Lecture 16: QoS and "

Transcription

1 Lecture 16: QoS and " CSE 123: Computer Networks Alex C. Snoeren HW 4 due now!

2 Lecture 16 Overview" Network-wide QoS IntServ DifServ Wireless CSMA/CA Hidden Terminals RTS/CTS CSE 123 Lecture 16: QoS and

3 Network-wide QoS" Integrated services Motivated by need for end-to-end guarantees On-line negotiation of per-flow requirements End-to-end per-router negotiation of resources Complex Differentiated services Motivated by economics (multi-tier pricing) No per-flow state Not end-to-end and not guaranteed services Simple CSE 123 Lecture 16: QoS and

4 Integrated Services" Example: guarantee 1MBps and < 100 ms delay to a flow Receiver Sender CSE 123 Lecture 16: QoS and

5 Integrated Services" Allocate resources - perform per-flow admission control Sender Receiver CSE 123 Lecture 16: QoS and

6 Integrated Services" Install per-flow state Sender Receiver CSE 123 Lecture 16: QoS and

7 Integrated Services" Install per flow state Sender Receiver CSE 123 Lecture 16: QoS and

8 IntServe: Data Path" Per-flow classification Sender Receiver CSE 123 Lecture 16: QoS and

9 IntServe: Data Path" Per-flow buffer management Sender Receiver CSE 123 Lecture 16: QoS and

10 IntServe: Data Path" Per-flow scheduling Sender Receiver CSE 123 Lecture 16: QoS and

11 Differentiated Services" Edge router Shape & police traffic Mark class of traffic in DS header field (e.g., gold service) Core router Schedule aggregates according to marks in header Drop lower-class traffic first during congestion A B Edge router Domain Core router CSE 123 Lecture 16: QoS and

12 Infrastructure vs. Ad hoc" infrastructure network AP AP wired network AP: Access Point AP ad-hoc network CSE 123 Lecture 16: QoS and

13 IEEE Infrastructure" mobile terminal fixed terminal application infrastructure network application TCP IP access point TCP IP LLC LLC LLC MAC MAC MAC MAC PHY PHY PHY PHY CSE 123 Lecture 16: QoS and

14 Frame Format" Synchronization synch., gain setting, energy detection, frequency offset compensation SFD (Start Frame Delimiter) Signal data rate of the payload (0A: 1 Mbit/s DBPSK; 14: 2 Mbit/s DQPSK) Service Length future use, 00: compliant q payload length HEC (Header Error Check) protection of signal, service and length, x 16 +x 12 +x variable bits synchronization SFD signal service length HEC payload PLCP preamble PLCP header CSE 123 Lecture 16: QoS and

15 WLAN: IEEE b" Data rate 1, 2, 5.5, 11 Mbit/s User data rate max. approx. 6 Mbit/s Transmission range 300m outdoor, 30m indoor Max. data rate ~10m indoor Frequency Free 2.4 GHz ISM-band CSE 123 Lecture 16: QoS and

16 Physical Channels" 12 channels available for use in the US Each channel is 22 MHz wide Only 3 orthogonal channels Using any others causes interference US (FCC)/Canada (IC) channel 1 channel 6 channel MHz [MHz] CSE 123 Lecture 16: QoS and

17 Carrier Sense Multiple Access" CSMA: listen before transmit: If channel sensed idle: transmit entire pkt If channel sensed busy, defer transmission Persistent CSMA: retry immediately with probability p when channel becomes idle (may cause instability) Non-persistent CSMA: retry after random interval CSE 123 Lecture 16: QoS and

18 Hidden Terminal Problem" A B C B can communicate with both A and C A and C cannot hear each other Problem When A transmits to B, C cannot detect the transmission using the carrier sense mechanism If C transmits, collision will occur at node B Solution Hidden sender C needs to defer CSE 123 Lecture 16: QoS and

19 CSMA/CA" Cannot detect collision w/half-duplex radios Wireless MAC protocols often use collision avoidance techniques, in conjunction with a (physical or virtual) carrier sense mechanism Collision avoidance Nodes negotiate to reserve the channel. Once channel becomes idle, the node waits for a randomly chosen duration before attempting to transmit. CSE 123 Lecture 16: QoS and

20 RTS/CTS (MACA)" A B C When A wants to send a packet to B, A first sends a Request-to-Send (RTS) to B On receiving RTS, B responds by sending Clear-to- Send (CTS), provided that A is able to receive the packet When C overhears a CTS, it keeps quiet for the duration of the transfer Transfer duration is included in both RTS and CTS CSE 123 Lecture 16: QoS and

21 Backoff Interval " Problem: With many contending nodes, RTS packets will frequently collide Solution: When transmitting a packet, choose a backoff interval in the range [0, CW] CW is contention window Wait the length of the interval when medium is idle Count-down is suspended if medium becomes busy Transmit when backoff interval reaches 0 Need to adjust CW as contention varies CSE 123 Lecture 16: QoS and

