Data in GPRS Networks

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Data in GPRS Networks

Antti Kantee <pooka@cubical.fi>

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Overview of our Slight Plight

Antti Kantee, T-110.456 Next Generation Cellular Networks. 2004 .

GSM Data

what is needed to provide data on top of GSM what’s wrong with the picture

GPRS:

overall architecture

components & interconnections protocols

roaming

problems with GPRS

PoC: GPRS use in "real life"

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GSM Data: packets in the circuit

GSM data is circuit switched

establish connection, use connection, disband connection setup time

charging is time-based

Internet-connection via (separate) ISP

GSM -> modem -> PSTN -> modem -> Internet GSM -> ISDN -> Internet

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High-Speed Circuit Switched Data (HSCSD)

one GSM TCH/F provides 9.6kbps user data total number of TCH’s available: 8

very simple idea: use several TCH/F’s

up to 57.6kbps total speed

8x9.6 = 76.8kbps, but A I/F is limited to 64kbps

de/multiplexing handled at MS and BSC+MSC

transparent to RF

transparent to outside network

transparent to whoever uses the stream

symmetric and asymmetric connections

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SMS: User and Control Plane Mixup

(ab)use signalling plane for transmitting user data

people have been doing this forever, e.g. non-answered phonecalls which "transmit" data using CID

main advantage: don’t need user-plane components for installations only wishing to do SMS

two types: broadcast and point-to-point

teenagers found mobile equipment, and the rest is history

SMS business view

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Quick Review of GPRS Itself

packet switched link

dynamic allocation of radio resources speeds up to 171.2kbps

that’s theory for you

"integration" with packet switched networks, such as the internet

billing on transfer volume, not connection duration

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image source: http://www.cs.hut.fi/~hhk/GPRS/gprs_own.html

GPRS Architecture: Overview

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GPRS: diffs to GSM

GPRS was designed as a plugin to GSM

business POV: easy to sell add-on to existing architecture

GGSN - GPRS Gateway Support Node

tunnel packets between SGSN and PSPDN using GTP

SGSN - Serving GPRS Support Node

mobility management and packet routing

PCU - Packet Control Unit

handle radio link layer for data frames

CGF - Charging Gateway Function

make sure nasty users are charged for all the trouble they cause

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GPRS Protocols: Signalling Plane (MS<->SGSN)

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GPRS Protocols: User Plane

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GPRS Protocols: SNDCP

Subnetwork Dependency Convergence Protocol used between MS and SGSN on user plane

maps external PDN to GPRS network

features:

multiplex PDPs onto LLC connections compression

de/fragmentation

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GPRS Protocols: LLC

Logical Link Control

provides a logical link between the MS and SGSN multiplexing between one SGSN and several MS’s abstracts radio characteristics

features:

un/acknowledged data transfer flow control

data confidentiality

variable-length frame support

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I heard it through the GPRSvine

as known, GPRS uses TDMA radio 52-multiframe structure

4 consecutive blocks in same slot = radio block RLC layer:

provide un/acknowledged transfer between MS and BSC

MAC layer:

provide medium access control (surprise!) collision avoidance, detection

arbitration

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Filling the slots

MS and BSS need to agree on which timeslots to fill - non-trivial problem

slots reserved for voice, but not used if silent slots reserved for data

slots available to GPRS users

predictive best-fit timeslot assignment, "tetris"

fixed allocation

dynamic allocation

extended dynamic allocation exclusive allocation

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GPRS Mobility Management

GPRS defines three different states for MM

tries to optimize battery consumption & uplink usage

idle:

network does not track MS location standby:

network knows MS location per RA busy:

network knows MS location per cell

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GPRS Session Management

before being able to actually transmit/receive PDU’s, the MS must establish a PDP context

e.g. dial "*99#" on your phone

MS sends activation request to SGSN, which selects GGSN for session

PDP context includes

PDP address for MS QoS parameters

PDP context will time out if not used

keepalives

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IP on GPRS (basically the previous slide again)

MS tells the SGSN that it wants to use IP

along with other parameters, such as desired IP address and QoS parameters

SGSN checks if MS has credentials for that and does authentication

also establish an encryption

SGSN selects GGSN for MS

establish a GTP tunnel

IP context created in MS, SGSN and GGSN

MS can now transfer packets into IP network and back

user plane protocol stack

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Roaming in GPRS

possibilities

1: exit PLMN as early as possible, use visitor location GGSN 2: exit PLMN from home location GGSN

the advantage of option number 1 is that it is smart from the POV of network topology

packets can directly be routed to the correct destination

however, option 1 makes billing difficult

therefore, option 2 is a winner

option 2 also makes mobility simple

you can always have the same PDP address

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Latency in GPRS

several factors contribute to latency in GPRS mobile station

process datagram and request radio resource

radio resource reservation

MS must request TBF

if none is active, negotiation can take a while usually source of significant portion of delay

transmit over air interface

MS -> BSS -> SGSN

transmit through core network

SGSN -> GGSN roaming?

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(source: SourceO2.com)

Some figures for latency

measured: 500 byte IP packet transmission MS delay: 50-200ms

TBF establishment

1 uplink TBF: 320-750ms

2 download TBF: 290-1700ms

once established, 0ms delay (surprise!)

air delay: 260-460ms

depends on number of TBF in use

core network delay: 20ms

non-roaming situation

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PoC: Push-to-talk over Cellular

use data channels to transmit voice

VoIP

structured so, that you can have calls "always open"

use tangent to signal when you’re actually transmitting GPRS: pay only for traffic

especially group calls benefit from this feature

GPRS & latency

"push-to-wait"?

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Wrapup

data on the Internet is packet-based

GSM data causes annoyances in Internet use

GPRS is packet-switched and easy to integrate with PDN’s

GPRS has been easy to plug into the existing GSM architecture

well, it was designed for that

GPRS falls short on a couple of aspects

data rates & latency issues

GPRS works "well ’nuf"

too well for 3G to be tempting?