
Gateway Serving RNC
GPRS GPRS Radio Node B Mobile
Internet Support Support Network Equipment
node node Controller Node B


Architecture of a Phase-1 GSM Network
VLR Base Base
MSC Mobile
HLR Center
Equipment
Internet EIR AuC A Abis Um
Interface Interface Interface

GSM Functional Entities and Network Elements
PLMN - Public Land Mobile Network HLR - Home Location Register
PSTN - Public Switched Telephone Network VLR - Visitor Location Register
ISDN - Integrated Services Digital Network BSC - Base Station Controller
GMSC - Gateway Mobile Switching Center BTS - Base Transceiver Station
MSC - Mobile Switching Center SIM - Subscriber Identity Module
EIR - Equipment Identity Register ME - Mobile Equipment
AuC - Authentication Center MS - Mobile Station

s The network elements and interfaces of GSM are standardized
s This provides for inter-vendor participation in operators’ networks
• Competition improves quality, provides economies of scale

GSM Network Evolution and History
GSM TODAY
VLR Base Base
MSC Mobile
HLR Center
Equipment
Internet

s The present GSM network architecture emerged from work of the
ETSI in the late 1980s
s The GSM network can be divided into three main domains
• The Network Switching Subsystem (GMSC, VLR, HLR, MSC)
• The Operations and Support Subsystem (not shown, includes
OMC-R)
• The Base Station Subsystem BSS (includes BSCs, BTSs)


GSM Evolution: General Packet Radio Service

s Around 1994, the GSM phase 2 standards were enhanced to
include a number of new and improved services. These
enhancements became known as GSM Phase 2 Plus.
s One of the new features proposed in 1994 was a new bearer
service, true packet radio service known as GPRS
s GPRS allows a user with suitable mobile station to occupy multiple
time slots on a TRX, culminating in the possible occupancy of all 8
timeslots if they are available
• Data rates supported per timeslot are 9.06, 13.4, 15.6, and
21.4 kb/s
• When all 8 timeslots are available, throughput can reach 8 x
21.4 kb/s = 171.2 kb/s, although realistic expectations are
around 115 kb/s due to BCCH and other requirements
s GPRS applications are expected to include internet access/web
browsing, video and Road Traffic and Transport Informatics
(RTTI), and e-commerce and point-of-sale accounting


GPRS Network Architecture
VLR LEGEND
PLMN
PSTN MSC
Existing GSM Core Network elements
ISDN A New GPRS elements and interfaces
User data & signaling
Gs Signaling only
SGSN
of a HLR EIR SMSC
different SIM
PLMN Gp Ater Mobile
Gc Gr Gf TCU BSC BTS Station
Gd Base Abis Base
Station Transceiver Mobile
Controller Station Eqpm’t
PSPDN PCUSN
GGSN Gn SGSN Gb Agprs
Gi
Um
Interface
s The GSM network architecture was modified to add packet services, through the
addition of the new network elements GGSN and SGSN
• GGSN Gateway GPRS Support Node
– Responsible for routing data packets entering and leaving the radio
network; also as a router for packets within the network
• SGSN Serving GPRS Support Node
– responsible for packet delivery to mobiles in its area
– a type of packet switch with capability to interrogate the GSM
databases HLR and VLR for location and service profiles of mobiles
s Data is “tunneled” from the GGSN to the SGSN using GTP, GPRS Tunneling
Protocol, encapsulating packets de-encapsulating on delivery


Understanding the Backbone Networks
VLR LEGEND
PLMN
PSTN MSC
Existing GSM Core Network elements
ISDN A New GPRS elements and interfaces
User data & signaling
Gs Signaling only
SGSN
of a HLR EIR SMSC
different SIM
PLMN Gp Ater Mobile
Gc Gr Gf TCU BSC BTS Station
Gd Base Abis Base
Station Transceiver Mobile
Controller Station Eqpm’t
PSPDN PCUSN
GGSN Gn SGSN Gb Agprs
Gi
FRAME RELAY Um
IP or X.25 Interface

s Gb between SGSN-PCUSN uses Frame Relay protocols
s Gn between SGSN-GGSN uses IP routing, GPRS Tunnel Protocol
s Gr between SGSN-HLR is an extension of MAP
s Gi between GGSN and PDNs uses IP and X.25
s Gd between SGSN-SMSC delivers SMS messages using MAP
s Gc between GGSN-HLR is optional, uses MAP
s Gs between SGSN-MSC/VLR is optional, uses BSSMAP


