Media, LAN Technologies, and
Network Topologies
Lecture 5
Introduction to Media
Network traffic must flow through some
form of media, whether it is a cable, or
is wireless.
The most common forms of network
media are twisted-pair, coaxial, and
fiber-optic cable.
Twisted-Pair Cable
T-P cable is the most common of all of the
media types in the average local area
network (LAN) environment.
Different categories of T-P cable exist. The
different categories of cable specify the
maximum data bandwidth that the cable can
withstand.
T-P comes in two forms, Unshielded (UTP) or
Shielded (Plenum/STP).
Twisted-Pair Categories
Category
Maximum
Data Rate
Usual
Application
CAT-1
< 1 Mbps
POTS & ISDN
4 Mbps
IBM Token
Ring
16 Mbps
Voice/Data 10baseT
CAT-2
CAT-3
Twisted-Pair Categories (cont.)
CAT-4
CAT-5
CAT-7
(in progress)
20 Mbps
16Mbps Token
Ring Networks
100 Mbps
100baseT,
155Mb ATM
1000 Mbps
1000baseT,
Gigabit
Ethernet
Twisted-Pair Comparison
Advantages





Cheap
Easy to implement
Easy to manage
LOTS of different
applications
Easy to terminate
Disadvantages


Susceptible to
EMF,RF interference
Limited distance –
100 meters
Twisted-Pair (cont.)
Twisted-pair cable
(CAT5 and up)
consists of 4
separate pairs of
wires, all wound
separately.
UTP is shown on the
right.
Coaxial Cable
Coaxial cable (coax) is almost the same
thing that carries your cable TV signal.
Data coax is just held to a higher
quality.
Historical Tidbit: Coax cable, although
not commonly seen nowadays, was how
Ethernet was developed!
Coax (cont.)
The physical
medium itself
consists of an inner
wire, surrounded by
an insulator, which is
also surrounded by
a shield.
Coax Applications
Local Area Networks
(LANs)



Thinnet (10base2) –
200 meters
Thicknet (10base5) –
500 meters
Baseband
transmissions only
Wide Area Networks
(WANs)


T3/DS3/E3
Broadband
transmissions
Baseband v. Broadband
Baseband is where the medium only
carries one signal on the line.
Broadband carries multiple signals on a
single line.
Coax Comparison
Advantages




Highly shielded from
EMF,RF interference
Signals propagate
much farther than TP
cable.
Conforms to
standards.
More channels than
TP cable.
Disadvantages





One cable for all
computers.
To add additional
computers, network must
be taken down.
MUST properly terminate.
Expensive.
Low channel count
compared to fiber.
Fiber Optic Cable
Fiber optic cable is where the future of
LAN wiring exists.
It is wicked fast.
It is WICKED fast!
Fiber Optic Cable (cont.)
Fiber comes in two
different types:


Multimode – a
channelized fiberoptic circuit. Multiple
carrier frequencies.
Singlemode – a
“clear channel”
circuit. One carrier
frequency.
Fiber Comparison
Advantages



Wicked fast!
Handles lots of
simultaneous B
channels.
Very reliable.
Disadvantages

Cost to implement.
 Splicing kit.
 Cable costs.
 Redundancy (FDDI)?


When disaster
strikes, it’s a major
ordeal.
Point-to-point only
Fiber Applications
High-bandwidth voice transmission.
“Backbone” applications.
Very fast data transfer between network
devices.
Other Media
Cellular
Infrared
Satellite


Geosynchronous
Low Earth Orbit
Packet Radio
Microwave
Applying It All To LANs
Point to point (between all computers)
is unmanageable. This only becomes
apparent when looking at multiple
machines.
If We Can’t Use Point-ToPoint?
That is where the concept of a shared
medium was derived.
If a shared medium was in use, then
you would only have to connect all of
the computers together, not each
individual computer!
Locality of Reference
A computer is more likely to
communicate with computers that it is
physically near than those that it is far
away from.
If a pair of computers communicates
once, the pair is likely to communicate
again in the near future, and then
periodically.
LAN Topologies
There are three main types of LAN
systems:



Star Topology
Ring Topology
Bus Topology
Some topologies are actually hybrids of
the above!
Star Topology
All computers
connect to a
centralized point.
The central point is
called the hub.
Ring Topology
One computer is
connected to the
two computers
adjacent to it.
In the traditional
case, if a system is
disconnected from
the ring, the
network fails.
Bus Topology
A bus topology consists of a single
cable to which each network device
attaches.
Bus topology has the same problems as
a ring topology.
Ethernet
This is the original Ethernet design by
Bob Metcalfe in 1976.
Ethernet (cont.)
The Ethernet specification details how devices
are supposed to interact on the segment,
distances between devices, and a whole other
multitude of other things.
Carrier Sense On Multi-Access Networks
(CSMA) with Collision Detection (CD) was the
most important part of the specification.
CSMA on Ethernet
CSMA indicates that computers wait
until the ether is free. In this case, no
electrical signal denotes when the ether
is “free.”
CSMA on Ethernet (cont.)
The “signal” is the carrier of the ether.
The act of waiting for the opportunity to
send on the ether is carrier sense.
Collision Detection
Since networks are not instantaneous, it is
possible for two stations to check the ether,
deem it time to send, and both send at the
same time.
This is called a collision.
When a collision occurs, the sender
immediately stops transmission, and waits
some random length of time, and then begins
transmission again.
Collision Detection (cont.)
A busy segment will always have collisions.
Collisions do not damage the equipment, but
it forces the two machines that caused the
collision to wait, and therefore it slows down
the network.
Fewer machines on a segment, smaller
segments, can improve the collision count.
LocalTalk
LocalTalk is another form of a bus network.
LocalTalk differs from Ethernet in a few ways:


When a machine senses the segment is free, it
holds it until the frame transmission is over. All
other machines know to wait for transmission to
complete.
Bandwidth is quite small at ~230Kbps. That’s
2.3% of the earliest Ethernet specification!
IBM Token Ring
Access mechanism is
called token passing.
Once the sent
information makes a
complete turn around
the ring, the sender
passes the token to the
next machine.
This is a strictalternation scheme
where all machines
have the same chance
to transmit information.
Fiber Distributed Data
Interconnect (FDDI)