英语翻译20

The

Safari

architecture

(Du

et

al.

2008)

also

provides

a

self-organizing

network

hierar-

chy and

a routing

protocol

with

similarity

to

the

landmark

approaches.

Such

landmarks,

called

drums

in

Safari,

use

a

self-election

algorithm

(a

distributed

algorithm

with

no

cen-

tralized

coordination)

to

form

subnets,

called

cells

and

supercells

.

At

the

lowest

level

of

the

cell

hierarchy

(level

0), each

individual

node

forms

its own cell.

At

level 1, Safari

de?nes

fundamental cells

, that is, cells that contain

multiple

individual

nodes,

but

no other

cells.

Higher-level cells

are then composed of multiple smaller cells at lower levels. Each

drum

periodically

broadcasts

beacon

messages

within

well-

de?ned

limited scopes in the network. These beacons aid the

hierarchy formation, give nodes an indication of their position in

the network topology, and provide routing information toward the

drum

’

s

cell.

Safari

uses

a

hybrid

approach

for

routing,

that

is,

routing within cells is based on the reactive DSR protocols, while

a proactive

routing

approach

is used to compute

routes

to more

distant

nodes.

The

inter-cell

communication

relies

on

a

destination node

’

s hierarchical address and on the beacon records

stored on each node. Basically,

inter-cell communication

follows

the

Network Layer

183

reverse

path

of

the beacons

issued

by

the

cells

’

drums.

The

main

advantage

of

Safari

over

other

approaches

is

its

scalability,

due

to

the

hybrid

routing

approach

and

the

hierarchical

addressing

scheme.

7.8

Location- Based

Routing

Location-based

or

geographic

routing

can

be

used

in

networks

where

sensor

nodes

are

able

to

determine their position using a variety of localization systems and algorithms (see Chapter

10 for examples of localization techniques). Instead of topological connectivity information, sensors

use geographic information to make forwarding decisions. In unicast location-based routing, packets

are sent directly to a single destination, which is

identi?ed

by its location. That is, a sender must be

aware not only of

its own

location,

but also the location of the des- tination. This

location

can be

obtained

either

via

querying

(e.g.,

?

ooding

a

query

to

request

a

response

from

the

destination

containing

its

location)

or

a

location

broker

,

that

is,

a

ser-

vice

that

maps

node

identities

to

locations.

In

broadcast

or

multicast

location-based

routing

approaches,

the

same

packet

must

be

disseminated

to multiple destinations.

Multicast pro- tocols take advantage of the known destination

locations to minimize resource consumption by reducing redundant links.

The

identity

of

a sensor

node

is

typically

less

important

than

its

location,

that

is,

data may be

disseminated

to

all

nodes

that

lie

within

a

certain

geographic

region.

This

approach

is

called

geocasting

and can, for example, be used to diffuse queries to

speci?c

regions of interest instead of

?

ooding the entire network,

signi?cantly

reducing both bandwidth and energy requirements. Once a

packet

reaches

the desired

region,

it

must

be either

dissemi- nated

(multicast)

to

all

nodes

within

this region or transmitted

to at least one node within this region (

anycast

).

Typically, location-based routing protocols require that every node in the network

knows

its own geographic

location and the identities and locations of its one-hop neighbors (e.g., obtained

via periodic beacon messages). The destination is expressed either as the location of a node (instead

of a unique

address)

or a geographic

region.

Compared

to other

routing solutions, an advantage

of

location- based routing is that only geographic information

is needed for forwarding decisions and it

is

not

necessary

to

maintain

routing

tables

or

to

establish end-to-end

paths

between

sources

and

destinations,

eliminating

the

need

for

control

packets

(apart

from

the

beacon

messages

among

neighbors).