CIS 725

1. CIS 725 Wireless networks

2. Wireless networks • Low bandwidth • High error rates

3. Issues • MAC protocols • Registration (handoff): must register care-of address with the home address • Route optimization • TCP

4. Hidden Terminal problem C wants to send to A but does not hear that A is busy

5. Exposed station problem C wants to send to D but thinks that transmission will fail

6. DIFS: distributed inter-frame space SIFS: Short inter-frame space RTS: request to send CTS: clear to send

7. Use of handshaking to prevent hidden station problem

8. Structure of wireless networks • MSS = mobility support stations or Access points (AP) • MH = mobile host or basic service station (BSS)

10. Handoff MSS1 MSS2

11. Models • Overlapping cells • 0-delay model • 1-sec delay

12. Models • Overlapping cells • 0-delay model • 1-sec delay MSS1 MSS1 MSS1 MSS2 MSS2 MSS2

13. TCP in wireless networks • Packet loss in wireless networks may be due to • Bit errors • Handoffs • Congestion (rarely) • Reordering (rarely, except for certain types of wireless nets) • TCP assumes packet loss is due to • Congestion • Reordering (rarely)

14. TCP in wireless networks • Timeout => reduce window size • Slow start - no movement = 100% - movement over overlapping cells 94% - movement over 0-delay cell 88% - movement over 1sec-delay 69%

15. Design Techniques for Mobile Networks • From mobile transmitter to base receivers - maintain timers at base stations - base station sends acks periodically - during handoff, new MSS keeps receiving by increasing W

16. From base sender to mobile receiver - MH sends selective acks - send acks after receiving a block of messages - Base station has a timer to detect loss of acks - no timer at MH

17. Other Solutions • Detect handoffs and notify transport layer of the handoff • Split TCP connections FH MSS MH

18. Split Connection Approach : Advantages • MSS-MH connection can be optimized independent of FH-MSS connection • Different flow / error control on the two connections • Local recovery of errors • Faster recovery due to relatively shorter RTT on wireless link • Good performanceachievable using appropriate MSS-MH protocol • Standard TCP on MSS-MH performs poorly when multiple packet losses occur per window

19. 39 40 38 37 FH MSS MH 36 40 Split Connection Approach : Disadvantages • End-to-end semanticsviolated • ack may be delivered to sender, before data delivered to the receiver • May not be a problem for applications that do not rely on TCP for the end-to-end semantics

20. 39 40 38 37 FH MSS MH 36 40 Split Connection Approach : Disadvantages • MSS retains hard state MSS failure can result in loss of data (unreliability) • If MSS fails, packet 40 will be lost • Because it is ack’d to sender, the sender does not buffer 40

21. 39 40 38 37 FH MSS MH 36 40 39 Hand-off 40 MH New MS station Split Connection Approach : Disadvantages • MSS retains hard state Hand-off latency increases due to state transfer • Data that has been ack’d to sender, must be moved to new base station

22. Split Connection Approach : Disadvantages • Buffer spaceneeded at MSS for each TCP connection • MSS buffers tend to get full, when wireless link slower (one window worth of data on wired connection could be stored at the base station, for each split connection)

23. TCP-Aware Link Layer • Snoop Protocol • observe TCP ACKs at the MSS • discard duplicate ACKs and retransmit • prevent fast retransmit at TCP sender • end-to-end reliability • soft state at MSS (only buffer packets)

24. Snoop Protocol • Buffers data packets at the base station BS • to allow link layer retransmission • When duplicate acks received by MSS from MH, retransmit on wireless link, if packet present in buffer • Prevents fast retransmit at TCP sender FH by dropping the duplicated acks at MSS FH MSS MH

25. Snoop : Example 35 TCP state maintained at link layer 36 37 38 40 39 38 37 FH MSS MH 36 Example assumes delayed ack - every other packet ack’d

26. Snoop : Example 35 39 36 37 38 41 40 39 38 36

27. Snoop : Example 37 40 38 41 39 43 42 41 40 36 36 36 Duplicate acks

28. Snoop : Example 37 40 38 41 39 42 44 43 37 41 FH MSS MH 36 36 Discard dupack Dupack triggers retransmission of packet 37 from base station MSS needs to be TCP-aware to be able to interpret TCP headers 36

29. Snoop Protocol : Disadvantages • Link layer at base station needs to be TCP-aware • Not useful if TCP headers are encrypted (IPsec) • Cannot be used if TCP data and TCP acks traverse different paths (both do not go through the base station)

30. Routing protocols • Proactive routing protocols Distance vector, Link state protocols * maintain routing paths at all times • Reactive routing protocols * create paths on demand • Hybrid protocols

31. Dynamic Source Routing • Initially, only next hop information is available. • Send packet to all neighbors • At some point, it will reach the destination and reverse path can be used to set up path 7 7 2 2 <1> <1,2> <1,3,5> 5 <1,3,5,7> 5 s s 1 3 1 3 <1,3> <1> <1,4,6> <1,4,6> d d <1> 8 8 <1,4> <1,4,6> 4 6 4 6 <1,4,6> Route reply with route record Building route record

32. Intermediate nodes may send replies if they already know a route • Cache management • Local repairs

33. Ad Hoc On Demand Routing (AODV) • Constructs routes on demand • Nodes maintain routing tables instead of source routes • Sequence numbers added to handle stale routes • Route discovery • Reverse path setup

34. Route Discovery RRep (a) Range of A's broadcast. (b) After B and D have received A's broadcast. (c) After C, F, and G have received A's broadcast. (d) After E, H, and I have received A's broadcast. RReq

35. Route Maintenance (a) D's routing table before G goes down. (b) The graph after G has gone down. D’s routing table before G goes down

36. Route maintenance • Route caching timeout used to purge old routes • Active_timeout period used to determine if neighboring node is active • If source moves, paths are re-established using RReq