Wireless Network

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Wireless Network TCP

• Dr. Chan Mun Choon
• School of Computing, NUS
• Jan 30, 2004
• CS 5229

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Admin

• About Me
• Joined SOC Dec 2003
• Member of Technical Staff in Bell Labs, Lucent
  Technologies from 1997- 2003
• Office S16 04-07
• Dr. Shorey will meet students on Feb 6 to talk
  about projects

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Overview

• Wireless Networks
• Cellular Network
• Wireless Local Area Network
• TCP over Wireless Networks
• Problems with TCP congestion control
• Solutions

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Wireless Comes of Age

• Guglielmo Marconi invented the wireless telegraph
  in 1896
• Communication by encoding alphanumeric characters
  in analog signal
• Sent telegraphic signals across the Atlantic
  Ocean
• Communications satellites launched in 1960s
• Advances in wireless technology
• Radio, television, mobile telephone

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Evolution of Cellular Wireless Network

• First Generation
• Analog
• AMPS North America
• Second Generation
• TDMA
• GSM (SingTel/M1, Europe, ATT)
• NA-TDMA IS-136 (ATT)
• CDMA (U.S.A.)
• Third Generation
• WCDMA (Europe, Singapore)
• CDMA2000 (U.S.A.)
• Fourth Generation
• OFDM, WLAN ???

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First Generation Analog System

• First Generation
• Advanced Mobile Phone Service (AMPS)
• Provide analog traffic channels
• Developed by ATT in 1970s
• Early deployment in 1980s
• gt 40 million users in 1997

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Going Beyond First Generation

• Capacity
• Increase capacity by operating with smaller
  cells, add spectrum, and/or use new technology to
  improve spectrum efficiency
• Roaming
• Requires information transfer and business
  arrangement between systems
• Introduce IS-41
• Security
• AMPS authentication procedures are weak
• Introduce robust network security technology
  based on encryption and secure key distribution
• Support for non-voice services

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Second Generation System

• Introduced in the early 1990s
• Digital traffic channel instead of analog
• Since data and control traffic are sent in
  digital form
• Encryption of traffic is simple
• Error detection and corrections can be applied,
  voice reception quality can be better
• Multiple channels per cell, as well as multiple
  users per channel (through TDMA or CDMA)

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Third Generation Systems

• Provides high-speed wireless communication for
  multimedia
• Voice quality comparable to PSTN
• Data 144kpbs for high-speed user (driving),
  384kpbs for slowly moving user (walking) and
  2.048Mbps for stationary user
• CDMA-based 3G systems more widely accepted
• CDMA 2000 in US
• UMTS in Europe
• 2.5G Systems
• EDGE, GPRS (GSM)
• 3G1x (2G CDMA)

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Multiple Access

• Wireless channel is broadcast channel, need to
  separate the desired signal from interfering
  signals
• Earliest approach is frequency division multiple
  access (FDMA)

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FDMA (Frequency Division Multiple Access)

• Similar to broadcast radio and TV, assign a
  different carrier frequency per call
• Modulation technique determines the required
  carrier spacing
• Each communicating wireless user gets his/her own
  carrier frequency on which to send data
• Need to set aside some frequencies that are
  operated in random-access mode to enable a
  wireless user to request and receive a carrier
  for data transmission

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TDMA(Time Division Multiple Access)

• Each user transmits data on a time slot on
  multiple frequencies
• A time slot is a channel
• A user sends data at an accelerated rate (by
  using many frequencies) when its time slot begins
• Data is stored at receiver and played back at
  original slow rate

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Frequency vs. time
FDMA
Frequency
Time

• In practical systems, TDMA is often combined
  with FDMA

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Duplex techniques

• Separates signals transmitted by base stations
  from signals transmitted by terminals
• Frequency Division Duplex (FDD) use separate
  sets of frequencies for forward and reverse
  channels (upstream and downstream)
• Time Division Duplex (TDD) same frequencies used
  in the two directions, but different time slots

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Examples

• FDD
• Cellular systems AMPS, NA-TDMA, CDMA, GSM
• TDD
• Cordless telephone systems CT2, DECT, PHS

