Huaihai Institute of Technology

curvyrawrNetworking and Communications

Oct 23, 2013 (3 years and 7 months ago)

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Computer Science & Technology Department

Huaihai Institute of Technology







Title:

Bridges,

Routers,

and Gateways




Subject:

Computer Network Concepts





Class:


计算机
056




ID Number:


XXXXXX







Name:


XXXXX










S
core
:



Remark:














Bridges,

Routers,

and Gateways



(By
your

name
)

Abstract:

Initially, the LAN was used to link common sets of hardware
. With their
increasing popularity among organizations,
coupled with the widespread availability of
the Transmission Control Protocol / Internet Protocol (TCP/IP), LANs began to be
used for linking diverse hardware and operating systems. Repeaters, bridges
, routers,
and gateways facilitate networking
; because they can be shared among many users,
these devices contribute substantially to lowering the cost of networking.
T
he latter
three relieve host computers
of the processing
-
intensive tasks of protocol con
version
and routing information to appropriate locations.

Keywords:

repeater; bridge; router; gateway.


Introduction

E
ach interconnection device is designed to operate in conjunction with different layer
of the Open Systems Interconnection (OSI ) re
ference

model, which provides specific
levels of network functions.
W
hen properly intergrated with the LAN, bridges, routers,
and gateways offer the following advantages:



E
xtended network reach.



S
implified
cabling.



I
mproved overall performance.



A
dditional configu
ration flexibility.



E
nhanced security and maintenance through
p
a
rti
ti
oning
.



S
imper network management.



R
educed operating costs.


Bridges

A
lthough the terms gateway and bridge are often used interchangeably, there is a subtle
difference
: a gateway connects
dissimilar networks, and a bridge

connects similar
networks.
A

bridge may connect two or more LANs within the same building or LANs
that are father apart.
L
ocal bridges are capable of operating at 16M bps; remote bridges
typically operate from 4.8K bps to
1.544M bps, depending on the type of leased line
used.


M
inor routing by bridges is achieved by using a hierarchical routing fearture
that
filters local data traffic without affecting local network performance.
T
he bridge
receives packets of data, scan
s only to the network address, and passes the packets to
the intended addressee.


A

self
-
lea
r
ning bridge can filer information by monitoring the traffic on the
networks connected
to it and learning the addresses that are associated with each
network.
I
n this way, the bridge isolates the traffic destined to remain on the local
segment of the network and broadcasts the rest to the other networks.
A
fter initial
installation, the bridge forwards all packets it receives.
A
s it learns which addresses
correspo
nd with each network or subnetwork, the level of filtering is increased.
T
he
bridge can even learn the entire topology of the network and be able

to automatically
implement reco
nfigurations


B
ridges are ideally suited to interconnecting similar network
s in which protocol
conversion is not required, security concerns are minimal, and only rudimentary
routing is required. For example, in a campuslike environment, bridges can be used to
connect each building

s local network to the fiber
-
optic backbone. The

bridge restricts

local traffic to a building or cluster of building and keeps it off the superhighway. The
bridge

s filtering capabilities enable the network manager to restrict the types of
packets that go over the bridge, thereby alleviating traffic bot
tlenecks and limiting
access to certain types of network resources.

Backbone traffic does not enter a building

s local traffic unless it is addressed to
a node there. The use of bridges at this level provides an effective way of expanding
the capacity and
physical reach of computer resources while minimizing the
performance interconnection costs at higher levels.


S
ubnetworks

M
any organizations find bridges useful in dividing large LANs into discrete
subnetworks that are easier to
contr
o
l

and manage.
B
ridge
s can be used to group
similar devices, protocols, and transmission media into communities of interest.
E
ach
community of interest constitutes a subnetwork.
S
uch partitioning can yield many
advantages.
F
or example, the elimination of congestion and the imp
rovement of
response

time for the entire network.
I
n addition, partitioning can make adding,
moving, and changing devices on the network much easier, because only the effects of
these activities on the subnetwork must be considered.
F
inally, partitioning m
akes
problems easier to diagnose and isolate and enhances overall security



M
ore sophisticated
bridges support
multiple

bridging between two networks,
providing the capacity required for high
-
volume traffic and the processing power to
implement redunda
nt, reliable configurations.
R
econfigurations and initial and
continuous opertions are often automatic.
F
or example, bridges that use the spanning
tree protocol can facilitate the design and implementation of flexible, reliable networks.
I
t allow
s networks

of bridges to be instructed when to accept or reject particular
messages so the data will flow only over specified routes.
In
the event of a failure on
the network, the bridge automatically selects alternate paths, ensuring continuous
network operation.




