CS8591 Computer Networks - Unit I

CS8591 – Computer Networks
Unit I – Introduction & Physical Layer
KAVIYA P, AP/IT
KAMARAJ COLLEGE OF ENGINEERING & TECHNOLOGY

UNIT I – Introduction & Physical Layer
Networks – Network Types – Protocol Layering – TCP/IP
Protocol suite – OSI Model – Physical Layer: Performance –
Transmission media – Switching – Circuit-switched Networks –
Packet Switching.
9/16/2020 KAVIYA P, AP/IT, KCET 2

Networks
• Network: The interconnection of a set of devices capable of communication.
• A device can be a host (or an end system) such as a large computer, desktop, laptop,
workstation, cellular phone, or security system.
• A device can also be a connecting device such as
– Router, which connects the network to other networks.
– Switch, which connects devices together.
– Modem (modulator-demodulator), which changes the form of data.
• Computer systems and peripherals are connected to form a network.

Networks – Network Criteria
• A network must be able to meet a certain number of criteria.
• The most important criteria are performance, reliability, and security.
• Performance
– Performance can be measured in terms of transit time and response time.
– Transit time: The amount of time required for a message to travel from one device
to another.
– Response time: The elapsed time between an inquiry and a response.
– The performance of a network depends on a number of factors, including the
number of users, the type of transmission medium, the capabilities of the connected
hardware, and the efficiency of the software.
– Performance is often evaluated by two networking metrics: Throughput and Delay.

Networks – Network Criteria
• Reliability
– Measured by
• The frequency of failure
• The time it takes a link to recover from a failure
• The network’s robustness in a catastrophe.
• Security
– Network security issues include
• Protecting data from unauthorized access
• Protecting data from damage and development
• Implementing policies and procedures for recovery from breaches and data
losses.

Networks – Physical Structure
• A network is two or more devices connected through links.
• Link: A communications pathway that transfers data from one device to
another.
• Types of Connection
– Point-to-Point
– Multipoint (also called multidrop)

Networks – Physical Structure
• Point-to-Point
– It provides a dedicated link between two devices.
– The entire capacity of the link is reserved for transmission between
those two devices.
• Multipoint (also called multidrop)
– More than two specific devices share a single link.
– The capacity of the channel is shared, either spatially or temporally.
– If several devices can use the link simultaneously, it is a spatially
shared connection.
– If users must take turns, it is a timeshared connection.

Networks – Physical Topology
• Physical topology refers to the way in which a network is laid out
physically.
• The topology of a network is the geometric representation of the
relationship of all the links and linking devices (usually called nodes) to
one another.
• There are four basic topologies possible:
– Mesh Topology
– Star Topology
– Bus Topology
– Ring Topology

Mesh Topology
• Every device has a dedicated point-to-point link to every other device.
• The term dedicated means that the link carries traffic only between the two devices it connects.
• No. of link to connect n nodes: n (n – 1) / 2.
• Application: Telephone regional offices
• Advantages
– The use of dedicated links guarantees that each connection can carry its own data load
– Robust
– Privacy or Security
• Disadvantages
– Amount of cabling and the number of I/O ports required is large & expensive
– Installation and reconnection are difficult
– The sheer bulk of the wiring can be greater than the available space

Star Topology
• Each device has a dedicated point-to-point link only to a central controller, usually called a hub..
• The devices are not directly linked to one another.
• The controller acts as an exchange: If one device wants to send data to another, it sends the data to
the controller, which then relays the data to the other connected device.
• Application: LAN, High-Speed LAN
• Advantages
– Star topology is less expensive than a mesh topology
– Easy to install and reconfigure
– Robustness
• Disadvantages
– If the hub goes down, the whole system is dead
– More cabling is required in a star than in some other topologies (such as ring or bus)

Bus Topology
• One long cable acts as a backbone to link all the devices in a network. (Multipoint connection)
• Nodes are connected to the bus cable by drop lines and taps.
• Drop line: A connection running between the device and the main cable.
• Tap: A connector that either splices into the main cable or punctures the sheathing of a cable to create
a contact with the metallic core.
• Application: Ethernet LANs
• Advantages
– Ease of installation
– A bus uses less cabling than mesh or star topologies
• Disadvantages
– Difficult reconnection and fault isolation
– A fault or break in the bus cable stops all transmission

