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CIS 725. Wireless networks. Wireless networks. Low bandwidth High error rates. Issues. MAC protocols Registration (handoff): must register care-of address with the home address Route optimization TCP. Hidden Terminal problem. C wants to send to A but does not hear that A is busy.

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cis 725

CIS 725

Wireless networks

wireless networks
Wireless networks
  • Low bandwidth
  • High error rates
issues
Issues
  • MAC protocols
  • Registration (handoff): must register care-of address with the home address
  • Route optimization
  • TCP
slide4

Hidden Terminal problem

C wants to send to A but does not hear that A is busy

slide5

Exposed station problem

C wants to send to D but thinks that transmission will fail

slide6

DIFS: distributed inter-frame space

SIFS: Short inter-frame space

RTS: request to send

CTS: clear to send

structure of wireless networks
Structure of wireless networks
  • MSS = mobility support stations

or Access points (AP)

  • MH = mobile host or

basic service station (BSS)

slide10

Handoff

MSS1

MSS2

models
Models
  • Overlapping cells
  • 0-delay model
  • 1-sec delay
models1
Models
  • Overlapping cells
  • 0-delay model
  • 1-sec delay

MSS1

MSS1

MSS1

MSS2

MSS2

MSS2

tcp in wireless networks
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)
tcp in wireless networks1
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%

design techniques for mobile networks
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

slide16
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

other solutions
Other Solutions
  • Detect handoffs and notify transport layer of the handoff
  • Split TCP connections

FH

MSS

MH

split connection approach advantages
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
split connection approach disadvantages

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
split connection approach disadvantages1

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
split connection approach disadvantages2

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
split connection approach disadvantages3
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)
tcp aware link layer
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)
snoop protocol
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

snoop example
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

snoop example1
Snoop : Example

35

39

36

37

38

41

40

39

38

36

snoop example2
Snoop : Example

37

40

38

41

39

43

42

41

40

36

36

36

Duplicate acks

snoop example3
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

snoop protocol disadvantages
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)
routing protocols
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
dynamic source routing
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

ad hoc on demand routing aodv
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
route discovery
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

route maintenance
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

route maintenance1
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