Media Access Control and Example Wireless Networks

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Media Access Control and Example Wireless Networks

• Y. Richard Yang
• 09/25/2006

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Outline

• Admin. and recap
• Basic media access control
• Example networks

3
Admin.

• Questions on hw1
• problem g)
• Change of office hours
• Monday  4-5 pm (Hong)
• Tuesday  930-1030 am (Richard)
• Wednesday 1030-1130 am (Hong)
• Friday Richard 2-3 pm (Richard)
• Project suggestions
• http//zoo.cs.yale.edu/classes/cs434/projects/sugg
  estions.pdf
• please form groups two per group
• after you have formed a group, please make an
  appointment with me to discuss selection of
  project

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Recap Media Access Control

• Implements resource allocation
• reduce/avoid interference
• efficient utilization of network resources by
  multiplexing transmissions
• Dimension for multiplexing
• space division multiple access
• time division multiple access
• frequency division multiple access
• code division multiple access

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Recap CDMA

• CDMA (Code Division Multiple Access)
• Unique code assigned to each user i.e., code
  set partitioning
• All transmissions share the same frequency and
  time
• Each transmission uses DSSS, and has its own
  chipping sequence (i.e., code) to encode data
• e.g. code -1 1 1 -1 1 -1 1

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CDMA Deal with Multiple-User Interference

• Two codes Ci and Cj are orthogonal, if
• , where we use . to denote inner
  product, e.g.
• If codes are orthogonal, multiple users can
  coexist and transmit simultaneously with
  minimal interference

C1 1 1 1 -1 1 -1
-1 -1 C2 1 -1 1 1
1 -1 1 1 ------------------------
-------------------------- C1 . C2 1 (-1)
1 (-1) 1 1 (-1)(-1)0
Analogy Speak in different languages!
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Example CDMA/DSSS
tb
user data d(t)
1
-1
X
tc
code c(t)
-1
1
1
-1
1
-1
1
-1
1
-1
-1
1
1
1

resulting transmitted signal
-1
1
1
-1
-1
1
-1
1
1
-1
1
-1
-1
1
tb bit period tc chip period
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Generating Orthogonal Codes

• The most commonly used orthogonal codes in
  current CDMA implementation are the Walsh Codes
• every row is orthogonal to every other row and
  the log NOT of every other row

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Summary

• SDMA, TDMA, FDMA and CDMA are basic media
  partitioning techniques
• divide wireless channel into smaller pieces
  (cells, time slots, frequencies, codes) for
  multiple transmissions to share
• Question how to divide?

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Allocation Constraints

• Each transmission i (si-gtdi) is specified by
• transmission power pi
• time ti
• frequency band fi
• code ci
• Given any two transmissions (si-gtdi pi, ti, fi,
  ci) and (sj-gtdj pj, tj, fj, cj), we can
  determine if they conflict
• We want a schedule
• to maximize resource efficiency
• to allocate resource fairly
• to satisfy app requirements
• e.g. real time app (such as VoIP) need access
  with a bounded delay
• e .g. s wants to talk to d, but cannot reach d
  directly

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MAC

• The general case is challenging and largely still
  open
• We start with the simplest scenario (e.g., 802.11
  or cellular up links) in this class
• time sharing the same frequency and code
• a single receiver (the Access Point)

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Time Division

• Fixed allocation
• Centralized authority, e.g., polling
• Taking turns, e.g., token passing
• Distributed random access

• discussion compare the schemes in terms of
• efficient utilization of resources
• delay to access channel
• quality of service (e.g., a guaranteed rate)

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A
Random Access Slotted Aloha
B
Time divided into slots get a packet from
upper layer repeat at each slottransmit
with probability p until no collision
A
B
C
C
S
E
S
S
Used in GSM initial access channel
Question what is the efficiency?
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A
Random Access Carrier-Sense Multiple
Access/Collision Detection
B
get a packet from upper layer
transmit prob p 1 repeat while (network
busy) wait transmit with probability p if
detect collision abort backoff p p/2
goto repeat else finish
A
B
The algorithm is the foundation of Ethernet
Question is the algorithm effective in wireless?
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The Hidden Terminal Problem
B
A
C
D
E

