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Jan 30, 20041 Chan, M.C. Wireless Network & TCP Dr. Chan Mun Choon School of Computing, NUS Jan 30, 2004 CS 5229.

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Presentation on theme: "Jan 30, 20041 Chan, M.C. Wireless Network & TCP Dr. Chan Mun Choon School of Computing, NUS Jan 30, 2004 CS 5229."— Presentation transcript:

1 Jan 30, 20041 Chan, M.C. Wireless Network & TCP Dr. Chan Mun Choon School of Computing, NUS Jan 30, 2004 CS 5229

2 Jan 30, 20042 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

3 Jan 30, 20043 Chan, M.C. Overview Wireless Networks –Cellular Network –Wireless Local Area Network TCP over Wireless Networks –Problems with TCP congestion control –Solutions

4 Jan 30, 20044 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

5 Jan 30, 20045 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 ???

6 Jan 30, 20046 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

7 Jan 30, 20047 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

8 Jan 30, 20048 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)

9 Jan 30, 20049 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)

10 Jan 30, 200410 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)

11 Jan 30, 200411 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

12 Jan 30, 200412 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 12341234

13 Jan 30, 200413 Chan, M.C. Frequency vs. time Frequency Time Carrier FDMA Time Frequency TDMA Time Frequency Hybrid FDMA/TDMA In practical systems, TDMA is often combined with FDMA

14 Jan 30, 200414 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

15 Jan 30, 200415 Chan, M.C. Examples FDD: –Cellular systems: AMPS, NA-TDMA, CDMA, GSM TDD –Cordless telephone systems: CT2, DECT, PHS

16 Jan 30, 200416 Chan, M.C. Frequency Band Usage Frequency RangeExample Usage 300Hz – 3000HzAnalog telephone 300kHz to 3MHzAM Radio 3 to 30MHzAmateur Radio, international broadcasting (e.g. BBC) 30 to 300MHzVHF television, FM Radio 300 to 3000MHzUHF television, cellular telephone, PCS 3 to 30GHzSatellite communication, radar, wireless local loop 30 to 300GHzExperimental; WLL 300GHz to 400THzInfrared LAN, consumer electronics 400 to 900 THzOptical communication

17 Jan 30, 200417 Chan, M.C. Frequency Bands Usage Example Frequency Range (MHz)Example Usage 824-849, 869-894AMPS NA-TDMA/IS-136 CDMA/IS-95 CDMA2000 3G1x 902-928, 2400-2484ISM (Industrial Scientific Medical) 890-915, 935-960GSM 1710-1785, 1805-18853G 1850-1910,1930-19903G

18 Jan 30, 200418 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?

19 Jan 30, 200419 Chan, M.C. 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 Jan 30, 200420 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

21 Jan 30, 200421 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

22 Jan 30, 200422 Chan, M.C. Hidden Terminal Problem D C B A 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 C cannot detects transmission from A and thus CSMA does not work when C starts transmission to B

23 Jan 30, 200423 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

24 Jan 30, 200424 Chan, M.C. Exposed Terminal Problem D C B A 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 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

25 Jan 30, 200425 Chan, M.C. IEEE 802 Protocol Layers

26 Jan 30, 200426 Chan, M.C. Protocol Stack

27 Jan 30, 200427 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

28 Jan 30, 200428 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=28  s, PIFS=78  s, DIFS=128  s, slot time=50  s

29 Jan 30, 200429 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

30 Jan 30, 200430 Chan, M.C. DCF mode transmission without RTS/CTS source destination other DIFS Data Ack SIFS NAV Defer access DIFS CW 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)

31 Jan 30, 200431 Chan, M.C. PCF Mode CP CFP Super-frame Variable Length CF-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

32 Jan 30, 200432 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

33 Jan 30, 200433 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 Service Set (BSS) Access points (AP) Extended Service Set (ESS)

34 Jan 30, 200434 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

35 Jan 30, 200435 Chan, M.C. TCP over wireless network

36 Jan 30, 200436 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

37 Jan 30, 200437 Chan, M.C. TCP Overview TCP – connection-oriented reliable transport protocol that adapts to congestion in the network u Assumes that losses are only caused by congestion in the network u Congestion is assumed in the network if TCP sender receives triple duplicate acks or when doesn’t receive acks (timeout ~ RTT) u TCP controls congestion by changing the congestion window size u 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 Jan 30, 200438 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

39 Jan 30, 200439 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

40 Jan 30, 200440 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. SenderReceiver Most traffic goes from wired network to wireless network

41 Jan 30, 200441 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

42 Jan 30, 200442 Chan, M.C. Improving TCP Performance Solves problem with transmission error over wireless links –Local recovery –End-to-end –Split connection

43 Jan 30, 200443 Chan, M.C. Local Recovery Internet Performs retransmission here if possible without getting TCP involves

44 Jan 30, 200444 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

45 Jan 30, 200445 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.

46 Jan 30, 200446 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

47 Jan 30, 200447 Chan, M.C. End-to-end Mechanisms WTCP 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

48 Jan 30, 200448 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

49 Jan 30, 200449 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

50 Jan 30, 200450 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

51 Jan 30, 200451 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)

52 Jan 30, 200452 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

53 Jan 30, 200453 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)chanmc@comp.nus.edu.sg

54 Jan 30, 200454 Chan, M.C. References W. Stallings, “Wireless Communications and Networks”, Prentice-Hall, 2002. http://www.ee.columbia.edu/~ramjee/ee69 50http://www.ee.columbia.edu/~ramjee/ee69 50 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 TCP over Wireless Links: Mechanisms and Implications,


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