GPRS Backbone Networks
s Two kinds of GPRS
backbones:
• Intra-PLMN among
GSNs of same PLMN
(private, IP-based)
• Inter-PLMN among
GSNs of different
PLMNs (roaming
agreements)
s Gateways between the
PLMNs and the external
inter-PLMN backbone are
called Border Gateways
• Border Gateways perform security functions to prevent unauthorized
access and attacks
s The Gn and GP interfaces are also defined between two SGSNs
• This allows exchange of user profiles as mobiles move around
s The Gf interface allows a SGSN to query the IMEI of a registering mobile
s The Gi interface connects the PLMN to external public or private PDNs
• Interfaces to IPv4, IPv6, and X.25 networks are supported
s The Gr interface allows an SGSN to communicate with an HLR

GPRS-GSM Coordination

s The MSC/VLR may be
extended with functions
and register entries for
efficient coordination
between GPRS packet
switched and GSM
circuit-switch services
• Combined GPRS
and non-GPRS
location updates
s Paging requests for circuit-switched GSM calls can be performed via
the SGSN
• The Gs interface connects the databases of SGSN and MSC/VLR
s The Gd interface allows short message exchanges via GPRS
• Gd interconnects the SMS gateway MSC (SMS-GMSC) with the
SGSN

GPRS Services

s GPRS bearer services provide end-to-end packet-switched data transfer.
There are two kinds:
s PTP Point-to-Point Service, available now, has two modes:
• PTP Connectionless Network Service (PTP-CLNS) for IP
• PTP Connection-oriented network Service (PTP-CONS) for X.25
s PTM Point-to-Multipoint Service (available in future releases)
• PTM-M Multicast Services broadcasts packets in certain geographical
areas; a group identified indicates whether the packets are intended
for all users or for a group
• PTM-G Group Call Service addresses packets to a group of users
(PTM group) and are sent out in geographical areas where the group
members are currently located
s SMS Short Message Services
s Supplemental Call Services:
• CFU Call Forwarding Unconditional, CFNRc Call Forwarding
Subscriber Not Reachable, CUG Closed User group
s Non-Standard Services may be offered at GPRS service providers
• Database access, messaging, e-transactions, monitoring, telemetry


The Stages of the GPRS Specifications

Stage 1
s The GPRS specification is 02.60 GPRS Service Description (overview)
built in three stages
s Stage 1 describes the basic Stage 2
GPRS Service Description
service capabilities 03.60
(System and Architecture)
03.64 Radio Interface Description
s Stage 2 describes the specific
system and network Stage 3
architectures and the radio 04.60 MS-BSS: RLC/MAC layer descriptions
interface description 04.64 MS-SGSN: Logical Link Control
04.65 MS-SGSN: SNDCP
s Stage 3 provides details of the 07.60 GPRS Mobile Stations
link control layer entities, 08.14 Gb (BSS-SGSN) layer 1
specifications of the mobile 08.16 Gb (BSS-SGSN) network service
08.18 Gb (BSS-SGSN) BSSGP
stations, and details of the
09.16 Gs (MSC/VLR-SGSN) layer 2
internal network element 09.18 Gs (MSC/VLR-SGSN) layer 3
interfaces and their protocols 09.60 Gn and Gp GPRS Tunneling Protocol