• A is sending to B, but C cannot detect the
  transmission
• Therefore C sends to B
• In summary, A is hidden from C

Question why is the hidden terminal problem bad?
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Some Techniques to Address The Hidden-Terminal
Problem

• CSMA/CD -gt CSMA/CA (congestion avoidance)
• default in 802.11
• even if media is not sensed busy, transmits with
  only a probability
• in real implementation, with a random delay
• Busy-tone multiple access
• used in CDPD (cellular digital packet data)
• the base station sends a busy tone on the down
  link when receiving data
• Virtual carrier sense

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Virtual Carrier Sense RTS/CTS

• Short signaling packets (virtual carrier sense)
• RTS (request to send) and CTS (clear to send)
• contain sender address, receiver address,
  transmission duration, called network allocation
  vector (NAV)
• A node keeps quiet for NAV in CTS

B
A
C
D
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Comparisons

• Slotted Aloha
• very simple to implement but need clock sync low
  efficiency
• CSMA/CA
• simple to implement
• low efficiency
• Busy tone
• simple to implement but need a channel for busy
  signal may generate exposed terminals (we will
  see it later)
• Virtual carrier sensing (RTS/CTS)
• high efficiency but add control overhead, and
  energy consumption can be high because a node
  needs to monitor the environment all the time
• Idlereceivesend 11.051.4 Stemm and Katz
  1997 Digitan 2 Mbps WLAN 122.5

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PHY/MAC of Some Wireless Systems

• The Phy/Link layers are also called radio
  interface
• Many deployed systems
• wide-area cellular networks
• 2G (e.g. GSM, CDMA)
• 3G (e.g., W-CDMA, CDMA2000 1X EV-DO)
• WiMAX (802.16)
• WiFi (802.11a, 802.11b, 802.11g)
• Bluetooth
• I will cover a few you will encounter several
  others in homework 2

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Evolution of Mobile Telecommunication Systems
CT0/1
AMPS
CT2
NMT
IMT-FT DECT
IS-136 TDMA D-AMPS
FDMA
TDMA
EDGE
IMT-SC IS-136HS UWC-136
GSM
GPRS
PDC
IMT-DS UTRA FDD / W-CDMA
IMT-TC UTRA TDD / TD-CDMA
IMT-TC TD-SCDMA
CDMA
IS-95 cdmaOne
IMT-MC cdma2000 1X EV-DO
cdma2000 1X
1X EV-DV (3X)
2G
3G
1G
2.5G
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GSM Overview

• Formerly Groupe Spéciale Mobile (founded 1982)
• Now Global System for Mobile Communication
• GSM is a PLMN (Public Land Mobile Network)
• several providers setup mobile networks following
  the GSM standard within each country
• Today many providers all over the world use GSM
• by June 2006 2 billion subscribers (78 mil in US)

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GSM Services

• Telematic services voice communication
• mobile phones (voice)
• fax
• short message service (SMS)
• Bearer services transfer data with access points
• synchronous 2.4, 4.8 or 9.6 kbit/s
• asynchronous 300 - 1200 bit/s
• Supplementary services
• e.g., forwarding of caller number suppression of
  number forwarding, automatic call-back,
  conferencing with up to 7 participants

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GSM Radio Subsystem (PHY/MAC)

• A GSM operator operates at a given band
• In US, GSM 1900 MHz was initially used, with GSM
  850 being added in 2001
• A US operator obtains a license from FCC

http//wireless.fcc.gov/uls/
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GSM Radio Subsystem (PHY/MAC)

• A GSM operator uses TDMA and FDMA to divide its
  allocated frequency
• divide allocated spectrum into different physical
  channels each physical channel has a frequency
  band of 200 kHz
• if BPSK, what is the transmission rate supported?
  
