Lecture Topics : #
1 Transmission Media
Guided Media
Unguided: Wireless
2 Switching
Circuit-switched Networks
Packet-switched Networks
Message Switching

Transmission Media
Outline
Guided Media
Unguided: Wireless
2 Switching
Message Switching

Transmission Media
Transmission Media
• A transmission medium can be broadly defined as anything that can
carry information from a source to a destination.
• Transmission media are actually located below the physical layer and
are directly controlled by the physical layer

Transmission Media
Transmission Media …
• The transmission medium is usually
• Free space
• Metallic cable
• Fiber-optic cable
• The information is usually a signal that is the result of a conversion of
data from another form

Transmission Media
Transmission Media …
• In telecommunications, transmission media can be divided into two
broad categories:

Transmission Media Guided Media
Outline
Guided Media
Unguided: Wireless
2 Switching

Guided Media
• Guided media, which are those that provide a conduit from one device
to another,
• …include twisted-pair cable, coaxial cable, and fiber-optic cable.
• A signal traveling along any of these media is directed and contained
by the physical limits of the medium.
• Guided Media are also known as bounded media , since the data
signals are bounded by the cabling system.
• Twisted-pair and coaxial cable use metallic (copper) conductors that
accept and transport signals in the form of electric current.
• Optical fiber is a cable that accepts and transports signals in the form
of light.

Twisted-pair cable
• A twisted pair consists of two conductors (normally copper), each
with its own plastic insulation, twisted together
• One of the wires is used to carry signals to the receiver, and
• the other is used only as a ground reference.
• The receiver uses the difference between the two.

Twisted-Pair Cable …
• the twists between wires is helpful in reducing noise (electro-magnetic
interference) and crosstalk
• If the two wires are parallel, the effect of these unwanted signals is
not the same in both wires
• …because they are at different locations relative to the noise or
crosstalk sources (e,g., one is closer and the other is farther).
• This results in a difference at the receiver.
• By twisting the pairs, a balance is maintained.
• There are two types of twisted pair cables available:
1 Shielded Twisted Pair (STP)
2 Unshielded Twisted Pair (UTP)

Shielded Twisted-Pair Cable(STP)
• STP cable has a metal foil or braided mesh covering that encases
each pair of insulated conductors.
• Although metal casing improves the quality of cable by preventing the
penetration of noise or crosstalk,
• …it is bulkier and more expensive.

Shielded Twisted-Pair Cable(STP) …
• This type of shielding protects cable from external Electromagnetic
Interferences (EMI).
• Therefore these cables can be used in noisy area.
• Shielded twisted pair cable (STP) is the standard cable specified for
IBM Token Ring networks and for Apple’s LocalTalk.

Unshielded Twisted Pair (UTP)
• most common twisted-pair cable used in communications.
• Unshielded twisted pair ( UTP ) cables contain no shielding.
• The UTP cables in computer networking have resistance rating of 100
ohm.
• UTP cable most commonly includes four pairs of wires enclosed in a
common sheath.
• UTP is a very flexible, low cost media, and can be used for either
voice or data communications.
• It is easy to install and maintain.
• It is widely used for cabling LAN in today’s computer network.

• RJ45: Twisted pair cables are connected by RJ45 connectors.
• The RJ45 is a keyed connector, meaning the connector can be
inserted in only one way.

• UTP cables are divided into different categories (Cat 1-6/7) which
vary in the number of twists per foot and quality.
• For example, Category 3 cables have 2 twists per foot whereas
Category 5 has 12.
• The higher categories allow transmitting data at higher speeds.

Table: Categories of unshielded twisted-pair cables
Cate-
gory
Specification Data
Rate
(Mbps
Use
CAT 1 Unshielded twisted-pair
used in telephone
> 0.1 Used for voice only, not
for data.
CAT 2 Unshielded twisted-pair
originally used in T-lines
2-4 Used for carrying data
at rate of 4 Mbps for Lo-
cal Talk
CAT 3 Improved CAT 2 used in
LANs .Contains 4 pair
of wires, with possible
maximum bandwidth of
16 Mbps
10 Used for 10 Mbps Eth-
ernet and 4 Mbps Token
Ring.

