CIS 725 Wireless networks. Low bandwidth High error rates.

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

Exposed station problem

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

RTS: request to sendCTS: clear to send

DIFS: distributed inter-frame spaceSIFS: Short inter-frame space

Use of handshaking to prevent hidden station problem

Structure of wireless networks

• MSS = mobility support stations

or Access points (AP)• MH = mobile host or

basic service station (BSS)

MSS1 MSS2

Handoff

Models

• Overlapping cells• 0-delay model• 1-sec delay

Models

• Overlapping cells• 0-delay model• 1-sec delay

MSS1 MSS2

MSS1 MSS2

MSS1 MSS2

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 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

• 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

• 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

• Detect handoffs and notify transport layer of the handoff

• Split TCP connections

FH MSS MH

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 performance achievable using appropriate MSS-

MH protocol– Standard TCP on MSS-MH performs poorly when

multiple packet losses occur per window

Split Connection Approach : Disadvantages

• End-to-end semantics violated– 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

FH MHMSS

40

39

3738

3640

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

FH MHMSS

40

39

3738

3640

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

FH MHMSS

40

39

3738

3640

MH

New MS station

Hand-off

40

39

Split Connection Approach : Disadvantages

• Buffer space needed 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

• 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

• 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 MHMSS

Snoop : Example

FH MHMSS

40 39 3738

36

Example assumes delayed ack - every other packet ack’d

36

37

38

35 TCP statemaintained at

link layer

Snoop : Example

41 40 3839

36

36

37

38

35 39

Snoop : Example

40

363636

Duplicate acks

4143 42

37

38

39

40

41

Snoop : Example

FH MHMSS

41

3636

3744 43

36

37

38

39

40

41

42

Discarddupack

Dupack triggers retransmissionof packet 37 from base station

MSS needs to be TCP-aware to

be able to interpret TCP headers

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

• 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

1

2

3

4

5

6

7

8

1

2

3

4

5

6

7

8

s s

d d

Building route record Route reply with route record

<1><1,2>

<1>

<1>

<1,3>

<1,4>

<1,3,5>

<1,4,6>

<1,3,5,7>

<1,4,6><1,4,6>

<1,4,6>

• 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

• Intermediate nodes may send replies if they already know a route

• Cache management• Local repairs

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

(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

RRep

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 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