Topic 7: LANs & Backbone
Networks
- Chapter 14: Local Area Network Technology
Business Data Communications,
4e
1
PC Networks
 Client/Server Communication
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Shared databases
Shared hardware resources
Shared Internet access
 Peer-to-Peer Communication
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Sharing work and information with colleagues
 Low cost is high priority
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Attachment costs in the hundreds of dollars
 A small LAN’s components:
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Computer, hub, cable, NIC, and network operating system.
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Tiered LANs
 Cost of attachment to a LAN tends to
increase with data rate
 Alternative to connecting all devices is
to have multiple tiers
 Multiple advantages
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Higher reliability
Greater capacity (less saturation)
Better distribution of costs based on need
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Tiered LAN Diagram
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LAN Topology
 Arrangement of workstations in a
shared medium environment
 Logical arrangement (data flow)
 Physical arrangement (cabling scheme)
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LAN Topologies: Bus
 Multipoint medium
 Stations attach to linear medium (bus)
using tap
 Full-duplex between station and tap
 Transmission from any stations travels
entire medium (both directions)
 Termination required at ends of bus
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Bus LAN Diagram
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LAN Topologies: Tree
 Generalization of bus topology
 Branching cable with no closed loops
 Cable(s) begin at headend, travel to
branches which may have branches of
their own
 Each transmission propagates through
network, can be received by any station
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Tree LAN Diagram
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Bus/Tree Topology Problems
 How do you identify who the
transmission is intended for?
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Data transmitted in frames
Each frame has header with addressing
info
 How do you regulate access?
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Stations take turns sending, by monitoring
control information in frames
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LAN Topologies: Ring
 Repeaters are joined by unidirectional
point-to-point links in a ring
 As a frame circulates past a receiver,
the receiver checks its address, and
copies those intended for it into a local
buffer
 Frame circulates until it returns to
source, which removes it from network
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Ring LAN Diagram
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LAN Topologies: Star
 Each station connected directly to
central node, usually with two
unidirectional links
 Central node can broadcast info, or can
switch frames among stations
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Star LAN Diagram
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Choosing a Topology
 Factors to consider include reliability,
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flexibility/expandability, and performance
Bus/tree is most flexible
Tree topology easy to lay out
Ring provides high throughput, but reliability
problems
Star can be high speed for short distances,
but has limited expandability
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Medium and Topology
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LAN Standards (IEEE802.x)
 Advantages of standards
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Assure sufficient volume to keep costs down
Enable equipment from various sources to
interconnect
 IEEE 802 committee developed, revises,
and extends standards
 Use a three-layer protocol hierarchy:
physical, medium access control (MAC),
and logical link control (LLC)
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IEEE LAN Standards
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IEEE 802.2: Logic link control (LLC) layer of data link layer
IEEE 802.3: Ethernet
IEEE 802.4: Token bus, an old protocol
IEEE 802.5: Token ring
IEEE 802.6: Distributed queue dual bus (DQDB) protocol, similar
to FDDI
IEEE 802.9: Integrated voice and data networking, including
ISDN, Iso-ethernet
IEEE 802.11: Wireless LAN
IEEE 802.12: 100Base-VG
IEEE 802.13: 100Base-X
IEEE 802.14: Cable modem
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*Logical Link Control
 Specifies method of addressing and controls exchange
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of data
Independent of topology, medium, and medium access
control
Unacknowledged connectionless service (higher layers
handle error/flow control, or simple apps)
Connection-mode service (devices without higher-level
software)
Acknowledged connectionless service (no prior
connection necessary)
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*Medium Access Control
 Provides a means of
controlling access to
a shared medium
 Two techniques in
wide use
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CSMA/CD
Token passing
 LLC frames data,
passes it to MAC
which frames it
again
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MAC control
(e.g. priority level)
Destination physical
address
Source physical
address
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Ethernet (IEEE 802.3)
The Ethernet LAN standard was originally
developed by DEC, Xerox, and Intel, but
has since become a formalized standard
by the Institute of Electrical and
Electronics Engineers as IEEE 802.3
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Ethernet Topology
Ethernet uses a bus topology (a high speed circuit and
a limited distance between the computers, such as
within one building).
From the outside, an ethernet LAN appears to be a star,
because all cables connect to the central hub.
Most ethernet LANs span sufficient distance to require
several hubs, but some ethernet LANs are build
without the use of hubs (coax bus).
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Ethernet Media Access Control
Carrier Sense Multiple Access with Collision
Detection (CSMA/CD)
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Wait until the bus is free and then transmit.
If no collision, transmission is completed.
If the collision is detected, send a jamming signal.
Wait a random amount of time, then re-broadcast.
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Token Ring (IEEE 802.5)
Token Ring was originally developed
by IBM, and have since been
standardized by IEEE as IEEE
802.5.
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Topology
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Token Ring Media Access
Control
 Token ring uses a controlled-access technique called token
passing.
 The “token” is a series of bits, travels between the computers in
a predetermined sequence.
 A computer with a message waits to transmit until it receives a
free token. The computer changes the free token to a busy
token and attaches its message to it. Then it retransmits it on
the circuit to the next computer in the sequence.
 The computer receiving the message, changes the
acknowledgement to ACK (or NAK) and sends the message back
to the sender, who creates a new free token.
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Token Ring Media Access
Control
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Token Ring Media Access
Control
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Token Ring Media Access
Control
Token loss:
 The token crashes before being transmitted - lost a free token
 A computer in the ring crashes - lost a busy token
 A token is always busy.
