GPRS and UMTS

T-110.300/301

Global Packet Radio Service

• GPRS uses the time slots not used for circuit switched services

• Data rate depends on the availability of free time slots– GPRS uses the multislot technique, but within one

frequency– It also can change the coding schema used for error

correction – Theoretical maximum rate is 171.2 kbps

– 10-40 kbps more realistic• Enhanced Data rates for Global Evolution (EDGE)

provides higher data rates

GPRS Network

• GPRS uses the same Radio Access Network (RAN) as GSM– A Packet Control Unit is added to the BSS

• Form the BSS the data packets are tunneled over the GSM backbone network

• Important new elements are– Serving GPRS Support Node (SGSN)

– Tracks the location of the MS – Provides routing and mobility management– Authenticates the MS– Manages the session– Collects billing data– IP packets from the MS are treated as IP packets first time here

– Gateway GPRS Support Node (GGSN)– Connects the GPRS network to other networks, e.g. the Internet

– GSM VLR and HLR are used

GPRS

MSBTS BSC MSC, VLR

HLR

GMSC

PSTN

Base Station Subsystem Network and SwitchingSubsystem

SGSN GGSN

Internet

GPRS Handovers

• There are no GPRS handovers as such– Since there is no circuit to hand over

• The MS requests a cell reselection and the packets are routed to the new cell

• Requires dynamic routing

GPRS Connection Protocol Stack

GSM RF

MAC

RLC

LLC

SNDCP

IP/X.25

Application

L1bisGSM RF

NetworkMAC

BSSGPRLCRelay

L1L1bisL2Network

IPBSSGP

UDP/TCP

LLC

GTPSNDCP

Relay

L1L2

IP

UDP/ TCP

GTPIP/X.25

MS BSS SGSN GGSN

GPRS Protocols

• BSSGP, BSS GPRS Protocol• GTP, GPRS Tunneling Protocol• LLC, Logical Link Protocol• MAC, Medium Access Control• GSM RF, GSM Radio Physical Layer• SNDCP, Subnetwork Dependent Convergence• UDP, TCP and IP in their usual roles• X.25 not widely implemented

GPRS Mobility Management

• A GPRS node may be in three states– Idle - no MS location information is kept by the network, no

active PDP (Packet Data Protocol) context – Busy - The location is known at cell level, an active PDP

context to an access point name is established– Standby - The location is known at the Routing Area Identifier

(RAI) level, the PDP context is active– A timeout moves the MS from Busy to Standby state, helps to

reduce battery consumption • GPRS attachment

– The MS request a PDP context from the SGSN– The SGSN selects the GGSN to be used based on the Access

Point Name– Internet, WAP gateway, a corporate network

– The SGSN queries the HLR to authenticate the MS and verify the request

EDGE

• Enhanced Data rates for Global Evolution• Up to 348 kbps• Initially developed for operators who failed to get

UMTS spectrum• Uses different modulation to squeeze more data

into same timeslot as GPRS• Requires hardware support in both the handset

and the BTS and software in the BSC

UMTS

• Universal Mobile Telecommunications System– A.k.a. 3G

• Provides:– More of the same as GSM/GPRS/EDGE

– Up to 2 Mbps promised data rate– 144 kbps a more realistic bit rate

– VoIP instead of PSTN calls?– Video phone, other multimedia services– Global roaming (almost)– Convergence for the operators– More effective radio spectrum usage

UMTS

MS BTS BSCMSC, VLR

HLR

GMSC

PSTNGERAN

SGSN GGSN

Internet

UE BS RNC

UTRAN

Circuit switched

Packet switched

UMTS RAN

• UMTS requires a new Radio Access Network– The Wideband CDMA-based UTRAN (Universal Terrestrial

RAN) provides the higher data rates– 1885-2025 MHz (uplink) – 2110-2200 MHZ (downlink)

– The GSM/EDGE GERAN can be used with UMTS– Might stay for a long time in the less populated areas

• New names for new components– Radio Network Controller– Base Station– User Equipment