GPRS Initial Release Features

s All network manufacturers are expected to support IP and
interworking with both internet and intranet in their first product
release
• To support this functionality, some form of server functionality
must be provided
– Domain Name Server (DNS) is required to translate
between domain names and IP addresses
– Dynamic Host Configuration Protocol (DHCP) is required to
allow automatic reassignment of addresses for mobile
hosts
s In early networks, a single SGSN will probably be sufficient due to
the gradual growth of users and traffic as mobiles become
available
s The connection between the GGSN and the MSC/VLR, HLR, and
SMSC will require a gateway using SS7/IP or SIG to link the IP
backbone with the interfaces to these network elements


GPRS

A Closer View of the GPRS
A Closer View of the GPRS
Internal Interfaces and Elements
Internal Interfaces and Elements


Serving GPRS Support Node (SGSN) Functions

s The Serving GPRS Support Node
(SGSN) is responsible for the following
to and from the mobile stations in its
service area:
• Packet Routing and Transfer
• Mobility management (attach/detach
and location management)
• Logical Link management
• Authentication and charging
functions, encryption
• Compression (optional)
• Location register of SGSN stores
location (cell, vlr) and user profiles
s A typical PLMN network will start with
only one SGSN
s Each BSC has a Packet
Communications Unit, PCU Several models of the
Nortel Passport Switch
• Similar hardware provides the for SGSN and PCUSN service
PCUSN function

Gateway GPRS Support Node (GGSN) Functions

s The Gateway GPRS Support Node
(GGSN) is the interface between
external packet data networks and
GSM backbone network
• Converts GPRS packets from
the SGSN into packet data
protocol format (IP, X.25) for Nortel’s GGSN:
the external networks Bay Contivity Extranet Switch
• Converts PDP addresses of CES-4500
incoming data packets to GSM
address of destination user,
and forwards to responsible s Initial GPRS traffic in a PLMN
SGSN network will be low, and a single
GGSN will suffice for first service
• GGSN stores the current SGSN and an appreciable time
address of the user and the thereafter
user’s profile in its location
register
• GGSN performs authentication
and charging functions
• Performs tunneling


GSM BTS Changes Required to Support GPRS
Three Possible GPRS BSS Configurations
s Since GPRS uses new coding Um Gb
schemes, a Channel Codec Interface Interface

Unit (CCU) is required CCU BTS BSC
PCU
• The CCU can normally be CCU
SGSN

implemented within BTS
software PCU in BTS
Advantage: short Round Trip Delay

s Timeslot allocation for GPRS Abis
is handled by a new Packet Interface
Controller Unit (PCU) which CCU BSC
also implements frame relay CCU
BTS PCU SGSN

connection with the GPRS
network PCU in BSC
• The PCU function can be Gb
physically implemented in Interface
the BTS, BSC, or at the CCU BSC
SGSN, but is conceptually CCU
BTS PCU SGSN
part of the BSS
PCU at SGSN
Advantage: Leverage -- 1 PCUSN can manage multiple BSCs


Channel Coding Implemented at the BTS
GPRS Coding Schemes
Pre- Infobits Parity Tail Output Punct Code Data
Coding Cod. Without Bits Bits Conv. ured Rate Rate
Scheme USF USF BC encoder Bits Kbit/s
CS-1 3 181 40 4 456 0 1/2 9.05
CS-2 6 268 16 4 588 132 ~2/3 13.4
CS-3 6 312 16 4 676 220 ~3/4 15.6
CS-4 12 428 16 456 1 21.4

s Channel coding is used to protect the transmitted GPRS data
packets against errors
• The channel coding in GPRS is very similar to that of GSM
– An outer block coding, an inner block coding, and an
interleaving scheme are used
s Four different coding schemes are defined in the table above
s As of mid-2001, network manufacturers were only implementing
CS-1 and CS-2