A solution for the “lost” token problem:
 Designate one computer to be the token monitor and another
computer to be a backup token monitor.
 If no token circulated through the network for a certain length of
time or if a busy token circulates too often, the token monitor will
create a new free token (and destroy the busy token if necessary.)
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Backbone Network Devices
Device
Operates at
Messages
Hub
Bridge
Physical
Data link
Switch
Data link
Router
Network
Gateway
Network
All transferred
Filtered using
data link layer add.
Switched using
data link layer add.
Routed using
network layer add.
Routed using
network layer add.
Physical Data Link Network
Layer
Layer Layer
S/D
S/D
Same Same
Same Same
S/D
Same Same
S/D
S/D
Same
S/D
S/D
S/D
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Hubs
 The active central element of the star
layout.
 When a single station transmits, the
hub repeats the signal on the outgoing
line to each station.
 Physically a star; logically a bus.
 Hubs can be cascaded in a hierarchical
configuration.
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Hubs
Operating at the physical layer, hubs are very
simple devices that pass all traffic in both
directions between the LAN sections they link.
They may connect different types of cable, but
use the same data link and network protocol.
Strictly speaking, hubs are not considered part
of a backbone network, but are usually
repeaters or amplifiers.
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Two-Level Star Topology
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Bridges
 Allow connections between LANs and to WANs
 Operates at Layer 2 (Data Link Layer) of OSI
 Used between networks using identical
physical and link layer protocols
 Provide a number of advantages
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Reliability: Creates self-contained units
Performance: Less contention
Security: Not all data broadcast to all users
Geography: Allows long-distance links
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Bridges
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Bridge Functions
 Read all frames from each network
 Accept frames from sender on one network that
are addressed to a receiver on the other network
 Retransmit frames from sender using MAC
protocol for receiver
 Must have some routing information stored in
order to know which frames to pass
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Bridges
If a bridge receives a packet with a destination
address that is not in the address table, it
forwards the packet to all networks or
network segments except the one on which it
was received.
Bridges are a combination of both hardware
and software, typically a “black box” that sits
between the two networks, but can also be a
computer with two NICs and special software.
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Bridge Operation
39
Ethernet Hubs and Switches
 Shared
medium
hubs
 Switched
x
LAN hubs
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Switches
Like bridges, switches operate at the data
link layer. Switches connect two or more
computers or network segments that
use the same data link and network
protocol. They may connect the same
or different types of cable.
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Switches
Switches operate at the same layers as bridges
but differ from them in two ways:
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First, most switches enable all ports to be in use
simultaneously, making them faster than bridges.
Second, unlike bridges, switches don’t learn
addresses, and need to have addresses defined.
Example: Intel Express 510 T switch.
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Switched Ethernet
 A simple concept behind switched
Ethernet - replace the LAN hub with a
switch. Each computer now has its own
dedicated point-to-point circuit.
 By increasing the number of
connections from the server to the
switch, the throughput of the server will
be improved because of more circuits.
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Switched Ethernet
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Types of Switches
 Store and forward switch
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Accepts a frame on input line
Buffers it briefly
Routes it to appropriate output line
 Cut-through switch
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Begins repeating the frame as soon as it
recognizes the destination MAC address
Higher throughput, increased chance of error
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Routers
Routers operate at the network layer. Routers connect
two or more LANs that use the same or different data
link protocols, but the same network protocol.
Routers may be “black boxes,” computers with several
NICs, or special network modules in computers.
In general they perform more processing on each
message than bridges and therefore operate more
slowly.
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Routers
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Routers vs Bridges
 Routers can choose the best route.
 Routers also only process messages
specifically addressed to it.
 Routers can connect networks using different
data link layer protocols. Therefore, routers
are able to change data link layer packets.
 Routers may split a message into several
smaller messages for transmission.
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Layer 3 Switches
 Problems With Layer 2 Switches
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Broadcast overload because of the single MAC broadcast address
(e.g. using ARP for Data Link Layer address resolution)
Lack of multiple links - only one path
 Normally, the above problems can be solved with several
subnets connected by routers. However,
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A MAC broadcast frame is then limited to only the devices and
switches contained in a single subnet.
A router does all IP-level processing, some of which could be not
necessary.
It is implemented in software and slow.
 Layer 3 switches implement the packet-forwarding logic of the
router in hardware.
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Gateways
Gateways operate at the network layer and use
network layer addresses in processing
messages.
Gateways connect two or more LANs that use
the same or different (usually different) data
link and network protocols. The may connect
the same or different kinds of cable.
Gateways process only those messages
explicitly addressed to them.
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Gateways
Gateways translate one network protocol into
another, translate data formats, and open
sessions between application programs, thus
overcoming both hardware and software
incompatibilities.
A gateway may be a stand-alone
microcomputer with several NICs and special
software, a FEP connected to a mainframe
computer, or even a special circuit card in the
network server.
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Gateways
One of the most common uses of gateways is to
enable LANs that use TCP/IP and ethernet to
communicate with IBM mainframes that use SNA.
The gateway provides both the basic system
interconnection and the necessary translation
between the protocols in both directions.
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Gateways
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A Caveat
The terminology used in the marketplace
may differ substantially. One vendor’s
bridge may provide the functions of a
router.
 Multiprotocol routers -can understand several
different network layer protocols.
 Brouters – Combine the functions of both bridges
and routers. They operate at both data link and
network layers.
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Cable Modem Configuration
20:02:43
IP Address
Subnet Mask
Gateway
Domain Name Server
68.1.216.37
255.255.248.0
68.1.216.1
68.1.208.30, 68.1.18.30
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