CAT 4 Improved CAT 3 used in
Token Ring networks
20 Used with 10 Mbps Eth-
ernet and 16 Mbps To-
ken Ring.
Contains 4 pair of wires;
provides more protec-
tion to crosstalk than
CAT 3
CAT 5 Cable wire is normally
24 AWG with a jacket
and outside sheath.
Contains 4 pair of
wires; it has better
protection for crosstalk.
100 Used for 100 Mbps Eth-
ernet (Fast Ethernet)

CAT 6 Contains 4 pair of wires;
it can carry data at rate
of 1Gbps.
1000 Used for Gigabit Ether-
net that can carry data
at rate of 1000Mbps

• There are three types of UTP cables
1 Straight-through cable
2 Crossover cable
3 Rollover cable

Straight-Through Cable
• This Ethernet network cable is made of 4 pair high performance cable
that consists twisted pair conductors that used for data transmission.
• Both end of cable is called RJ45 connector.
• they are primarily used to connect unlike devices, like Router to
Hub/Switch, Computer to Hub/Switch
• The cable can be categorized as Cat 5, Cat 5e, Cat 6 UTP cable.

Straight-Through Cable …
• You usually use straight cable to connect different type of devices.
• This type of cable will be used most of the time and can be used to:
• Connect a computer to a switch/hub’s normal port.
• Connect a computer to a cable/DSL modem’s LAN port.
• Connect a router’s WAN port to a cable/DSL modem’s LAN port.
• Connect a router’s LAN port to a switch/hub’s uplink port. (normally
used for expanding network)
• Connect 2 switches/hubs with one of the switch/hub using an uplink
port and the other one using normal port.

Crossover Cable
• …the only difference is each type will have different wire arrangement
in the cable for serving different purposes.
• in Crossover cable the wires orange-white and green-white, and then
the orange and green are switched
• they are primarily used to connect like devices, like Router to
Router/Computer, Computer to Computer, Hub to Hub, and Switch
to Switch

Applications
• Twisted-pair cables are used in telephone lines to provide voice and
data channels.
• The local loop-the line that connects subscribers to the central
telephone office—commonly consists of unshielded twisted-pair cables.
• The DSL lines that are used by the telephone companies to provide
high-data-rate connections also use the high-bandwidth capability of
unshielded twisted-pair cables.
• Local-area networks, such as lOBase-T and lOOBase-T, also use
twisted-pair cables.

Rollover Cable
• in Rollover cable one end of the cable wired exactly opposite from the
other
• it connects a device to a router or switchs console port.
• it allows a programmer to make a connection to the router or switch,
and program it as needed

Coaxial Cable
• Coax has a central core conductor of solid or stranded wire (usually
copper) enclosed in an insulating sheath,
• …which is, in turn, encased in an outer conductor of metal foil, braid,
or a combination of the two.
• The outer metallic wrapping serves both as a shield against noise and
as the second conductor, which completes the circuit.
• This outer conductor is also enclosed in an insulating sheath, and the
whole cable is protected by a plastic cover

Coaxial Cable …
• Coaxial cable (or coax) carries signals of higher frequency ranges than
those in twisted pair cable
• To connect coaxial cable to devices, we need coaxial connectors.
• The most common type of connector used today is the
Bayone-Neill-Concelman (BNe), connector
• Three popular types of BNe connectors

Coaxial Cable …
1 BNC connector
• …used to connect the end of the cable to a device, such as a TV set.
2 BNC T connector
• …used in Ethernet networks to branch out to a connection to a
computer or other device.
3 BNC terminator
• …used at the bend of the cable to prevent the reflection of the signal.

Coaxial Cable …
Applications
• Coaxial cable was widely used in analog telephone networks where a
single coaxial network could carry 10,000 voice signals.
• it was used in digital telephone networks where a single coaxial cable
could carry digital data up to 600 Mbps.
• However, coaxial cable in telephone networks has largely been
replaced today with fiber-optic cable
• Cable TV networks also use coaxial cables.
• Another common application of coaxial cable is in traditional
Ethernet LANs

Fiber-Optic Cable
• A fiber-optic cable is made of glass or plastic and transmits signals in
the form of light.
• It transfers data in the form of light rather than electrical signals.
• Optical fibers use reflection to guide light through a channel.
• A glass or plastic core is surrounded by a cladding of less dense glass
or plastic.
• The difference in density of the two materials must be such that a
beam of light moving through the core is reflected off the cladding
instead of being refracted into it.