22 Non-symmetric ranges" Interference range Carrier sense range DATA A B C D E F Transmit range CSE 123 Lecture 16: QoS and

23 MAC Modes" Distributed Coordination Function (DCF) CSMA/CA collision avoidance via randomized back-off mechanism minimum distance between consecutive packets ACK packet for acknowledgements (not for broadcasts) DCF w/ RTS/CTS Distributed Foundation Wireless MAC avoids hidden terminal problem Point Control Fuction (PCF) - optional Access point polls terminals according to a list Were not going to discuss CSE 123 Lecture 16: QoS and

24 IEEE DCF " DCF is CSMA/CA protocol Uses a Network Allocation Vector (NAV) to implement collision avoidance DCF suitable for multi-hop ad hoc networking Optionally uses RTS/CTS exchange to avoid hidden terminal problem Any node overhearing a CTS cannot transmit for the duration of the transfer Uses ARQ to provide reliability CSE 123 Lecture 16: QoS and

25 IEEE " RTS = Request-to-Send RTS A B C D E F Pretending a circular range CSE 123 Lecture 16: QoS and

26 IEEE " NAV = remaining duration to keep quiet RTS = Request-to-Send RTS A B C D E NAV = 10 F CSE 123 Lecture 16: QoS and

27 IEEE " CTS = Clear-to-Send CTS A B C D E F CSE 123 Lecture 16: QoS and

28 IEEE " CTS = Clear-to-Send CTS A B C D E F NAV = 8 CSE 123 Lecture 16: QoS and

29 IEEE " DATA packet follows CTS. Successful data reception acknowledged using ACK. DATA A B C D E F CSE 123 Lecture 16: QoS and

30 IEEE " Reserved area ACK A B C D E F CSE 123 Lecture 16: QoS and

31 Binary Exponential Backoff" When a node fails to receive CTS in response to its RTS, it increases the contention window CW is doubled (up to an upper bound) More collisions è longer waiting time to reduce collision When a node successfully completes a data transfer, it restores CW to CW min CSE 123 Lecture 16: QoS and

32 Backoffs" SIFS (Short Inter Frame Spacing) highest priority, for ACK, CTS, polling response PIFS (PCF IFS) medium priority, for time-bounded service using PCF DIFS (DCF, Distributed Coordination Function IFS) lowest priority, for asynchronous data service DIFS medium busy DIFS PIFS SIFS contention next frame direct access if medium is free DIFS t CSE 123 Lecture 16: QoS and

33 DCF Example" B1 = 25 wait B1 = 5 data B2 = 20 data B2 = 15 wait B2 = 10 cw = 31 B1 and B2 are backoff intervals at nodes 1 and 2 CSE 123 Lecture 16: QoS and

34 Fragmentation" sender receiver DIFS RTS SIFS CTS SIFS frag 1 SIFS ACK SIFS 1 frag 2 SIFS ACK2 other stations NAV (RTS) NAV (CTS) NAV (frag 1 ) NAV (ACK 1 ) DIFS contention data t CSE 123 Lecture 16: QoS and

35 MAC management" Association/Reassociation integration into a LAN roaming, i.e. change networks by changing access points scanning, i.e. active search for a network Power management sleep-mode without missing a message periodic sleep, frame buffering, traffic measurements CSE 123 Lecture 16: QoS and

36 Scanning" Goal: Find a network to connect Passive scanning Not require transmission Move to each channel, and listen for Beacon frames Active scanning Require transmission Move to each channel, and send Probe Request frames to solicit Probe Responses from a network CSE 123 Lecture 16: QoS and

37 Association in " 1: Association request 2: Association response Client 3: Data traffic AP CSE 123 Lecture 16: QoS and

38 Reassociation in " 1: Reassociation request 3: Reassociation response Client 5: Send buffered frames 6: Data traffic New AP 2: verify previous association Old AP 4: send buffered frames CSE 123 Lecture 16: QoS and

39 Roaming" No or bad connection? Then perform: Scanning scan the environment, i.e., listen into the medium for beacon signals or send probes into the medium and wait for an answer Reassociation Request station sends a request to one or several AP(s) Reassociation Response success: AP has answered, station can now participate failure: continue scanning AP accepts Reassociation Request signal the new station to the distribution system the distribution system updates its data base (i.e., location information) typically, the distribution system now informs the old AP so it can release resources CSE 123 Lecture 16: QoS and

40 Summary" Network-wide QoS requires coordination IntServ and DiffServ two different approaches Either in-band signaling or out-of-band classification Wireless Common technology for local-area wireless Uses CSMA/CA Needs to handle hidden terminal problem Challenges due to asymmetric ranges CSE 123 Lecture 16: QoS and

41 For next time " Finish up Project 2 Complete your CAPE survey online We ll review the entire term next time CSE 123 Lecture 16: QoS and