Soft Real-Time Wireless Communication for Sensor Networks, Lecture notes of Wireless Networking

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An overview of soft real-time wireless

communication for Sensor Networks

Marco Caccamo

Outline

Wireless LANs background

Enforce priorities with CSMA/CA access

Speed routing protocol

Real-time chains

Wireless LAN preliminaries

Two kind of wireless network topologies

Both supported by IEEE 802.11 MAC protocols

Ad hoc network

• Mobile terminals communicate with each other in an

independent basic service set (BSS)

-^

BSS: single hop ad hoc network

• A packet may need to traverse multiple links to reach a

destination

5

Two kind of network topologies

• Infrastructure network
  • Mobile terminals communicate

with the backbone networkthrough an access point (AP)

• APs can be bridged supporting

range extension betweenmultiple BSSs, forming anextended service set (ESS)

(1) Hidden Terminal Problem

[Tobagi75]

Node B can communicate with A and C both

A and C can not hear each other

When A transmits to B, C cannot detect thetransmission using the

carrier sense

mechanism

If C transmits, collision will occur at node B

A

B

C

The wireless LAN background is based on Nitin Vaidya’swireless MAC tutorial

A solution for Hidden Terminal Problem

[MACA , Karn90]

When node A wants to send a packet to node B, node Afirst sends a

Request-to-Send (RTS)

to B

On receiving RTS, node B responds by sending

Clear-to-

Send (CTS)

, provided node B is able to receive the packet

When a node (such as C) overhears a CTS, it keeps quietfor the duration of the transfer

Transfer duration is included in both RTS and CTS

A

B

C

IEEE 802.

C

F

A

B

E

D

RTS

RTS = Request-to-Send

IEEE 802.

C

F

A

B

E

D

RTS

RTS = Request-to-Send

IEEE 802.

C

F

A

B

E

D

CTS

CTS = Clear-to-Send

IEEE 802.

C

F

A

B

E

D

DATA

MACs to transmit time-sensitive data

Reservation based schemes

• Allow time-bounded traffic to reserve a periodic slot on the

channel that they can access alone

• Need a central agent to decide slot schedule.– Waste unused but reserved resource– E.g. IEEE 802.11 PCF (Point Coordination Function)

Priority based schemes

• Enforce priorities to let some stations to have precedence

over others when accessing the wireless channel.

IEEE 802.11 wireless MAC

MAC options:

• Distributed and centralized MAC components– DCF (Distributed Coordination Function):
  • DCF is suitable for multi-hop ad hoc networking• DCF is a Carrier Sense Multiple Access/Collision

Avoidance (CSMA/CA) protocol

• short IFS -> high priority (ACKs, CTS)
  • PCF (Point Coordination Function):
    • Uses polling to grant stations their turn to transmit.• Polling performed by Access Point (AP)• PCF and DCF intervals are combined building a

periodic frame

CSMA/CA in details

Carrier sense in IEEE 802.

• Physical carrier sense– Virtual carrier sense using Network Allocation Vector

(NAV)

• NAV is updated based on overheard

RTS/CTS/DATA/ACK packets, each of whichspecified duration of a pending transmission

Collision avoidance

• Nodes stay silent when channel is sensed busy

(physical/virtual)

• Backoff intervals used to reduce collision probability

Backoff interval (Collision Avoidance)

When transmitting a packet, choose a randombackoff value in the range [0,cw-1]

• cw is called contention window– cw begins with 8

Count down the backoff interval when medium isidle

Î

Count-down is suspended (frozen) if medium becomesbusy

When backoff interval reaches 0, transmit RTS