Fiber-Optic Cable …
Fiber-Optic Cable Connectors
• There are three types of connectors for fiber-optic cables
1 Subscriber channel (SC) connector
2 Straight-tip (ST) connector
3 MT-RJ

• The subscriber channel (SC) connector is used for cable TV.
• …It uses a push/pull locking system.
• The straight-tip (ST) connector is used for connecting cable to
networking devices.
• …It uses a bayonet locking system and is more reliable than SC.
• MT-RJ is a connector that is the same size as RJ45.

Applications
• Fiber-optic cable is often found in backbone networks because its
wide bandwidth is cost-effective.
• Today, with wavelength-division multiplexing (WDM), we can transfer
data at a rate of 1600 Gbps.
• The SONET network that provides such a backbone
• Some cable TV companies use a combination of optical fiber and
coaxial cable,
• …thus creating a hybrid network.
• …Optical fiber provides the backbone structure while coaxial cable
provides the connection to the user premises
• Local-area networks such as 100Base-FX network (Fast Ethernet) and
1000Base-X also use fiber-optic cable.

• Fiber-optic cable has several advantages over metallic cable
(twisted-pair or coaxial).
• Higher bandwidth.
• Fiber-optic cable can support dramatically higher bandwidths (and
hence data rates) than either twisted-pair or coaxial cable.
• Currently, data rates and bandwidth utilization over fiber-optic cable
are limited not by the medium but by the signal generation and
reception technology available.
• Less signal attenuation
• Fiber-optic transmission distance is significantly greater than that of
other guided media.
• A signal can run for 50 km without requiring regeneration.
• We need repeaters every 5 km for coaxial or twisted-pair cable.
• Immunity to electromagnetic interference.
• Electromagnetic noise cannot affect fiber-optic cables.
• Resistance to corrosive materials
• Glass is more resistant to corrosive materials than copper.

• There are some disadvantages in the use of optical fiber
• Installation and maintenance
• Fiber-optic cable is a relatively new technology.
• Its installation and maintenance require expertise that is not yet
available everywhere.
• Unidirectional light propagation.
• Propagation of light is unidirectional.
• If we need bidirectional communication, two fibers are needed.
• Cost
• The cable and the interfaces are relatively more expensive than those of
other guided media.
• If the demand for bandwidth is not high, often the use of optical fiber
cannot be justified

Transmission Media Unguided: Wireless
Outline
Guided Media
Unguided: Wireless
2 Switching

Unguided: Wireless
• Unguided media transport electromagnetic waves without using a
physical conductor.
• Signals are normally broadcast through free space and thus are
available to anyone who has a device capable of receiving them.

Unguided: Wireless …
• Unguided signals can travel from the source to destination in several
ways:
1 Ground propagation
2 Sky propagation
3 Line-of-sight propagation

Ground propagation
• radio waves travel through the lowest portion of the atmosphere,
hugging the earth.
• These low-frequency signals emanate in all directions from the
transmitting antenna and follow the curvature of the planet.
• Distance depends on the amount of power in the signal:
• …The greater the power, the greater the distance.
Sky propagation
• higher-frequency radio waves radiate upward into the ionosphere(the
layer of atmosphere where particles exist as ions)
• …where they are reflected back to earth.
• This type of transmission allows for greater distances with lower
output power.

Line-of-sight propagation
• …very high-frequency signals are transmitted in straight lines directly
from antenna to antenna.
• Antennas must be directional, facing each other, and either tall
enough or close enough together not to be affected by the curvature
of the earth.
• Line-of-sight propagation is tricky because radio transmissions cannot
be completely focused.
• The section of the electromagnetic spectrum defined as radio waves
and microwaves is divided into eight ranges, called bands,
• …each regulated by government authorities.
• These bands are rated from very low frequency (VLF) to extremely
high frequency (EHF)

Figure: Bands

• We can divide wireless transmission into three broad groups:

Radio Waves
• Although there is no clear-cut demarcation between radio waves and
microwaves,
• …electromagnetic waves ranging in frequencies between 3 kHz and 1
GHz are normally called radio waves;
• …waves ranging in frequencies between 1 and 300 GHz are called
microwaves.
• Radio waves, for the most part, are omnidirectional.
• When an antenna transmits radio waves, they are propagated in all
directions.
• This means that the sending and receiving antennas do not have to
be aligned.
• A sending antenna sends waves that can be received by any receiving
antenna.

