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Introduction to Computer
Networks
Elements of Network
Connectivity
This lesson explores the devices and technologies
available to expand networks beyond the scope of
simple local area networks.
Connectivity Devices
• Devices used to expand LANs include modems,
repeaters, bridges, routers, brouters, and gateways.
• Modem Technology: Modems convert digital signals
to analog waves, and vice versa.
Modem Hardware
• Modems are known as data communications
equipment (DCE) and share the following
characteristics:
• A serial (RS-232)
communications
interface
• An RJ-11
telephone-line
interface (a fourwire telephone
plug)
Modem Standards
• Some of the common industry standards for
modems include:
• Hayes-Compatible: manufacturers currently offer
modems with speeds of 56,600 bps or more.
• International Standards: the International
Telecommunications Union (ITU) has developed
standards for modems.
• These specifications, known as the V series,
include a number that indicates the standard.
ITU Modem Standards
• V.22bis
V.32
14,400
28,800
57,600
56,600
2400
9600
1991
1994
1995
1998
1984
1984
V.34
V.42
V.90
V.32b
• Several of the newer modems feature industry
standards, such as V.42bis/MNP5 data compression,
and have transmission speeds of 57,600 bps; and
some modems go up to 76,800 bps.
Types of Modems
• There are different types of modems because
different types of communication environments
require different methods of sending data.
• These environments can be divided roughly into
two areas related to the timing of
communications:
•
Asynchronous
•
Synchronous
Asynchronous Communication (Async)
• Async was developed in order to make use of
common telephone lines.
• Communication is not synchronized: There is no
clocking method to coordinate the transmission
between the sender and the receiver.
• Speed can be up to 28.8K bps. However, the data
compression can boost the rate to 115.2K.
Synchronous Communication
• Synchronous communication relies on a timing
scheme to transmit bits in block known as frames.
• Special characters are used to synchronize and
check its accuracy periodically.
Synchronous protocols
• Synchronous protocols perform a number of jobs
that asynchronous protocols do not.
• Format data into blocks.
• Add control information.
• Check the information to provide error control.
• The primary protocols in synchronous
communication are:
• Synchronous Data Link Control (SDLC), Highlevel Data Link Control (HDLC), Binary
Synchronous Communications Protocol (bisync).
Asymmetric Digital Subscriber Line
(ADSL)
• This technology converts existing twisted-pair
telephone lines into access paths for multimedia and
high-speed data communications.
• These new connections can transmit more than 8
Mbps to the subscriber and up to 1 Mbps from the
subscriber.
• The technology requires special hardware, including
an ADSL modem on each end of the connection.
• It also requires broadband cabling, which is not
available in every locations, and there is a limit to
the connection length.
Expanding a Network Using
Components
• There are components that can be installed to
increase the size of the network within its existing
environment. These components include:
• Hubs.
• Repeaters.
• Bridges.
• Routers.
• Gateways.
Hubs
• Connecting or adding hubs to a LAN can effectively
increase the number of workstations.
• This method of growing a LAN is popular, but does
come with many design limitations.
• Uplink
crossover
cables are
wired
differently
than standard
patch cables.
Repeaters
• Repeaters regenerate weakened signals
• Repeaters do not translate or filter signals.
• A repeater
cannot
connect
segments
using
different
access
methods.
Implementing a repeater
• Use a repeater to:
• Connect segments of similar or dissimilar media,
• To increase the distance transmitted.
• Pass all traffic in both directions.
• Connect in the most cost-effective manner.
• Do not use a repeater when:
• There is heavy network traffic.
• Segments are using different access methods.
• Data filtering is needed.
Bridges
• Bridges use routing table to segment network
traffics, and make each segment more efficient.
Remote Bridges
• Because bridges can expand and segment
networks, they are often used in large networks tha
have widely dispersed segments.
Spanning tree algorithm (STA)
• Because remote LAN segments can be joined over
telephone lines, it is possible for multiple LANs to
be joined by more than one path.
• In this situation, it is possible that data might get
into a continuous loop. To handle this possibility,
the 802.1 Network Management Committee of
IEEE has implemented the spanning tree algorithm
(STA).
• STA can sense the existence of more than one
route, determine which would be the most efficient,
and then configure the bridge to use that one.
Routers
• Router works in an environment that consists of
several network segments with differing protocols
and architectures
• Router is a device that not only knows the address
of each segment, but can also determine the best
path for sending data and filtering broadcast
traffic.
• Routers work at the network layer of the OSI
reference model, it can exchange protocol-specific
information between separate networks.
Routing Algorithms
• OSPF ("open shortest path first") is a link-state
routing algorithm. Link-state algorithms control the
routing process and allow routers to respond
quickly to changes in the network. NetWare Link
Services Protocol (NLSP) is a link-state algorithm
to be used with IPX.
• RIP (Routing Information Protocol) uses
distance-vector algorithms to determine routes.
Transmission Control Protocol/Internet Protocol
(TCP/IP) and IPX support RIP.
Types of Routers
• Static Routers.
• Static routers require an administrator to
manually set up and configure the routing table
and to specify each route.
• Dynamic Routers.
• Dynamic routers discover routes automatically
and examine information from other routers to
make decisions about how to send data across
the network.
Distinguishing Between Bridges and
Routers
• Bridges work at the data-link layer MAC sublayer,
and routers work at the network layer
Brouters
• A brouter can act as a router for one protocol and
as a bridge for all the others.
• Brouters can:
• Route selected routable protocols.
• Bridge nonroutable protocols.
• Deliver more cost-effective and more manageable
internetworking than separate bridges and
routers.
Gateways
• Gateways enable communication between different
architectures and environments.
• A gateway links two systems that do not use the
same:
• Communication protocols.
• Data-formatting structures.
• Languages.
• Architecture.
How Gateways Work
• Gateways
are taskspecific,
typically
perform
protocol
conversion
at the
application
layer.