Ring Topology
• Each device has a dedicated point-to-point connection with only the two devices on either side of it.
• A signal is passed along the ring in one direction, from device to device, until it reaches its
destination.
• Each device in the ring incorporates a repeater.
• Application: IBM LAN, Token Ring
• Advantages
– Easy to install and reconfigure
– Fault isolation is simplified. (By generating an alarm, after a specific period of time)
• Disadvantages
– Unidirectional traffic
– Break in the ring (such as a disabled station) can disable the entire network (Solved by using
dual ring)

Network Types
• Network types can be distinguished using few criteria such as size, geographical
coverage, and ownership.
1. Local Area Network (LAN)
– Interconnects hosts within limited area and has its network equipment and
interconnects locally managed.
– Privately owned and connects some hosts in a single office, building, or
campus.
– Each host in a LAN has an identifier, an address, that uniquely defines the host
in the LAN.
– A packet sent by a host to another host carries both the source host’s and the
destination host’s addresses.

Network Types
1. Local Area Network (LAN)

Network Types
2. Wide Area Network (WAN)
– WAN has a wider geographical span, spanning a town, a state, a country, or even the world.
– A LAN interconnects hosts; a WAN interconnects connecting devices such as switches, routers,
or modems.
– A WAN is normally created and run by communication companies and leased by an
organization that uses it.
– Two distinct examples of WANs: Point-to-point WANs and Switched WANs.
– Point-to-point WAN: A network that connects two communicating devices through a
transmission media (cable or air).
– Switched WAN: A combination of several point-to-point WANs that are connected by
switches.
– Internetwork: When two or more networks are connected, they make an internetwork, or
internet.

Network Types
2. Wide Area Network (WAN)
Point-to-Point WAN
Switched WAN
An internetwork made of two LANs and one point-to-point WAN

Network Types
3. Switching
• An internet is a switched network in which a switch connects at least two links together.
• A switch needs to forward data from a network to another network when required.
• The types of switched networks are circuit-switched and packet-switched networks.
• Circuit-Switched Network
– A dedicated connection, called a circuit, is always available between the two end
systems; the switch can only make it active or inactive.
• Packet-Switched Network
– The communication between the two ends is done in blocks of data called packets.
– Here the switches function for both storing and forwarding because a packet is an
independent entity that can be stored and sent later.

Network Types
3. Switching
• Circuit-Switched Network
• Packet-Switched Network

Network Types
4. Internet
• Composed of thousands of interconnected networks.
• Internet has several backbones, provider networks, and customer networks.
• Backbones and provider networks are also called Internet Service Providers
(ISPs).
• The backbones are often referred to as international ISPs; the provider networks are
often referred to as national or regional ISPs.
• The customer networks are networks at the edge of the Internet that actually use the
services provided by the Internet.
• They pay fees to provider networks for receiving services.

Network Types
4. Internet

Network Types
5. Accessing the Internet
• Using Telephone Networks: Telephone networks have connected themselves to the
Internet.
– Dial-up Service: Add modem to the telephone line that converts data to voice.
– DSL Service: Allows the line to be used simultaneously for voice and data
communication.
• Using Cable Networks: The cable companies have been upgrading their cable
networks and connecting to the Internet. (Eg: TV Broadcasting)
• Using Wireless Networks: A household or a small business can be connected to the
Internet through a wireless WAN.
• Direct Connection to the Internet: A large organization or a large corporation can
itself become a local ISP and be connected to the Internet.

Protocol Layering
• In data communication and networking, a protocol defines the rules that both the
sender and receiver and all intermediate devices need to follow to be able to
communicate effectively.
• If communication is simple, one simple protocol is needed.
• If the communication is complex, there need to divide the task between different
layers, in which case a protocol at each layer is needed, or protocol layering.