B
ridges come in handy when multiple versions of the same LAN product are
interconnected.
F
or example, AT&T

s original version of StarLAN can be bridged to
StarLAN 10, the company

s product for users needing high bandwidth(10M
-
bps)
LANs.
A
T&T implements

bridging through a dedicated bridge unit, its Information
Systems Network, or a router.


I
n addition, bridges may be used to link mixed
-
media and dissimilar
-
speed
networks ( e.g., Ethernet and token rings )

but are capable only of moving individual
pa
ckets between them. Such bridges operate at the media
-
access control (MAC)
sublayer within the data link layer of the OSI reference model. These devices respond
only to the addresses of the packets on the networks they bridge, however, and not to
the size
of the packets or the speed of transmission. Therefore, using bridges to link
Ethernet and token
-
ring LANs, even at the data link level, may present problems with
reliability. Because a high
-
level protocol conversion is involved when connecting these
netwo
rks, a gateway is usually required.


Routers

A router is similar to a bridge in that both provide filtering and bridging functions
across the network. Routers, however, are different from bridges because they offer
more
sophisticated

network
management

and

traffic control capabilities. In terms of
complexity, a router falls between a bridge and a gateway.


The function of a router is to join LANs at the network layer of the OSI
reference model. This layer has two levels: internet and subnetwork. Because

DECnet,
for example, does not have the
internet layer, its routers work at the subnetwork level
only. Digital Equipment Corp

s terminal
protocol
, local area transport, cannot be
routed because it does not conform to the
specification
s of the network layer
. Therefore,
to be routed, an application must use a
protocol

that performs the functions associated
with the network layer.


Each network protocol has a routing
protocol

built into it.

Through this, the
router accesses the addressing
information

and s
hares and with other routers and hosts
on the network. The information the router needs to route data is
built

into the packet
itself.


To send

packets to their destinations, a router must perform several functions.
When a packet arrives at the router,

it holds the packet in queue until it is finished
handling the previous packet. The router then scans the destination address and looks it
up in its routing table. The routing table lists the various nodes on the network, the
paths between those

nodes, an
d how much it costs to transmit over these paths.


I
f a particular node has more than one path, the router selects the one that is the
most economical. If the packet is too large for the destination node to accept,

the router
breaks it down into a mana
geable frame size.

T
his capability is especially important in
wide area networking, in which telephone lines provide the link between LANs.
W
ith
smaller

packets, there is less chance that the data will be corrupt
ed by noise on the line.
E
ven if that occurs

and a retransmission is necessary, the smaller packet size reduces
information delay.


G
ateways

B
ecause organizations generally consist of specialized work groups, different networks
may be needed to meet the requirements of different users.
W
hen a devic
e
performs

protocol conversions that allow information to be exchanged between the various
networks, it is called a gateway.


A
s organizations become more complex, the ability to share files and
communicate information across diverse networks becomes n
ecessary to improve
effici
ency and productivity.
T
he need to connect dissimilar LANs may also arise as the
result of corporate mergers or acquisitions, or it may stem from the desire to
interconnect LANs with WANs
(e.g. , packet
-
switched networks) for econ
omical data
transport over long distances.
W
hatever
the justification for
linking dissimilar
networks, gateways are designed to do the job.


A

gateway can physically consist of a two
-
port card that plugs into the expansion
slots of the microcomputer th
at has been designated as the server.
T
he two
-
port
configuration allows gateways to perform speed conversion.
F
or example, one port
on
the

board
might provide a 64K
-
bps connection, whereas the other might provide a
19.2K
-
bps connection.
T
ogether, the two p
ort can support up to 32 concurrent sessions.
S
ome gateways even include a built
-
in packet assembler
-
disassembler to provide
logical access, eliminating the need for separate units.


A
ccess to the gateway is controlled by assigning specific ports to ce
rtain
microc
omputer can access the port , security is enforced.
U
nder this scheme, each port
may have provide access to all mainframe applications, for example, whereas another
port may be limited to only one application.
T
he problem with dedicated access
is that
idle ports cannot be used by anyone else, which means that efficiency is sacrificed for
the sake of security.


W
hen security is not an issue, gateway access may be on a contention basis,
providing more
opportunities for users to link with the m
ainframe or other network
resources because users are not limited to specific gateway ports.
S
ome gateways
permit both shared and dedicated access, allowing some ports to be reserved for
specific microcomputers and the rest pooled for general use.