• New functions– Cell breathing– Soft handover

Cell Breathing

• The CDMA technology enables multiple transmitters to use the same frequency and timeslot, since the transmitter and receiver are synchronized by the code

• Each transmission appears like background noise to other receivers

• However this reduces the signal to noise ratio– Usually referred as Signal to Interference Ratio (SIR)

• In practice CDMA based cells shrink when more traffic is added to the cell

• This is known as cell breathing and it adds a new challenge to the network design– The cell can handle more customers, but its size changes

Soft Handover

• As the UE is promised a certain Quality of Service in the beginning of an active service connection the handover issue is more complex than in 2G services, where the only service is the basic voice call– Multimedia calls have variable QoS requirements

• In a soft handover the UE has multiple radio connections to base stations – Due to CDMA technology all the base stations are using

the same frequencies (wideband transmission)• A more complex procedure than hard handover,

intended to provide seamless service quality

How to Make More of Little?

• The IP header is about 20 bytes, TCP too• The MAC layer of the RAN makes all connections point to

point• So do we really need to transport all these bytes?• Instead the two parties can agree on the header content and

set up a set of labels or abbreviated headers– Makes IP stateful over at least one hop– Enables also fast retransmission– IP and TCP or UDP headers can be replaces with 1-3 bytes

• Several methods, see RFCs 1144, 2507, 2508, 3095• Recommended in UMTS

IP Addresses and user's rights

• The current IPv4 address space will not support all cellular users

• If we use NAT/PAT and 10.0.0.0/8 we get IP connectivity to outside services, BUT– It is difficult to have servers inside the cellular packet network– Or peer-to-peer services

• So extending IPv4 with NAT creates an un-equal network of clients and servers

• IPv6 is the most obvious solution • Also note that having a public IP address is not always an

advantage, traffic flooding attacks not only provide DoS, they also show up in the communications bill

UMTS Releases

• 3GPP R99 a.k.a R3– The first specification– An UMTS RAN adjacent to existing GSM/GPRS network– Packet switching based backbone for all traffic– Otherwise uses existing components (MSC, SGSN etc.) with

some upgrades• R4

– Minor enhancements– Packet switching based backbone for GSM/GPRS also– MSC becomes an MSC server

• R5– All IPv6 network and services– IP Multimedia Subsystem provides access to the PSTN

– Session Initiation Protocol used for signalling– GSM/GPRS RAN exists still

UMTS Services

• UMTS produces new service architectures• Customized Application for Mobile-Enhanced Logic

(CAMEL) is a standard for providing IN services to GSM and UMTS – Handles the roaming issue, when a customer is under a

Visiting MSC and wants to use an IN service specified in the HLR

• Wireless Application Protocol and other information browsing methods

• Location based services• Rich calls• Presence services• Open Mobile Alliance standardizes the service platforms,

http://www.openmobilealliance.org/– Roughly modeled after IETF, but corporate driven

Convergence

• With the UMTS R5 all IP backbone the telecoms industry will move over to using TCP/IP technology

• The phenomenon of traditional telecom and datacom technologies converging is a major issue to the industry

• 4G ideas will continue to drive this convergence – Can a VoIP telephone call be handed over from the

WLAN/Internet/SIP/RTP technologies to UMTS/PSTN technologies?

UMTS International Separation

• WCDMA vs. CDMA2000• The "European" UMTS is based on a set of standards called

WCDMA and developed by the 3GPP (3:rd Generation Partnership Project), http://www.3gpp.org/

• The "American" CDMA2000 is standardized by 3GPP2, http://www.3gpp2.org/

• Asian standards bodies, manufacturers and operators are active in both

• The reasons behind the division are political and commercial

• Both organizations are providing open standards and try to use work from the IETF as much as possible

• Also national standardization in large Asian countries, mostly China, Korea and Japan

4G - after UMTS

• A combination of UMTS, WLAN, Bluetooth and other similar technologies is often interpreted as 4G

• No formal definition yet• Data rates of 20 - 100 Mbps?• WLAN is already coming to the new high end

terminals, enabling e.g. VoIP• 4G is more about the services than the actual

networking technologies