Protocol Layering: OSI Model
• The ISO defined a common way to connect computers, called the Open
Systems Interconnection (OSI) architecture. (eg. public X.25 network).
• It defines partitioning of network functionality into seven layers.
• The bottom three layers, i.e., physical, data link and network are
implemented on all nodes on the network including Switches, Routers.
• The top four layers are transport, session, presentation and application
layers.

Exchange of Data in OSI Model

OSI Model - Physical Layer
• The physical layer is responsible for movements of individual bits from one hop
(node) to the next.
• The physical layer defines the characteristics of the interface between the devices and
the transmission medium. It also defines the type of transmission medium.
• Representation of bits: Defines the type of encoding (how 0’s and 1’s are changed to
signals).
• Transmission rate: (Number of bits sent per second) is defined by the physical layer.
• Synchronization: Sender and receiver clock is synchronized by physical layer.
• Topology : It defines how the machines are connected to make a network.
• Protocols/Standards: Ethernet, Fast Ethernet, FDDI, 10 Base T, IEEE 802.11.
Transmission of bits in Physical Layer

OSI Model - Data Link Layer
• The data link layer is responsible for moving frames from one hop (node) to the
next.(i.e.) hop to hop delivery of frames
• Framing: Divides the stream of bits received from the physical layer into manageable
data units called frames.
• Physical Addressing: If frames are to be distributed to different systems on the network,
the data link layer adds a header to the frame to define the sender and/or receiver of the
frame. If the frame is intended for a system outside the sender's network, the receiver
address is the address of the device that connects the network to the next one.
• Flow Control: If the rate at which the data are absorbed by the receiver is less than the
rate at which data are produced in the sender, the data link layer imposes a flow control
mechanism to avoid overwhelming the receiver.
• Error Control: The data link layer adds reliability to the physical layer by adding
mechanisms to detect and retransmit damaged or lost frames. It also uses a mechanism to
recognize duplicate frames. Error control is normally achieved through a trailer added to
the end of the frame.
• Access Control: When two or more devices are connected to the same link, data link
layer protocols are necessary to determine which device has control over the link at any
given time.

• Protocols: HDLC, PPP, ARP and RARP.
OSI Model - Data Link Layer
Frame transfer in Data Link layer

OSI Model - Network Layer
• Responsible for the source to destination delivery of the packets across
multiple networks. If two systems are connected to the same link, there is usually
no need for a network layer.
• Logical Addressing: If a packet passes the network boundary, we need to
distinguish the source and destination systems. The network layer adds a header to
the packet that includes the logical addresses of the sender and receiver.
• Routing: When independent networks or links are connected to create
internetworks (network of networks) or a large network, the connecting devices
(called routers or switches) route or switch the packets to their final destination.
One of the functions of the network layer is to provide this mechanism.
• Protocols: IPv4,IPv6,ICMP,IPsec,IGMP, X.25, RIP and OSPF.

OSI Model - Network Layer
Packet transfer in Network layer

OSI Model - Transport Layer
• Responsible for process-to-process delivery of the entire message.
• Process: An application program running on a host.
• Service point Addressing: Computers often run several programs at the same time. For
this reason, source-to- destination delivery means delivery not only from one computer to
the next but also from a specific process (running program) on one computer to a specific
process (running program) on the other. The transport layer header must include a type of
address called a service-point address (or port address). The network layer gets each
packet to the correct computer; the transport layer gets the entire message to the correct
process on that computer.
• Segmentation and Reassembly: A message is divided into transmittable segments, with
each segment containing a sequence number. These numbers enable the transport layer to
reassemble the message correctly upon arriving at the destination and to identify and
replace packets that were lost in transmission.

• Connection Control: The transport layer can be either connectionless or connection
oriented. A connectionless transport layer treats each segment as an independent packet
and delivers it to the transport layer at the destination machine. A connection oriented
transport layer makes a connection with the transport layer at the destination machine first
before delivering the packets. After all the data are transferred, the connection is
terminated.
• Flow Control: Similar to flow control in data link layer, however, transport layer
performing end to end flow control.
• Error Control: Transport layer provides end to end error control. The sending transport
layer makes sure that the entire message arrives at the receiving transport layer without
error (damage, loss, or duplication). Error correction is usually achieved through
retransmission.
• Protocols: TCP and UDP.