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Frequency Band Usage
Frequency Range Example Usage
300Hz 3000Hz Analog telephone
300kHz to 3MHz AM Radio
3 to 30MHz Amateur Radio, international broadcasting (e.g. BBC)
30 to 300MHz VHF television, FM Radio
300 to 3000MHz UHF television, cellular telephone, PCS
3 to 30GHz Satellite communication, radar, wireless local loop
30 to 300GHz Experimental WLL
300GHz to 400THz Infrared LAN, consumer electronics
400 to 900 THz Optical communication
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Frequency Bands Usage Example
Frequency Range (MHz) Example Usage
824-849, 869-894 AMPS NA-TDMA/IS-136 CDMA/IS-95 CDMA2000 3G1x
902-928, 2400-2484 ISM (Industrial Scientific Medical)
890-915, 935-960 GSM
1710-1785, 1805-1885 3G
1850-1910,1930-1990 3G
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Issues

• Cellular networks have been traditionally
  designed mainly for voice applications. Next
  generation high speed wireless networks are
  expected to be data-centric. What are some of
  the components or assumptions that needs to be
  changed?

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Wireless MAC protocols
Wireless MAC protocols
Fixed-assignment schemes (GSM)
Random-access schemes (802.11)
Demand assignment schemes (HDR)
Circuit-switched
CL packet-switched
CO packet-switched
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Random access MAC protocols

• Comparable to connectionless packet-switching
• No reservations are made instead a wireless
  endpoint simply starts sending data packets
• Access to control channels in GSM uses random
  access protocols
• 802.11 uses CSMA/CA

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CSMA

• Carrier Sense Multiple Access
• sense carrier
• if idle, send
• wait for ack
• If there isnt one, assume there was a collision,
  retransmit

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Hidden Terminal Problem
A can hear B but not C and D B can hear A and C
but not D C can hear B and D but not A
D
B
C
A
C cannot detects transmission from A and thus
CSMA does not work when C starts transmission to B
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Mechanisms for CA

• Use of Request-To-Send (RTS) and Confirm-to-Send
  (CTS) mechanism
• When a station wants to send a packet, it first
  sends an RTS. The receiving station responds with
  a CTS. Stations that can hear the RTS or the CTS
  then mark that the medium will be busy for the
  duration of the request (indicated by Duration ID
  in the RTS and CTS)
• Stations will adjust their Network Allocation
  Vector (NAV) time that must elapse before a
  station can sample channel for idle status
• this is called virtual carrier sensing
• RTS/CTS are smaller than long packets that can
  collide

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Exposed Terminal Problem
A can hear B but not C and D B can hear A and C
but not D C can hear B and D but not A D can hear
C but not A and B
D
B
RTS
CTS
CTS
C
A
C cannot transmit to B even if it will not
interfere with transmission from B to A. As a
result, network throughput is reduced.
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IEEE 802 Protocol Layers
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Protocol Stack
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802.11 MAC

• IEEE 802.11 combines a demand-assignment MAC
  protocol with random access
• PCF (Point Coordination Mode) Polling
• CFP (Contention-Free Period) in which access
  point polls hosts
• DCF (Distributed Coordination Mode)
• CP (Contention Period) in which CSMA/CA is used

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Interframe Space (IFS) Values

• Short IFS (SIFS)
• Shortest IFS
• Used for immediate response actions
• Point coordination function IFS (PIFS)
• Midlength IFS
• Used by centralized controller in PCF scheme when
  using polls
• Distributed coordination function IFS (DIFS)
• Longest IFS
• Used as minimum delay of asynchronous frames
  contending for access
• SIFS lt PIFS lt DIFS
• e.g. in 802.11, SIFS28ms, PIFS78ms, DIFS128ms,
  slot time50ms

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IFS Usage

• SIFS
• Acknowledgment (ACK)
• Clear to send (CTS)
• Poll response
• PIFS
• Used by centralized controller in issuing polls
• Takes precedence over normal contention traffic
• DIFS
• Used for all ordinary asynchronous traffic

