Simulation of GPRS MS-BS Interface

Using ns-Network Simulator

Dual Degree Presentationby

Richa Jain (96D07007)

Department of Electrical Engineering,Indian Institute of Technolgy,

Mumbai, India.

Outline

• Introduction to GPRS• About ns-Network Simulator• Scope of the Project• Implementation details• Experiments• Concluding remarks..

The General Packet Radio Service

• Packet switched radio transmission • Suited for bursty applications (eg Internet)• Data rates upto 170 kbps• Re-uses GSM resources• Lower call set-up times; cheaper

GPRS Architecture

GPRS Protocol Stack

About ns

• Public domain• Event driven simulator• Split language – OTcl and C++• Nodes and links; routing algorithms; queuing

schemes• Protocols – TCP, UDP, IP• Traffic generators – CBR, Exponential, Pareto

• Basic mobility support.

A simple ns scriptset ns [new Simulator]#Creating two nodesset n0 [$ns node]; set n1 [$ns node]

#Creating a link btwn the nodes; prop delay=10ms; Drop tail.$ns duplex-link $n0 $n1 1Mb 10ms DropTail

#Nodes themselves don't send/rx data. Create an agent to do soset cbr0 [new Agent/CBR]$ns attach-agent $n0 $cbr0#create sinkset null0 [new Agent/Null]$ns attach-agent $n1 $null0

#connect the two agents $ns connect $cbr0 $null0

#Tell cbr0 when to start and stop$ns at 0.5 "$cbr0 start"$ns at 4.5 "$cbr0 stop“$ns run

Wireless Features

• Mobile node (ad-hoc node)• Air interface• Radio propagation model• Adhoc routing protocols• Mobile IP

Structure of a wireless node in ns

Sample script for a wireless simulation

# Set up hierarchical routing.# Specify topology.# Create ‘God’

# Create a Base Station$ns_ node-config -adhocRouting DSR -llType LL -macType 802.11 -ifqType DropTail -ifqLen 100 -antType OmniAntenna -propType TwoRayGround -phyType WirelessPhy

-topoInstance $topo -wiredRouting ON

-agentTrace ON -routerTrace ON -macTrace ON

-movementTrace OFF -channel $chan_

Sample script for a wireless simulation

set BS(0) [$ns_ node 1.0.0]$BS(0) random-motion 0 $BS(0) set X_ 1.0$BS(0) set Y_ 2.0

# create mobilenodes in the same domain as BS(0) $ns_ node-config -wiredRouting OFFset node_(0) [ $ns_ node 1.0.1]$node_(0) base-station [AddrParams addr2id [$BS(0) node-addr]]

#create and attach Agents – TCP/UDP/CBR : :

#include movement..$ns_at 10.0 “$node(0) setdest 200.0 150.0 15.0 : : :

Scope of the project

We simulate the network stack for the MS-BS interaction and focus on the handling of radio resources.

GPRS Air interface

• TDMA• Uplink-downlink skew of 3 slots• Physical channel = a slot on a frequency.• Logical channels – Broadcast/Control/Data.• Multiplexing logical channels onto physical

channels• Transmission bursts

A radio block(RLC PDU)

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3

200bytes

TDMAframes

Transmission over the air interface (in GPRS)

Features of our GPRS simulator

• The Set-up• A single cell• The wireless node now has LL, RLC, MAC for

GPRS.• Supports traffic in either direction : MS to BS or

BS to MS• BS can transmit/receive on multiple frequencies

GPRS sim

Air Interface / Channels

• Separate uplink/downlink frequencies.• TDMA structure with 8 slots per frame.• Uplink-downlink skew of 3 slots.• User defined options:

- number of frequencies- number of GPRS slots per frame

• Signaling channel – slot0, on uplink/downlink frequency 0.

GPRS sim

Transmission and reception over the air-interface

• RLC PDU size set to 50 bytes => 50 bytes transmitted per slot .• Propagation delay set to 0.• Timers model transmission and reception.• A ‘radio’ : switched ON during

transmission/reception, OFF otherwise.

GPRS sim

Signaling for Call Set-Up and Handling

• Resource Request Message• from MS to BS• on the Random Access channel (slot0, freq0) • lock IFQ• wait only one TDMA frame for reply

• Resource Reply Message• from BS to MS on allotted channel• update vlr_ tables• wait till MS receives reply before

transmitting

GPRS sim

Signaling….• Resource Release Message

• from MS to BS• on the dedicated slot• clear entries• later requests will be treated on par with fresh requests

GPRS sim

Resource Handling

• Slot Allocation • first free slot is alloted.• allocation symmetric on uplink and

downlink.• Slot Release

• MS initiates message on expiry of slot_release_timer.• BS deletes vlr_ entries on receipt of

message.• timer value based on experiments.

GPRS sim

Dealing with Exceptions

• Collisions• happen only on the Random Access

channel.• BS drops all colliding requests.• each MS waits one TDMA frame, then backs off for a

random interval.

• Slot level errors• error model introduced.• users may include/not; user defined rate.

• Multiple ARP requests• secondary requests dropped.

GPRS sim

Experiments

• Studying the average packet delay• Scenario• Observations• Inferences

GPRS sim

Average packet delay for various data rates

GPRS sim

Average packet delay for various data rates

GPRS sim

Average packet delay vs data generation rates

GPRS sim

Simulations …

• Resource release timer• Scenario• Observations• Inferences

• Interaction of LL/RLC and TCP’s ARQ• GPRS vs GSM capacity analysis

GPRS sim

Summarizing ….

• A simulator for the GPRS MS-BS interface was developed.

• Features of the simulator were tested.• Experiments on the average packet delay, slot release timer etc were performed.