Process Delivery in Transport layer

OSI Model - Session Layer
• The session layer is responsible for dialog control and synchronization.
• Provides a name space that is used to tie together the potentially different transport
streams that are part of a single application.
• The session layer is the network dialog controller. It establishes, maintains, and
synchronizes the interaction among communicating systems.
• Dialog Control: The session layer allows two systems to enter into a dialog. It
allows the communication between two processes to take place in either half duplex
(one way at a time) or full-duplex (two ways at a time) mode.
• Synchronization: The session layer allows a process to add checkpoints, or
synchronization points, to a stream of data.
• Protocols: SAP (Session Announcement Protocol), RTP (Real Time Protocol), and
SOCKS( Socket Secure).

OSI Model - Presentation Layer
• The presentation layer is responsible for translation, compression, and
encryption.
• It is concerned with syntax and semantics (format) of the information exchanged
between peers.
• Translation: Different computers use different encoding systems, the presentation
layer is responsible for interoperability between these encoding methods.
• Encryption: To carry sensitive information, a system ensures privacy by
encrypting the message before sending and decrypting at the receiver end.
• Compression: Data compression reduces the number of bits contained in the
information. It is particularly important in multimedia transmission.
• Protocols: MIME and XDR

OSI Model - Application Layer
• The application layer is responsible for providing services to the user.
• The application layer enables the user, whether human or software, to access the network.
• It provides user interface and support for services such as electronic mail, remote file access,
shared database management and several types of distributed services.
• File transfer, access, and management: This application allows a user to access files in a
remote host (to make changes or read data), to retrieve files from a remote computer for use in
the local computer, and to manage or control files in a remote computer locally.
• Mail services: This application provides the basis for e-mail forwarding and storage.
• Directory services: This application provides distributed database sources and access for
global information about various objects and services.
• Protocols: DNS, FTP, TFTP, HTTP, SMTP, SNMP, Telnet, DHCP, POP3 and IMAP.

Summary of OSI Model

Protocol Layering: TCP/IP Protocol Suite
• Also called as “Internet Architecture”.
• The “Sub Network” layer shown here is sometimes referred to as “Network
Layer” or “Link Layer” or “Host to Network Layer”.

Host to Network layer
• Wide variety of protocols denoted NET1, NET2,… NETn implemented by a combination
of hardware (eg. Network Adapter) and software (Eg. Network Device Driver).
• Example: Ethernet and FDDI
Internet Layer
• Consist of a single protocol called “Internet Protocol” (IP) that supports the
interconnection of multiple networking technologies into a single, logical network.
• Supporting protocols,
– ARP (Address Resolution Protocol): Associates the logical address (IP) with a
physical address.
– RARP (Reverse Address Resolution Protocol): Allows host to discover the logical
address when it knows the physical address.
– ICMP (Internet Control Message Protocol): Querying and Error reporting
protocol.
– IGMP (Internet Group Message Protocol): Facilitate the simultaneous
transmission of a message to a multiple recipients.

Transport Layer
• Contains two main end to end protocols,
– TCP: Provides a reliable byte stream channel (Connection oriented)
– UDP: Provides a unreliable datagram channel (Connectionless)
Application Layer
• Provides wide range of protocols for different applications.
• Protocols provided by this layer are,
– FTP (File Transfer Protocol)
– TFTP (Trivial File Transfer Protocol)
– SMTP (Simple Message Transfer Protocol)
– Telnet (Remote Login)
– HTTP (Hyper Text Transfer Protocol)
– DNS (Domain Name System)
– SNMP (Simple Network Management Protocol)
– HTTP (Hypertext Transfer Protocol), etc,...

