Local Area Networks, 3rd Edition
David A. Stamper
Part 5: Connecting to Other Systems
and Networks
Chapter 14
Making Network
Connections
© 2001 by Prentice Hall
14-1
Chapter Preview
In this chapter you will study:
• LAN-to-LAN connections
• Repeaters, bridges, routers, and
gateways
• Switches and virtual LANs
• Remote access
• Common carrier services
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LAN-to-LAN Connections
• Companies that have LANs in separate geographical
locations or LANs that cover distances greater than the
maximum medium distance allowed, or companies with
more nodes that can be accommodated by one LAN, must
segment their network into two or more LANs.
• Having separate LANs allows a company to split functions
and gives an additional level of security.
• LANs may also be connected in order to consolidate
independent LANs that may have been formed in an ad-hoc
manner.
• Responsiveness of the system can be maintained while the
number of users increases by adding more resources to an
existing LAN—memory, disks, another server, etc. —or by
splitting the LAN into two or more smaller LANs.
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Repeaters
• Every LAN has a distance restriction. IEEE
standards specify a maximum segment length of
500 meters. If you want to span longer distances,
you can use a repeater to connect two segments.
• As signals travel along the medium, they lose
strength through attenuation. Weak signals can
cause transmission errors. A repeater accepts a
signal, regenerates it, and passes it along at full
strength.
• A repeater does not separate one segment of the
network from another.
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A Repeater, Bridge, and Router
and the OSI Reference Model
Processor 1
Processor 2
Application Layer
Application Layer
Presentation Layer
Presentation Layer
Session Layer
Session Layer
Transport Layer
Transport Layer
Network Layer
Router
Network Layer
Data Link Layer
Bridge
Data Link Layer
Physical Layer
Repeater
Physical Layer
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Bridges
• Early bridges were used to connect two networks,
each of which used the same MAC protocol.
Today, bridges also connect LANs having
different MAC protocols. This device may be
called a ‘brouter’ or multiprotocol bridge.
• Most bridges being sold today are called learning
bridges, or transparent bridges. A learning bridge
builds its routing table from messages it receives
and does not need to be loaded with a predefined
routing table.
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Basic Bridge Functions
Packet Routing Function 1. Accept packet from LAN A.
2. Examine address of packet.
3. If packet address is a LAN A address, allow the
packet to continue on LAN A.
4. If packet address is a LAN B address, transmit
the packet onto the LAN B medium.
5. Do the equivalent for LAN B packets.
Additional Functions
Media conversion
Learning
Remote connection
Signal conversion
Speed conversion
Packet statistics
Token ring to ethernet conversion
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Token Rings Connected by a
Bridge
LAN A
Token-Ring Network
Bridge
LAN B
Token-Ring Network
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Spanning Tree Algorithms
• Spanning tree algorithms, in which bridges
exchange routing information with each other,
can be used on any type of LAN.
• The advantages of the spanning tree algorithm
are that it is MAC-layer-independent, bridges can
learn the topology of the network without manual
intervention, and paths can change if an existing
path becomes inoperable or if a better path is
introduced. The algorithm overhead is the size of
the routing table for networks with many
communicating nodes, and the extra network
traffic resulting from status messages and
flooding.
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Source Routing
• Source routing is an IEEE standard used as a
routing algorithm for token-passing networks.
• The advantage of the source routing algorithm is
that bridges are not responsible for maintaining
large routing tables for extensive networks. Each
node is responsible for maintaining routing
information only for the nodes with which it
communicates. The disadvantages are the
overhead of sending numerous packets during
discovery and the extra routing data that must be
appended to each message.
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Remote Bridge Connection
Alternatives
RS-232 serial lines
Synchronous transmission at 56 Kbps or 64 Kbps
Fractional T-1 at multiples of 64 Kbps
Integrated services digital network (ISDN)
Digital subscriber lines (DSL)
RS-422 serial lines at 19.2 Kbps to 2 Mbps
T-1 Line at 1.5 Mbps
X.25 packet-switching network
Frame relay
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Routers
• Routers operate at the network layer of the OSI Reference
Model. The network layer is responsible for packet routing
and for collecting accounting information.
• Some networks use a static routing algorithm, meaning that
packet routing between two nodes never changes. In a
CSMA/CD bus LAN, a packet is broadcast to every node; in
a token ring, a packet is transmitted from one node to the
next node in the ring.
• A Novell network uses a protocol called sequenced packet
exchange/internetwork packet exchange (SPX/IPX) to
transfer packets between nodes. SPX operates at the
transport layer and IPX at the network layer.
• Another protocol used by many networks is the
transmission control protocol/internet protocol (TCP/IP).
TCP/IP is the protocol used on the Internet.
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Gateways
• The interface between two dissimilar networks is
called a gateway. A gateway is basically a
protocol converter. It reconciles the differences
between the networks it connects.
• The components of a gateway are the network
interfaces and the logic that carries out the
conversion necessary when moving messages
between networks. The conversion must change
the header and trailer of the packet to make it
consistent with the protocol of the network or
data link to which the message is being
transferred.