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DCF mode transmission without RTS/CTS
Data
source
Ack
destination
NAV
other
Defer access

• Send immediately (after DIFS) if medium is idle
• If medium was busy when sensed, wait a CW after
  it becomes idle (because many stations may be
  waiting when medium is busy if they all send the
  instant the medium becomes idle, chances of
  collision are high)

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PCF Mode
CP
CFP
CFP
Super-frame
CF-Burst, asynchronous traffic defers
Variable Length

• Allows time sensitive data to be transfer using
  a centralized scheduler (AP)
• Makes use of PIFS, and can lock out all
  asynchronous traffic which uses DIFS (PIFS lt
  DIFS)
• Occupies the initial portion of a super-frame
  asynchronous traffic contents for the rest of the
  super-frame

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IEEE 802.11 Architecture

• Access point (AP)
• Basic service set (BSS)
• Stations competing for access to shared wireless
  medium
• Isolated or connected to backbone DS through AP
• Distribution system (DS)
• Extended service set (ESS)
• Two or more basic service sets interconnected by
  DS

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Infrastructure based architecture

• Independent BSS (IBSS) has no AP
• adhoc mode only wireless stations
• Infrastructure BSS defined by stations sending
  Associations to register with an AP

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Transition Types Based On Mobility

• No transition
• Stationary or moves only within BSS
• BSS transition
• Station moving from one BSS to another BSS in
  same ESS
• ESS transition
• Station moving from BSS in one ESS to BSS within
  another ESS

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• TCP over wireless network

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The wireless dimension

• Naturally broadcast medium
• communications among some hosts are interference
  for the other hosts
• Poor/Unreliable link quality
• Harsh environment
• continuously changing characteristics uses
  adaptation
• high error rate uses FEC-based channel coding
• bursty errors due to sudden fades uses
  interleaving
• Mobility
• signal strength varies with location
• motion affects signals
• must change channels during handoff
• Low/limited power

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TCP Overview

• TCP connection-oriented reliable transport
  protocol that adapts to congestion in the network
  
• Assumes that losses are only caused by
  congestion in the network
• Congestion is assumed in the network if TCP
  sender receives triple duplicate acks or when
  doesnt receive acks (timeout RTT)
• TCP controls congestion by changing the
  congestion window size
• If there is a loss the sender reduces the window
  (and its sending rate) alleviating the
  congestion in the intermediate nodes.

TCP always reduces the throughput to alleviate
congestion (losses)
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TCP (Reno) Overview
loss (dup. Ack)
losses/disconnect
linear
timeout
Slow start
Fast retransmission
TCP Congestion Window Evolution, AIMD
Congestion avoidance phase
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TCP Overview

• Losses congestion is an assumption valid for
  fixed networks but not for wireless networks
• Fading channels have high bit error rate (BER),
  producing momentary losses that are not caused by
  congestion and doesnt necessarily mean a future
  reduction in available bandwidth
• TCP congestion control results in a unnecessary
  reduction in end-to-end throughput

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Wireless Network Architecture
Most traffic goes from wired network to wireless
network
Sender
Receiver
The wireless link is assumed to be the last hop
where most of the loss and delay occurs.
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Transport Layer Loss in Wireless Networks

• Transmission errors
• Harsh wireless link
• Handoffs
• Misrouted packets during handoff
• Possible in Mobile IP
• Mobile transceiver out of range

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Improving TCP Performance

• Solves problem with transmission error over
  wireless links
• Local recovery
• End-to-end
• Split connection

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Local Recovery
Performs retransmission here if possible without
getting TCP involves
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Local Recovery

• Snoop (ACM Mobicom 95)
• Caches unacknowledged TCP packets in base station
• Performs local retransmission using packets in
  local cache
• Detects packet loss by snooping on sequence
  number of acknowledgement packets (triple
  duplicate acks)
• Suppress duplicate acks during local
  retransmission
• Works better if transmission time over the
  wireless link is significantly smaller than the
  coarse grain TCP timer and round trip time (in
  LAN environment)
• Performance improves through faster
  retransmission and less TCP congestion control

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End-to-End Mechanism

• Modifies TCP endpoints to differentiate between
  congestion and transmission loss.
• Help from intermediate router/base-station to
  differentiate between congestion and transmission
  loss.

