Advanced Computer Networks 263-3501-00

Exercise Session 4

Qin Yin

Spring Semester 2013

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Administration

• If you haven't received any email about your submission – We got your solutions for A1 & A2

• About solutions – Use your own words, minimize copy-paste

– Be precise about your points

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COPE: Wireless Network Coding

• Consider the following scenario:

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COPE Approach

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Increased throughput: saved transmission can be used to send more data

COPE – Snooping

• Every node snoops on all packets

• A node stores all heard packets for a limited time

• Node sends Reception Reports to tell its neighbors what packets it heard – Reports are piggybacked on packets

– If no packets to send, periodically send reports

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COPE – Coding

• To send packet p to neighbor A, XOR p with packets already known to A – Thus, A can decode

• But how can multiple neighbors benefit from a single transmission?

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Question 1

• What are the assumptions that COPE makes?

• What are the implementation related tricks that COPE plays?

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Basic loss estimation and rate change algorithm

• From “Robust Rate Adaptation for 802.11 Wireless Networks”, Mobicom'06: – Measure loss ratio over a window of packets

– Adjust the data rate accordingly

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Question 2

• In the paper, the author also proposed an improved algorithm with adaptive RTS filter. Explain briefly: – how that algorithm works?

– how it improves over the basic algorithm?

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Beacon transmission in a busy 802.11 infrastructure network

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Beacon transmission in a busy 802.11 ad-hoc network

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Power Management in 802.11

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Power Management in 802.11 (cont.)

• 802.11 access points periodically send a beacon with a timestamp to synchronize all stations

• Power management implemented using beacons: beacons may contain a TIM or DTIM

• TIM = Traffic Indication Map, announces a set of unicast packet transmissions – Contains the list of destinations of the buffered packets

• DTIM announces broadcast packet • Stations always awake to receive 'T' and 'D' • Stations stay awake if they are listed as receiver in 'D'

or 'T' packet

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Question 3

• You've seen power management in IEEE 802.11 infrastructure networks – Explain why this mechanism doesn't work for ad-

hoc networks.

– Propose a power management solution for IEEE 802.11 ad-hoc networks.

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The Advent of Cellular Networks

• Prior to cellular, mobile radio telephone system was based on: – High power/transmitter/receivers – Could support about 25 channels – In a radius of 80 km

• To increase the network capacity – Multiple low-power transmitters (100W or less) – Small transmission radius → area split in cells – Each cell with its own frequencies – The same frequency can be reused at sufficient

distance

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Question 4

• Why do mobile network providers install several thousands of base stations which is quite expensive, throughout a country, and do not use powerful transmitters with huge cells like for example radio stations use?

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The Hexagonal Pattern

• Used as an approximation for a cell • Equidistant access to neighboring

cells • Center to center distance d = ,

center to corner radius R • Each cell features on base station • Through power control cover the cell

area while limiting power leaking to co-frequency cells

• Frequency re-use not possible in direct adjacent cells

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3R

Minimum cell separation

• D = minimum distance between centers of co-channels cells

• N = # of cells in a repetitious pattern, i.e. reuse factor

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N = 4 N = 7

System capacity and interference

• S = Total # of duplex channels available for use • K = total # of duplex channels per cell • S = k*N

• If a cluster is replicated M times within a system, the total #

of duplex channels C can be used as a measure of capacity – C = MkN = MS

• Tradeoff: – If N decreases → k increases (since S is a constant) – If N decreases → M increases (for a fixed geographical area) – M increases → system capacity C increases – Price: D/R decreases → co-channel interference increases

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Cell splitting

• Smaller cells have greater system capacity – Better spatial reuse

• As traffic load grows, larger cells could split into smaller cells

• Requires careful power control and possibly more frequent handoffs for mobile stations

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Question 5

• Consider a cellular system in which total available voice channels to handle the traffic are 960. The area of each cell is 6 km2 and the total coverage area of the system is 2000 km2 – Calculate the system capacity if the cluster size, N

(reuse factor), is 4. – Do the same with N = 7 – How many times would a cluster size 4 have to be

replicated to cover the entire cellular area – Does decreasing the reuse factor N increase the

system capacity? Explain.

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GSM Architecture

• MS: Mobile Station • BTS: Base Transceiver

Station • BSC: Base Station Controller • MSC & GMSC: Switching

Centers • HLR & VLR: Location

Registers • RSS: Radio Subsystem • NSS: Network and switching

subsystem • OSS: Operation subsystem

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NSS Databases

• Home Location Register (HLR) – Contains permanent and semi-dynamic data of all

subscribers • Mobile subscriber ISDN number

• International mobile subscriber identity (IMSI)

• Current VLR and MSC

• Mobile subscriber roaming number (MSRN)

– This user-specific information exists once for each user in one HLR

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NSS Databases

• Visitor Location Register (VLR) – Local (dynamic) database for a subset of the user data – Includes data about all users currently in the domain

of the VLR – When an MS enters into the area of the VLR, the VLR

copies all relevant information from the HLR • Avoid frequent HLR updates and long-distance signaling with

the HLR

– VLR gets updated at MS startup and then periodically • If no update is received by VLR for some time it's considered

detached (e.g., roaming, new VLR takes over)

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Question 6

• How is localization, location update, roaming done in GSM and reflected in the databases? What are the typical roaming scenarios?

• (Optional) Looking at the HLR/VLR database approach used in GSM, how does this architecture limit the scalability in terms of users, and especially moving users?

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