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Ways to Increase LAN Media
Throughput
•
•
•
•
•
•
Faster hardware
Higher network speeds
Lower bandwidth demands
LAN segmentation
Full duplex
Switching hubs
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LAN Switches
• LAN switches work similarly to the way in which a
telephone switch works. In the idle state, you are not
connected to anything except the telephone switch and
cannot communicate with anyone until a circuit is set up.
When you dial someone’s number, the complete
transmission capacity of the telephone network is not
dedicated to your call; instead, a connection is made
between your telephone and the telephone of the person
you are calling and a single circuit is used, leaving other
circuits available to other subscribers.
• LAN switches, also called switching hubs, look much like
standard wiring hubs. The switching hub examines the data
link header of the packet and obtains the destination
address. The switch then establishes a dedicated
connection between the sender’s port and the recipient’s
port, and the two communicate.
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A LAN Switch
Common Server
Common Server
Switching Hub
10/100 Mbps
To/From
Other Hubs
Conventional Hub-10/100 Mbps
To Workstations or
Segment Servers
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Conventional Hub-10/100 Mbps
To Workstations or
Segment Servers
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Some Benefits of Switching Hubs
Efficient network segmentation to balance traffic.
Good price/performance.
Availability of shared and dedicated bandwidth.
Support of new technologies such as asynchronous transfer mode
and other network protocols.
Preservation of investment in cabling and LAN adapters.
Devices can communicate at NIC speeds as long as necessary.
Provide bridging and routing as well as integration with faster
technologies, such as FDDI, 100 Mbps ethernet.
Multiple connections can be established concurrently.
Ability to define virtual LANs (VLANs)
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Virtual LANs
• The benefit of VLANs is the ability to collect existing
computers into a VLAN and to move workstations while
maintaining their VLAN connection. Because VLANs are
identified by addresses or switch ports and not by physical
connections, LAN nodes can be moved from one location
to another and remain on the same VLAN without having to
change connections at the wiring hubs.
• A variety of methods are used to determine how users are
grouped into a VLAN:
–
–
–
–
Port
MAC Address
Packet Tagging
Network layer addresses
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LAN-to-Host Connections
• The Host as a LAN Node
– Some hosts have the ability to connect to the LAN as a node. This is the
most effective way to establish the connection.
• Asynchronous Connections
– Virtually every computer has the ability to send and receive by using an
asynchronous data link protocol. Because most computers support the
asynchronous data link protocol, it is sometimes used to link a
microcomputer to a host. Usually, a microcomputer attached to a host
asynchronously operates in one of two modes: file transfer or terminal
emulation.
• Dedicated Connection Per Microcomputer
– One way to connect a LAN node to a host is to provide a dedicated
connection between a port on the host and each microcomputer needing a
host connection. A dedicated connection provides direct host access, and
the microcomputer does not use LAN resources for communicating with the
host.
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LAN-to-Host Connections (cont.)
– A dedicated connection has several disadvantages. First, as with all
asynchronous connections, the sped of the link is slow. These connection
speeds can be over 100,000 bps, but typically for microcomputer
connections they are 33.6 Kbps or less. If many LAN nodes must
communicate with the host, many host ports are required.
• Multiplexing
– A multiplexer is a hardware device that allows several devices to share one
communication channel. Multiplexing is typically used to consolidate the
message traffic between a computer and several remotely located terminals.
This technique can also be used to allow several microcomputers to share a
communication link to a host processor.
• Shared Asynchronous Connections
– In some applications, each LAN node needs occasional access to the host,
but the number of concurrent connections is far fewer than the number of
LAN nodes. In such situations, a dedicated line per node is excessive. A
better solution is to share asynchronous connections.
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Common Features of Asynchronous
Communications Software
Scripts
Mouse support
File transfers (CompuServe, Xmodem, Ymodem, Kermit)
Terminal emulation (ANSI, DEC VT220, IBM 3101, TTY)
Electronic mail
Phone directory
Capture of data to a disk
Text editor
Password security
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Interconnection Utilities
• File Transfer Utilities
– File transfer utilities allow you to move files between network nodes. File
transfer capabilities are an intrinsic part of many routers; part of the TCP/IP
protocol suite is a file transfer capability.
• Remote Logon
– A remote logon essentially establishes a remote user as a local user on the
remote node. Once a user has successfully logged onto the remote node,
commands issued by that user are processed and acted on by the remote
node rather than by the local node.
• Remote Access
– Through remote connections, LAN administrators can resolve problems
from home or other work locations; users can perform some of their work at
home and telecommute; and travelers can conduct work while away form
the office.
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Interconnection Utilities (cont.)
• Integrated Services Digital Networks (ISDNs)
– One objective of ISDNs is to allow international data exchange. This requires
interfaces between a number of national and regional providers of such
services. The first mission of the ISDN program has been to define the
functions and characteristics of the network and to establish
implementation standards.
– ISDN was the first high-speed alternative to switched, analog connections
for Internet access. ISDN when used for Internet access usually provides
speeds of 128 Kbps.
• Digital Subscriber Lines (DSL)
– Digital subscriber lines are an emerging service that provides much faster
transmission rates than analog modems and ISDN.
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