jan 30, 20041 chan, m.c. wireless network & tcp dr. chan mun choon school of computing, nus jan...
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Jan 30, 2004 1 Chan, M.C.
Wireless Network & TCP
Dr. Chan Mun Choon
School of Computing, NUS
Jan 30, 2004
CS 5229
Jan 30, 2004 2 Chan, M.C.
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
Jan 30, 2004 3 Chan, M.C.
Overview
• Wireless Networks– Cellular Network– Wireless Local Area Network
• TCP over Wireless Networks– Problems with TCP congestion control– Solutions
Jan 30, 2004 4 Chan, M.C.
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
Jan 30, 2004 5 Chan, M.C.
Evolution of Cellular Wireless Network
• First Generation – Analog– AMPS: North America
• Second Generation– TDMA
• GSM (SingTel/M1, Europe, AT&T)• NA-TDMA IS-136 (AT&T)
– CDMA (U.S.A.)• Third Generation
– WCDMA (Europe, Singapore)– CDMA2000 (U.S.A.)
• Fourth Generation– OFDM, WLAN ???
Jan 30, 2004 6 Chan, M.C.
First Generation Analog System
• First Generation– Advanced Mobile Phone Service (AMPS)– Provide analog traffic channels– Developed by AT&T in 1970s– Early deployment in 1980s– > 40 million users in 1997
Jan 30, 2004 7 Chan, M.C.
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
Jan 30, 2004 8 Chan, M.C.
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)
Jan 30, 2004 9 Chan, M.C.
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)
Jan 30, 2004 10 Chan, M.C.
Multiple Access
• Wireless channel is broadcast channel, need to separate the desired signal from interfering signals
• Earliest approach is frequency division multiple access (FDMA)
Jan 30, 2004 11 Chan, M.C.
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
Jan 30, 2004 12 Chan, M.C.
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
Jan 30, 2004 13 Chan, M.C.
Frequency vs. timeF
requ
ency
Time
CarrierFDMA
Time
Fre
quen
cy
TDMA
Time
Fre
quen
cy
Hybrid FDMA/TDMA
• In practical systems, TDMA is often combined with FDMA
Jan 30, 2004 14 Chan, M.C.
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
Jan 30, 2004 15 Chan, M.C.
Examples
• FDD:– Cellular systems: AMPS, NA-TDMA, CDMA,
GSM
• TDD– Cordless telephone systems: CT2, DECT,
PHS
Jan 30, 2004 16 Chan, M.C.
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
Jan 30, 2004 17 Chan, M.C.
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
Jan 30, 2004 18 Chan, M.C.
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?
Jan 30, 2004 19 Chan, M.C.
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
Jan 30, 2004 20 Chan, M.C.
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
Jan 30, 2004 21 Chan, M.C.
CSMA
• Carrier Sense Multiple Access– sense carrier– if idle, send– wait for ack
• If there isn’t one, assume there was a collision, retransmit
Jan 30, 2004 22 Chan, M.C.
Hidden Terminal Problem
D
C
B
A
A can hear B but not C and DB can hear A and C but not DC can hear B and D but not A
C cannot detects transmission from A and thus CSMA does not work when C starts transmission to B
Jan 30, 2004 23 Chan, M.C.
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
Jan 30, 2004 24 Chan, M.C.
Exposed Terminal Problem
D
C
B
A
A can hear B but not C and DB can hear A and C but not DC can hear B and D but not AD can hear C but not A and B
C cannot transmit to B even if it will not interfere with transmission from B to A. As a result, network throughput is reduced.
RTS
CTS CTS
Jan 30, 2004 25 Chan, M.C.
IEEE 802 Protocol Layers
Jan 30, 2004 26 Chan, M.C.
Protocol Stack
Jan 30, 2004 27 Chan, M.C.
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
Jan 30, 2004 28 Chan, M.C.
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 < PIFS < DIFS– e.g. in 802.11, SIFS=28s, PIFS=78s, DIFS=128s, slot time=50s
Jan 30, 2004 29 Chan, M.C.
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
Jan 30, 2004 30 Chan, M.C.
DCF mode transmission without RTS/CTS
source
destination
other
DIFSData
AckSIFS
NAV
Defer access
DIFSCW
Random backoff time• 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)
Jan 30, 2004 31 Chan, M.C.
PCF Mode
CPCFP CFP
Super-frame
Variable LengthCF-Burst, asynchronous traffic defers
• 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 < DIFS)• Occupies the initial portion of a super-frame; asynchronous traffic contents for the rest of the super-frame
Jan 30, 2004 32 Chan, M.C.
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
Jan 30, 2004 33 Chan, M.C.
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
Distribution System (DS)
Basic ServiceSet (BSS)
Access points (AP)Extended Service Set
(ESS)
Jan 30, 2004 34 Chan, M.C.
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
Jan 30, 2004 35 Chan, M.C.
TCP over wireless network
Jan 30, 2004 36 Chan, M.C.
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
Jan 30, 2004 37 Chan, M.C.
TCP OverviewTCP – 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 doesn’t 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)
Jan 30, 2004 38 Chan, M.C.
TCP (Reno) Overview
Slow start
~ linear
loss (dup. Ack)
Fast retransmission
losses/disconnect
timeout
Congestion avoidance phase
TCP Congestion Window Evolution, AIMD
Jan 30, 2004 39 Chan, M.C.
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 doesn’t necessarily mean a future reduction in available bandwidth
• TCP congestion control results in a unnecessary reduction in end-to-end throughput
TCP Overview
Jan 30, 2004 40 Chan, M.C.
Wireless Network Architecture
Internet
The wireless link is assumed to be the last hop where most of the loss and delay occurs.
Sender ReceiverMost traffic goes from wired network to wireless network
Jan 30, 2004 41 Chan, M.C.
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
Jan 30, 2004 42 Chan, M.C.
Improving TCP Performance
• Solves problem with transmission error over wireless links– Local recovery– End-to-end – Split connection
Jan 30, 2004 43 Chan, M.C.
Local Recovery
Internet
Performs retransmission here if possible without getting TCP involves
Jan 30, 2004 44 Chan, M.C.
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
Jan 30, 2004 45 Chan, M.C.
End-to-End Mechanism
Internet
• Modifies TCP endpoints to differentiate between congestion and transmission loss. • Help from intermediate router/base-station to differentiate between congestion and transmission loss.
Jan 30, 2004 46 Chan, M.C.
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
Jan 30, 2004 47 Chan, M.C.
End-to-end MechanismsWTCP
• Wireless TCP (INFOCOM’99)
• WAN Environment assumed– Non-congestion related packet loss– Very low bandwidth (<19.2Kbps)– Large round trip time (800ms – 4sec)– Asymmetric Channel which leads to ack compression– Occasional blackouts lasting 10s or more
Jan 30, 2004 48 Chan, M.C.
WTCP (Cont’d)
• 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 >> receiving rate, decrease send rate• Else If sending rate << 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
Jan 30, 2004 49 Chan, M.C.
Split Connection
Internet
TCP sesssion from sender but terminates on BS
A separate transport session between base station and mobile device
Buffer
Jan 30, 2004 50 Chan, M.C.
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
Jan 30, 2004 51 Chan, M.C.
Split Connection (Cont’d)
• 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.venturiwireless.com)
Jan 30, 2004 52 Chan, M.C.
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 TCP’s 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
Jan 30, 2004 53 Chan, M.C.
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@comp.nus.edu.sg) or Dr. Shorey (rajeev@comp.nus.edu.sg)
Jan 30, 2004 54 Chan, M.C.
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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