Simulation of GPRS
MS-BS Interface
Using
ns-Network Simulator
Dual Degree Presentation
by
Richa Jain (96D07007)
Department of Electrical Engineering,
Indian Institute of Technolgy,
Mumbai, India.
Outline
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Introduction to GPRS
About ns-Network Simulator
Scope of the Project
Implementation details
Experiments
Concluding remarks..
The General Packet Radio Service
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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
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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 script
set ns [new Simulator]
#Creating two nodes
set 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 so
set cbr0 [new Agent/CBR]
$ns attach-agent $n0 $cbr0
#create sink
set 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
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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 OFF
set node_(0) [ $ns_ node 1.0.1]
$node_(0) base-station [AddrParams addr2id [$BS(0) node-addr]]
#create and attach Agents – TCP/UDP/CBR
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#include movement..
$ns_at 10.0 “$node(0) setdest 200.0 150.0 15.0
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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
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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
Transmission over the air
interface (in GPRS)
200bytes
A radio block
(RLC PDU)
TDMA
frames
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1
2
3
4
5
6
7
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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
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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
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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
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Studying the average packet delay
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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.