The MS-SGSN Interface

s Packet Controller functions are provided by
the PCU, which is implemented in a
Physical BSC A VLR
physical PCUSN in the BSS TCU
Ater MSC
• The PCUSN handles the GPRS-
specific packet processing using frame Gb
relay protocols BSC PCUSN SGSN
Agprs
• The PCUSN connects to the BSC with
network manufacturers’ proprietary Agprs
BTS
interfaces
• The PCUSN connects to the SGSN via
the standard-defined Gb interface MS
s Although a PCUSN can optionally serve
more than one BSC, all channels from one PCUSN and PCU Distinction
•A PCUSN (Packet Controller Unit Serving
BSC must pass through the same PCUSN Node) is the hardware unit which implements
s TRAU frames from the mobile pass through the PCU (Packet Controller Unit) function

the BTS to the BSC and on into the PCUSN


MS-SGSN Logical Link Control (LLC)
GMM SNDCP SMS GMM SNDCP SMS

LLC LLC
Relay
RLC RLC BSSGP BSSGP

MAC MAC Network Svc Network Service
GSM/RF GSM RF L1 L1

MS Um BSS Gb SGSN

s LLC provides the reliable link between MS and SGSN
s LLC supports these layer-3 Protocols:
• SNDCP Sub-Network Dependent Convergence Protocol
• GMM/SM GPRS Mobility & Session Management
• SMS Short Message Service
s Protocols supported by the LLC provide:
• Data ciphering for security
• Flow control; sequential order of delivery; error detection/recovery
• Acknowledged and Unacknowledged data transfer modes
s The LLC provides transparency - the lower level radio link protocols are
not involved and do not affect the GPRS applications running above


MS-SGN Service Access Points (SAP) for LLC

s The LLC provides six service access points (SAP) to the upper
layers
• Each SAP has its own Service Access Point Identity (SAPI)
s The SAPs include:
• GMM/SM - Service for Signaling for Session/Mobility
Management
• SMS - Short Message Service
• QoS1 Packet Transmission SNDCP access
s Frames are assembled/disassembled using a multiplex procedure
• A logical link management entity (LLME) manages resources


The Gb Interface: PCU-SGSN
SMS-GMSC OTHER
SMS-IWMSC GPRS PLMN
s The SGSN and the PCUSNs of each BSC Gd Gp GG
SN
are linked by a backbone network using SG
Frame Relay protocol over the Gb interface BTS BSC
Gb SN
GG Gi
• Data rate can be up to 2 Mbps Gf Gn SN
BTS Gs Gr PDN
• Frame relay protocol implementation is
actually simpler than X.25 Gc
EIR
s Layers at each node of the Gb : MS
MSC D HLR
• Physical Layer VLR

• Network Service Layer (NS)
• Base Station Subsystem GPRS
Protocol (BSSGP)
• Network Management (NM)
– GPRS Mobility Management
(GMM)
– LLC/Relay


PCU and SGSN Operation over the Gb Interface
s BSSGP: Base Station Subsystem
GPRS Protocol
User Packets User Packets
• Provides flow control, manages Via RLC and Via RLC and
buffers, provides services for
MAC layers MAC layers

the higher layers
s GMM - GPRS Mobility Management Relay GMM NM LLC GM NM
• Manages mobility features for
users, such as location updating BSSGP L3 BSSGP
and paging L2
s NM - Network Management Network Services Frame
Relay
Network Services

• Manages flow control, buffers, Gb
virtual pathways between Physical Physical
PCU/SGSN PCU SGSN
s Network Services
• Implements the communications
protocol for the Gb interface
(Frame Relay)
s Physical Layer
• Hardware and physical nature
of the interface


The Gn Interface: SGSN-GGSN
SMS-GMSC OTHER
SMS-IWMSC GPRS PLMN
s The SGSN and GGSN are linked by a Gd Gp GG
SN
GPRS backbone using IP routing SG
BTS BSC
s The Gn interface creates and operates Gb SN
GG Gi
through secure tunnels, using the Gf G SN
Gr n
GPRS Tunneling Protocol (GTP) BTS Gs PDN

s The GTP packet headers include EIR
Gc

• Tunnel endpoint and group identity MS
MSC D HLR

• PDU type VLR

• QoS parameters
• Routing protocol identification
– Static, RIP2, OSPF
s Beneath IP, any transport architecture
can be used
• Ethernet, Token-Ring, FDDI, ISDN,
ATM