• how about QPSK?
• partition the time of each physical channel into
  frames each frame has a duration of 4.615 ms
• divides each frame into 8 time slots (also called
  a burst)
• each slot is a logical channel
• user data is transmitted through a logical channel

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GSM - TDMA/FDMA
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Backup Slides
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Many Types of Logical Channels

• Control channels
• Broadcast control channel (BCCH)
• from base station, announces cell identifier,
  synchronization
• Common control channels (CCCH)
• paging channel (PCH) base transceiver station
  (BTS) pages a mobile host (MS)
• random access channel (RACH) MSs for initial
  access, slotted Aloha
• access grant channel (AGCH) BTS informs an MS
  its allocation
• Dedicated control channels
• standalone dedicated control channel (SDCCH)
  signaling and short message between MS and an MS
• Traffic channels (TCH)

• Example call setup from an MS

BTS
MS
SDCCH message exchange
Communication
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GSM Coding and Modulation Speech
Viterbis alg
Use GMSK, a variant of binary frequency shift
keying
See A brief Overview of the GSM Radio Interface
by Thierry Turlettihttp//citeseer.ist.psu.edu/5
15017.html
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GSM Coding and Modulation Speech

• Source speech coding
• one sample (260 bits) per 20 ms

Audio sample (20ms)
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Channel Coding

• Encode the audio sample using convolutional coding

Decoding using Vibertis algorithm, again !
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Interleaving

• Spread a single audio sample (20 ms) into 8 bursts

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GSM Data Services

• An MS can reserve a (logical) traffic channel
  (slot) to send data
• data rate standardized at 9.6 kbps
• Use High Speed Circuit Switched Data (HSCSD) to
  improve data rate
• allocate multiple traffic channels
• higher bandwidth
• different amount of bandwidth
• Problem of HSCSD
• data traffic is bursty reserving multiple slots
  is not efficient
• Solution General Packet Radio Service (GPRS)
• allocate multiple slots from the same frame
• only request slots when needed

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GPRS
PRACH Pkt. Random Access Channel PAGCH Pkt.
Access Grant Channel PTCH Pkt. Traffic Channel
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GPRS User Data Rates in kbps
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W-CDMA

• Spectrum
• 1920-1980 MHz uplink
• 2110-2170 MHz downlink
• Modulation QPSK
• CDMA coding
• CDMA with chipping rate fixed at 3.840 Mchip/s

Question how to allocate codes?
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Orthognal Variable Spreading Factor (OSVF)
1,1,1,1,1,1,1,1
...
1,1,1,1
1,1,1,1,-1,-1,-1,-1
1,1
1,1,-1,-1,1,1,-1,-1
...
1,1,-1,-1
X,X
1,1,-1,-1,-1,-1,1,1
1
X
1,-1,1,-1,1,-1,1,-1
X,-X
...
1,-1,1,-1
1,-1,1,-1,-1,1,-1,1
1,-1
SFn
SF2n
1,-1,-1,1,1,-1,-1,1
...
1,-1,-1,1
1,-1,-1,1,-1,1,1,-1
SF1
SF2
SF4
SF8
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GPRS Coding
g(1)(D) 1 D3 D4 g(2)(D) 1 D D3 D4
,
1000 ms / 240 ms/frame 48frame 114
bytes/frame 181/456 9.05kbps
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Random Access Ethernet
get a packet from upper layer K 0 cw
0 // K random wait time cw congestion
window repeat wait for K unit of transmission
time while (network busy) wait wait for
clearance time after the previous signal stops
transmit and detect collision if detect
collision stop and transmit a jam signal
double cw choose K randomly from 0,
1, 2, , cw. goto repeat
Question are there any things questions in
wireless networks?
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CSMA Exponential Backoff RTS/CTS/DATA/ACK

• Protocol
• begin
• while (channel busy) wait
• if channel idle
• send RTS
• if receive CTS
• send DATA
• wait for ACK
• if ACK
• return
• exponential backoff
• wait for the backoff period
• goto begin

• Summary
• Use RTS/CTS for virtual carrier sense
• Use ACK to improve reliability
• Missing CTS/ACK triggers exponential backoff

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GSM Elements
MS (mobile station) BSC (base station
controller) BTS (base transceiver station) MSC
(mobile switching center) GMSC (gateway MSC) HLR
(home location register) VLR (visiting location
register