A
dvanta
ges

W
hen a separate server is used as a gateway, cabling costs and installation time are
reduced and moves and changes are easier.
I
n fact, users can change the physical
location of the
ir equipment and retain their logical address on the network.
W
ith
comm
unications functions off
-
loaded from the host, valuable processing resources are
made available for more important tasks.


A
nother advantage of using gateways is simplified network management.
I
nstead of monitoring the traffic of 100 microcomputers on
the network, managers
would have to monitor only a single gateway, which appears to
the host as a single
peripheral

device.
I
n this case, a separate cluster controller is
unnecessary

because the
gateway replaces it.

Gateways are capable of extracting detai
led information about the data traffic
that passes through them and about the status of the data links it interfaces with. The
gateway can ensure that the links are handing data reliably, without exceeding
user
-
defined error rate thresholds. In addition, i
t can monitor the various protocols
being used, making sure that enough protocol conversion processing power is available
for any given application. The gateway’s management system can generate a variety of
reports that can be output automatically at speci
fied times or as demand warrants using
a keyboard command. Network statistics can be archived for trend analysis, which can
assist in long
-
range planning.

In wide area networking environments, the gateway balances load levels,
bypasses failed links, and fi
nds the most economical route. With some gateways, all of
these functions are performed automatically as the result of a single connection request
from a user, regardless of the equipment location or the protocols in
volved
. In this
environment, the ability

to detect, isolate, and diagnose problems becomes important.
The network management tools that are available with today’s sophisticated gateways
allow the remote configuration of channels, links, and such network interconnection
devices as bridges and rou
ters. Through the network management systems, gateway
ports may be brought online or offline as required.



Because gateways perform protocol conversion at every layer of the OSI

reference model, performance bottlenecks may become a problem.
E
very new
c
onnection, hop, and protocol that is added to the network not only intensifies the
problem

but invites new problems, which complicates network management.
W
ith so
much networking overhead devoted to protocol translation, gateways have been
relegated to suc
h specific applications as electronic mail and batch file transfers.


I
ntelligent Gateways

S
ome vendors are developing so
-
called intelligent gateways that communicate with
each other to determine the best way to route information, taking into consideration

such factors as congestion, priority, performance, security, and even cost.
B
uilding
such capabilities into intelligent gateways relieves users of having to make these
decisions.


A
n inherent weakness of such schemes, however, is congestion, which may

affect the performance of the entire network or of only one gateway of the network.
C
ongestion may be caused by an inefficient routing scheme, causing traffic to stay on
the primary data link longer than necessary, which slows down the entire network.
A
lt
ernatively, congestion may actually be in the gateway itself, which
occasionally
occurs when there are too many packets to filter.
T
o minimize the chance of such
bottlenecks, the gateway protocols much be able to perform flow control and respond
to congest
ion indicatiors.
Investigating the cause of the congestion can make a
diff
ere
nce in determining
whether it is worthwhile to reroute through other gateways
attached to the network.



When congestion is detected, the intelligent gateway can assign priorit
y to the
information that is to be routed. Ranking information is also important in the
management of the network. Intelligent gateways have the ability to the let diagnostic
information pass through or around conges
ted areas, providing real
-
time
s
tatus re
ports
on each link.



If the entire network is congested, the packets can bypass the alternative
gateways located on the other side of the network in favor of hopping through a
different network. Hopping to another network may pose a thread to security,

however.

Intelligent gateways maintain security by distinguishing routine and sensitive
information during the routing decision.


Conclusion

Bri
d
ges,

routers,

and gateways provide varying levels of connectivity,

efficiency,

and
economy to corporate networ
king. The choice of interconnection device hinges largely
on the topology of the network and the types of applications being run on the network.
Such devices are increasingly appreciated not only for providing in
t
ernetworking
connectivity but to helping to

unify the organization into an enterprisewide utility.


References

[1]


永田
,
黄厚宽.关于计算机专业英语教学的几个问题.
1996
年全国计算机教育研究会
论文集.
1990

11
月,重庆.

[2]


杨永田
,
张丽珂.计算机专业英语-
Fundamental Computer Concepts
.哈尔滨船舶工程学
院出版社,
1991

[
3
]

杨永田
,
王慧强.计算机专业英语-
Fundamental and New Computer Concepts
.哈尔滨工
程大学出版社,
1995