The Gp Interface: SGSN - Other PLMN GGSN
SMS-GMSC OTHER
SMS-IWMSC GPRS PLMN

s The Gp interface connects an SGSN of Gd Gp GG
SN
one PLMN with a GGSN of another SG
BTS BSC
PLMN Gb SN
GG Gi
Gf Gn SN
s This interface forms an inter-PLMN BTS Gs Gr PDN
backbone providing mobile IP Gc
capability for roaming mobiles EIR
MS
s Specific configuration of this link MSC D HLR
VLR
depends on the features intended by
the two PLMN operators, as well as
dimensioning issues


More About GPRS Tunneling on Gn and Gp
GPRS TUNNELING PROTOCOL (GTP) SMS-GMSC OTHER
SMS-IWMSC GPRS PLMN
PROTOCOL STACK
Gd Gp GG
IP SN
GMM/SM SNDCP GTP GTP SG
IP BTS BSC SN
LLC UDP/TCP Gn, Gp UDP/TCP Gb GG Gi
BSSGP IP IP Gf Gn SN
NS L2 L2 L2 BTS Gs Gr PDN
L1B1s L1 L1 L1 Gc
EIR
SGSN GGSN
MS
MSC D HLR
VLR
s GPRS Tunneling Protocol (GTP) is used to carry user packets
between nodes
• GTP allows various protocols and is adaptable to both inter-
and intra-PLMN GGSN interfaces
– UDP/IP if reliable link is not required, TCP/IP if required


The Gn Interface: SGSN-GGSN
SMS-GMSC OTHER
SMS-IWMSC GPRS PLMN
s The SGSN and GGSN are linked by a Gd Gp GG
SN
GPRS backbone using IP routing SG
BTS BSC
s The Gn interface creates and operates Gb SN
GG Gi
through secure tunnels, using the Gf G SN
Gr n
GPRS Tunneling Protocol (GTP) BTS Gs PDN

s The GTP packet headers include EIR
Gc

• Tunnel endpoint and group identity MS
MSC D HLR

• PDU type VLR

• QoS parameters
• Routing protocol identification
– Static, RIP2, OSPF
s Beneath IP, any transport architecture
can be used
• Ethernet, Token-Ring, FDDI, ISDN,
ATM


The Gi Interface: GGSN-PDN
SMS-GMSC OTHER
SMS-IWMSC GPRS PLMN
s If the Gi interface is implemented via a Gd Gp GG
SN
public network, IP Security Protocol SG
(IPSEC) can be used to provide link BTS BSC
Gb SN
GG G
authentication and encryption Gf Gn SN i
• This allows use of public networks BTS Gs Gr PDN
such as the internet while Gc
maintaining confidentiality of data EIR
MS
s The GGSN creates VPN tunnels using MSC D HLR
VLR
security protocols like IPSEC if needed
s Four tunneling protocols are available:
• PPTP (client-initiated)
• L2F, L2TP (implemented on ISP
side)
• IPSec (layer-3 secure protocol)
s Transparent and Non-Transparent
modes are available


The Gr Interface: SGSN-HLR
SMS-GMSC OTHER
SMS-IWMSC GPRS PLMN

s The Gr interface is an extension of the Gd Gp GG
SN
GSM-MAP (mobile application part) SG
BTS BSC SN
s Most network manufacturers use an Gb GG Gi
Gf Gn SN
SS7 gateway element to provide BTS GsG PDN
interworking between the GPRS r
Gc
network and the SS7-based voice EIR
network MS
MSC D HLR
VLR
• This relieves the SGSN from having
to do SS7 processing
• The SS7 gateway can be a
conventional server, usually with
redundancy features on both the
SGSN (IP) and SS7 sides


