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Wireless Network

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Title: Wireless Network


1
Wireless Network TCP
  • Dr. Chan Mun Choon
  • School of Computing, NUS
  • Jan 30, 2004
  • CS 5229

2
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

3
Overview
  • Wireless Networks
  • Cellular Network
  • Wireless Local Area Network
  • TCP over Wireless Networks
  • Problems with TCP congestion control
  • Solutions

4
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

5
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 ???

6
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

7
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

8
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)

9
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)

10
Multiple Access
  • Wireless channel is broadcast channel, need to
    separate the desired signal from interfering
    signals
  • Earliest approach is frequency division multiple
    access (FDMA)

11
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

12
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

1
2
3
4
1
2
3
4
13
Frequency vs. time
FDMA
Frequency
Time
  • In practical systems, TDMA is often combined
    with FDMA

14
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

15
Examples
  • FDD
  • Cellular systems AMPS, NA-TDMA, CDMA, GSM
  • TDD
  • Cordless telephone systems CT2, DECT, PHS

16
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
17
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
18
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?

19
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
20
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

21
CSMA
  • Carrier Sense Multiple Access
  • sense carrier
  • if idle, send
  • wait for ack
  • If there isnt one, assume there was a collision,
    retransmit

22
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
23
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

24
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.
25
IEEE 802 Protocol Layers
26
Protocol Stack
27
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

28
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

29
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

30
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)

31
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

32
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

33
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

34
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

35
  • TCP over wireless network

36
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

37
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)
38
TCP (Reno) Overview
loss (dup. Ack)
losses/disconnect
linear
timeout
Slow start
Fast retransmission
TCP Congestion Window Evolution, AIMD
Congestion avoidance phase
39
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

40
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.
41
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

42
Improving TCP Performance
  • Solves problem with transmission error over
    wireless links
  • Local recovery
  • End-to-end
  • Split connection

43
Local Recovery
Performs retransmission here if possible without
getting TCP involves
44
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

45
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.

46
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

47
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

48
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

49
Split Connection
Buffer
TCP sesssion from sender but terminates on BS
A separate transport session between base station
and mobile device
50
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

51
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)

52
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

53
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)

54
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
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