Applications
• Radio waves are used for multicast communications
• The omnidirectional characteristics of radio waves make them useful
for multicasting,
• …in which there is one sender but many receivers.
• AM and FM radio, television, maritime radio, cordless phones, and
paging are examples of multicasting.

Microwaves
• Electromagnetic waves having frequencies between 1 and 300 GHz are
called microwaves.
• Microwaves are unidirectional.
• When an antenna transmits microwave waves, they can be narrowly
focused.
• This means that the sending and receiving antennas need to be
aligned.
• The unidirectional property has an obvious advantage.
• A pair of antennas can be aligned without interfering with another
pair of aligned antennas.

Microwaves
Unidirectional Antenna
• Microwaves need unidirectional antennas that send out signals in one
direction.
• Two types of antennas are used for microwave communications: the
parabolic dish and the horn

Microwaves
Applications
• Microwaves, due to their unidirectional properties, are very useful
when unicast (one-to-one) communication is needed between the
sender and the receiver.
• They are used in cellular phones , satellite networks , and wireless
LANs

Infrared
• Infrared waves, with frequencies from 300 GHz to 400 THz
(wavelengths from 1 mm to 770 nm),
• …can be used for short-range communication.
• Infrared waves, having high frequencies, cannot penetrate walls.
• This advantageous characteristic prevents interference between one
system and another;
• …a short-range communication system in one room cannot be affected
by another system in the next room.
• When we use our infrared remote control, we do not interfere with the
use of the remote by our neighbors.
• However, this same characteristic makes infrared signals useless for
long-range communication.
• In addition, we cannot use infrared waves outside a building because
the sun’s rays contain infrared waves that can interfere with the
communication.

Infrared
Applications
• The infrared band, almost 400 THz, has an excellent potential for
data transmission.
• Such a wide bandwidth can be used to transmit digital data with a
very high data rate.
• The Infrared Data Association (IrDA),
• …an association for sponsoring the use of infrared waves,
• …has established standards for using these signals for communication
between devices such as keyboards, mice, PCs, and printers.
• For example, some manufacturers provide a special port called the
IrDA port that allows a wireless keyboard to communicate with a PC.

Switching
Outline
Guided Media
Unguided: Wireless
2 Switching
Message Switching

Switching
Basic Idea
• A network is a set of connected devices
• Whenever we have multiple devices, we have the problem of how to
connect them to make one-to-one communication possible.
• One solution is to make a point-to-point connection between each
pair of devices
• (a mesh topology) or between a central device and every other device
(a star topology).
• These methods, however, are impractical and wasteful when applied to
very large networks.
• The number and length of the links require too much infrastructure to
be cost-efficient,

Switching
Basic Idea …
• A better solution is switching.
• A switched network consists of a series of interlinked nodes, called
switches.
• Switches are devices capable of creating temporary connections
between two or more devices linked to the switch.
• In a switched network, some of these nodes are connected to the end
systems (computers or telephones, for example).
• Others are used only for routing.

Switching
Three Methods of Switching

Switching Circuit-switched Networks
Outline
2 Switching
Message Switching

• A circuit-switched network consists of a set of switches connected by
physical links.
• A connection between two stations is a dedicated path made of one
or more links.
• However, each connection uses only one dedicated channel on each
link.
• Each link is normally divided into n channels by using FDM or TDM

Circuit-switched Networks …
• Three Phases
1 Setup phase
• Before the two parties (or multiple parties in a conference call) can
communicate,
• …a dedicated circuit (combination of channels in links) needs to be
established.
• The end systems are normally connected through dedicated lines to the
switches,
• …so connection setup means creating dedicated channels between the
switches.
2 Data Transfer Phase
• After the establishment of the dedicated circuit (channels), the two
parties can transfer data.
3 Teardown Phase
• When one of the parties needs to disconnect, a signal is sent to each
switch to release the resources.