The Gd Interface: SGSN-SMCS GMSC/IWMSC
SMS-GMSC OTHER
SMS-IWMSC GPRS PLMN

s The Gd interface delivers SMS Gd Gp GG
SN
messages via GPRS in the same SG
BTS BSC
manner as the GSM-MAP Gb SN
GG Gi
Gf Gn SN
BTS Gs Gr PDN
Gc
EIR
MS
MSC D HLR
VLR


The Gf Interface: SGSN-EIR
SMS-GMSC OTHER
SMS-IWMSC GPRS PLMN

s The Gf interface connects the SGSN Gd Gp GG
SN
and the Equipment Identity Register SG
BTS BSC
(EIR) Gb SN
GG Gi
Gf Gn SN
BTS Gs Gr PDN
Gc
EIR
MS
MSC D HLR
VLR


The Gs Interface: SGSN-MSC/VLR
SMS-GMSC OTHER
SMS-IWMSC GPRS PLMN

s The Gs interface is optional Gd Gp GG
SN
• Provides simultaneous GPRS and BTS BSC
SG
SN
GSM operation between SGSN and Gb GG Gi
Gf Gn SN
MSC/VLR (same as BSSMAP but BTS Gs Gr PDN
optional) Gc
EIR
MS
MSC D HLR
VLR


The Gc Interface: GGSN-HLR
SMS-GMSC OTHER
SMS-IWMSC GPRS PLMN

s The Gc interface is optional Gd Gp GG
SN
• Provides the same functions as the BTS BSC
SG
SN
MAP between GGSN and HLR Gb GG Gi
Gf Gn SN
BTS Gs Gr PDN
Gc
EIR
MS
MSC D HLR
VLR


The D Interface: MSC/VLR - HLR
SMS-GMSC OTHER
SMS-IWMSC GPRS PLMN

s The MAP-D interface is used by both Gd Gp GG
SN
GSM and GPRS networks to SG
BTS BSC
communicate between the HLR and Gb SN
GG Gi
the VLR in the MSC Gf Gn SN
BTS Gs Gr PDN
s This link is specified in the GSM-MAP Gc
and is not changed in GPRS EIR
MS D HLR
MSC
VLR


Quality of Service
Reliability
Probability of
Service Precedence Lost Dupli- Out-of Corrupt-
Class
High Packet cated Sequence ed
Packet Packets Packets
Medium 1 109 109 109 109
2 104 105 105 106
Low 3 102 105 105 102

s Mobile packet applications have a wide range of reliability expectations --
real-time multimedia, Web browsing, email transfer
s QoS Classes settable per session are a very important feature
• Service Precedence
– Priority of a service in relation to other services
• Reliability
– Required transmission characteristics (3 classes defined)
• Delay
– Maximum values for mean delay and 95-percentile delay
• Throughput
– Maximum-Peak bit rate and the mean bit rate

Quality of Service: the Delay Parameter

Delay
128 byte packet 1024 byte packet
Class Mean 95% Mean 95%
Delay Delay Delay Delay
1 <0.5s <1.5s <2s <7s
2 <5s <25s <15s <75s
3 <50s <250s <75s <375s
4 Best Effort Best Effort Best Effort Best Effort

s Using these QoS Classes, QoS profiles can be negotiated
between the user and the network for each session, depending
on QoS demand and currently available resources.
• Billing is based on data volume, type of service, and QoS
profile


Mobile Classes and Simultaneous Usage
s In a GSM network, two classes of service can run concurrently:
• Circuit-Switched Services (speech, data, and SMS)
• Packet-Switched Services (GPRS)
s Three Classes of Mobile Stations are defined:
• Class A mobiles
– Support simultaneous operation of GPRS and conventional GSM
services, but two separate radio chains are required
• Class B mobiles
– Able to register with the network for both GPRS and conventional
GSM services simultaneously, but can only use one of the two
services at a given moment - voice can pre-empt data
• Class C mobiles
– Able to attach for either conventional GSM or GPRS, manually
switched
– Simultaneous registration (and usage) is not possible, except for
SMS messages which can be received and sent at any time


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