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End-to-end Mechanisms

• Explicit Loss Notification
• RFC 2481
• Use bit 6 and 7 in TOS field of IP header to
  indicate congestion
• Use some of the 6-bits in the reserved field of
  TCP header
• TCP Hack (INFOCOM 2001)
• TCP checksum covers both TCP header and data
• Add separate checksum for TCP header
• If data is corrupted, it is likely that header is
  fine since data size is usually much larger than
  header size
• Information in the header can be used to relay to
  the sender that there is packet error due to
  transmission error instead of congestion

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End-to-end MechanismsWTCP

• Wireless TCP (INFOCOM99)
• WAN Environment assumed
• Non-congestion related packet loss
• Very low bandwidth (lt19.2Kbps)
• Large round trip time (800ms 4sec)
• Asymmetric Channel which leads to ack
  compression
• Occasional blackouts lasting 10s or more

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WTCP (Contd)

• Congestion Control
• Use the ratio of the actual rate of the sender to
  the observed rate at the receiver as the primary
  metric for rate control
• Additive increase/multiplicative decrease
• If sending rate gtgt receiving rate, decrease send
  rate
• Else If sending rate ltlt receiving rate, increase
  send rate
• Else maintain
• Reliability
• SACK
• No retransmission time-out. Instead send probe
  packet to request for highest sequence number
  received to aid SACK

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Split Connection
Buffer
TCP sesssion from sender but terminates on BS
A separate transport session between base station
and mobile device
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Split Connection

• Indirect-TCP and M-TCP
• Split TCP connections into two TCP sessions
• One TCP session is from sender (in the wireline
  network) to base-station and the other session
  from base-station to receiver (in the wireless
  network)
• Packets are buffered at the base-stations until
  transmitted across the wireless connection
• Assumption is that latency over the wireless
  network is not a significant part of the
  end-to-end delay
• Violates end-to-end semantics

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Split Connection (Contd)

• Another popular variation of the split connection
  approach is to used UDP between base station and
  mobile device and TCP between base station and
  wireline host.
• Avoid using TCP congestion control over the
  wireless links completely
• Performs separate flow/congestion control in the
  last hop (usually using a rate-estimation
  algorithm)
• Violates end-to-end semantics
• Example Venturi Wireless (http//www.venturiwirel
  ess.com)

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TCP over 3G Cellular

• Trends in High-Speed 3G Wireless Network Design
• Extensive local retransmission to reduce impact
  of loss (particular useful for TCP)
• Earlier work in TCP focuses primarily on the
  issue of TCPs problem in differentiating between
  congestion and link loss
• Improvement comes at the expense of increased
  delay variability
• Using scheduling to improve bandwidth utilization
• High-speed wireless network uses channel-state
  based scheduling to improve throughput
• Schedule users with higher SNR to improve channel
  usage efficiency
• Improvement comes at the expense of increased
  rate variability
• What is the impact on TCP and how to improve
  throughput?
• Chan, M.C., Ramjee R, TCP/IP Performance over 3G
  Wireless Links with Rate and Delay Variation,
  ACM Mobicom 2002

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Summary

• There are still many interesting and open problem
  on TCP over wireless networks.
• If you are interested in working in this area,
  please contact me (chanmc_at_comp.nus.edu.sg) or Dr.
  Shorey (rajeev_at_comp.nus.edu.sg)

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References

• W. Stallings, Wireless Communications and
  Networks, Prentice-Hall, 2002.
• http//www.ee.columbia.edu/ramjee/ee6950
• Sonia Fahmy, Venkatesh Prabhakar, Srinivas R.
  Avasarala, Ossama Younis, TCP over Wireless
  Links Mechanisms and Implications, Technical
  report CSD-TR-03-004, Purdue University, 2003