Switching Packet-switched Networks
Outline
2 Switching
Message Switching

• With packet switching, messages are broken into discrete units called
packets that are routed to their destination.
• Each packet contains data and headers.
• Headers contain information such as source and destination address.
• The channel can be shared among many sessions to achieve higher
utilization.

Packet-switched Networks …
• In packet switching, there is no resource allocation for a packet.
• This means that there is no reserved bandwidth on the links, and
there is no scheduled processing time for each packet
• Resources are allocated on demand.
• The allocation is done on a first come, first-served basis.
• When a switch receives a packet, no matter what is the source or
destination, the packet must wait if there are other packets being
processed.
• As with other systems in our daily life, this lack of reservation may
create delay.

• Packet switching is divided into two approaches:
1 Datagram approach – mainly used in the Network Layer
2 Virtual Circuit approach – a Data Link Layer technology

Datagram network
• In a datagram network, each packet is treated independently of all
others.
• Even if a packet is part of a multipacket transmission, the network
treats it as though it existed alone.
• Packets in this approach are referred to as datagrams
• Datagram switching is normally done at the network layer.

Datagram network …
• The datagram networks are sometimes referred to as connectionless
networks.
• …connectionless here means that the switch (packet switch) does not
keep information about the connection state.
• There are no setup or teardown phases.
• Each packet is treated the same by a switch regardless of its source or
destination.

Routing Table
• If there are no setup or teardown phases, how are the packets routed
to their destinations in a datagram network?
• In datagram network, each switch (or packet switch) has a routing
table
• …which is based on the destination address.
• The routing tables are dynamic and are updated periodically.
• The destination addresses and the corresponding forwarding output
ports are recorded in the tables.
• This is different from the table of a circuit-switched network
• …in which each entry is created when the setup phase is completed
and deleted when the teardown phase is over.
• A switch in a datagram network uses a routing table that is based on
the destination address

Destination Address
• Every packet in a datagram network carries a header that contains,
• …among other information, the destination address of the packet.
• When the switch receives the packet, this destination address is
examined;
• …the routing table is consulted to find the corresponding port through
which the packet should be forwarded.
• This address, unlike the address in a virtual-circuit-switched network,
remains the same during the entire journey of the packet.

Virtual-Circuit Networks
• A virtual-circuit network is a cross between a circuit-switched network
and a datagram network.
• It has some characteristics of both.
1 As in a circuit-switched network, there are setup and teardown phases
in addition to the data transfer phase
2 Resources can be allocated during the setup phase, as in a
circuit-switched network, or on demand, as in a datagram network.
3 As in a datagram network, data are packetized and each packet carries
an address in the header
4 As in a circuit-switched network, all packets follow the same path
established during the connection.
5 A virtual-circuit network is normally implemented in the data-link layer,
6 while a circuit-switched network is implemented in the physical layer
and a datagram network in the network layer.

Virtual-Circuit Networks …

Switching Message Switching
Outline
2 Switching
Message Switching

Message Switching
• An alternative switching strategy is message switching.
• In this form of switching, no physical path is established in advance
between sender and receiver.
• Instead, when the sender has a block of data to be sent, it is stored in
the first switching office (i.e., router) and then forwarded later, one
hop at a time.
• Each block is received in its entirety, inspected for errors, and
• …then retransmitted.
• Since messages can be very large, this can cause buffering problems
and high mean delay times

Switching and TCP/IP Layers
1 Switching at Physical Layer
• At the physical layer, we can have only circuit switching.
• There are no packets exchanged at the physical layer.
• The switches at the physical layer allow signals to travel in one path or
another
2 Switching at Data-Link Layer
• At the data-link layer, we can have packet switching.
• However, the term packet in this case means frames or cells.
• Packet switching at the data-link layer is normally done using a
virtual-circuit approach.
3 Switching at Network Layer
• …either a virtual-circuit approach or a datagram approach can be used.
• Currently the Internet uses a datagram approach,
4 Switching at Application Layer
• At the application layer, we can have only message switching.
• The communication at the application layer occurs by exchanging
messages

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