Internet Protocol Graph

Three Features of Internet Architecture
• Does not imply strict layering
• The application is free to bypass the transport layer and to directly use IP or one of the
underlying layers.
• Hourglass shape protocol graph
• Wide at top, narrow in middle and wide at bottom.
• IP serves as the focal point for architecture.
• It defines a common method for exchanging packets among different networks.
• Above IP can be arbitrarily many transport protocols offering a different channel
abstraction to application layer.
• It separates the Host to Host delivery with Process to Process delivery.
• Below IP, the architecture allows for different network technologies, ranges from
Ethernet to wireless.
• In order to add a new protocol to the architecture, it needs to be a protocol specification
and at least one representative implementations of the specification.

Physical Layer: Performance
1. Bandwidth
• Network performance is measured in terms of bandwidth.
• In networking, the term bandwidth is used in two contexts:
– Bandwidth in Hertz: The range of frequencies a channel can pass.
– Bandwidth in Bits per Seconds: Refers to the speed of bit transmission in a channel
or link.

2. Throughput
• Measure of how fast we can actually send data through a network.
• The bandwidth is a potential measurement of a link; the throughput is an actual
measurement of how fast we can send data.
• Example:
A network with bandwidth of 10 Mbps can pass only an average of 12,000 frames per minute
with each frame carrying an average of 10,000 bits. What is the throughput of this network?
Solution:
Throughput = (12,000 x 10,000) / 60 = 2,000,000 bps = 2 Mbps

3. Latency (Delay)
• It defines how long it takes for an entire message to completely arrive at the destination
from the time the first bit is sent out from the source.
• It is made of four components: propagation time, transmission time, queuing time and
processing delay.
Latency = propagation time + transmission time + queuing time + processing delay

3. Latency (Delay)
• Propagation Time: Measures the time required for a bit to travel from the source to the
destination.
Propagation time = Distance / (Propagation Speed)
• Example:
What is the propagation time if the distance between the two points is 12,000 km? Assume the
propagation speed to be 2.4 × 108 m/s in cable.
Solution:
Propagation time = (12,000 x 1000) / (2.4 x 108) = 0.05 Sec = 50 ms

3. Latency (Delay)
• There is a time between the first bit leaving the sender and the last bit arriving at the
receiver.
• The first bit leaves earlier and arrives earlier; the last bit leaves later and arrives later.
• Transmission Time: Depends on the size of the message and the bandwidth of the channel.
Transmission time = (Message size) / Bandwidth
• Example:
What are the propagation time and the transmission time for a 2.5-KB (kilobyte) message (an
email) if the bandwidth of the network is 1 Gbps? Assume that the distance between the sender
and the receiver is 12,000 km and that light travels at 2.4 × 108 m/s.
Solution:
Propagation time = (12,000 x 1000) / (2.4 x 108) = 0.05 Sec = 50 ms
Transmission time = (2500 x 8) / 109 = 0.00002 Sec = 0.02 ms

3. Latency (Delay)
• Example - Transmission Time
What are the propagation time and the transmission time for a 5-MB (megabyte) message (an
image) if the bandwidth of the network is 1 Mbps? Assume that the distance between the
sender and the receiver is 12,000 km and that light travels at 2.4 × 108 m/s.
Solution:
Propagation time
= (12,000 x 1000) / (2.4 x 108)
= 0.05 Sec = 50 ms
Transmission time
= (5,000,000 x 8) /106
= 40 Sec

3. Latency (Delay)
• Queuing Time: The time needed for each intermediate or end device to hold the
message before it can be processed.
• The queuing time is not a fixed factor; it changes with the load imposed on the network.
• When there is heavy traffic on the network, the queuing time increases.
• An intermediate device, such as a router, queues the arrived messages and processes
them one by one.
• If there are many messages, each message will have to wait.

4. Bandwidth-Delay Product
• It defines the number of bits that can fill the link.
• Case 1: Assume that we have a link with a bandwidth of 1 bps and the delay of the link is
5 s. The product 1 × 5 is the maximum number of bits that can fill the link. There can be no
more than 5 bits at any time on the link.
• Case 2: Assume we have a bandwidth of 5 bps. There can be maximum 5 × 5 = 25 bits on
the line. The reason is that, at each second, there are 5 bits on the line; the duration of each
bit is 0.2 s.

5. Jitter
• Refers to small intermittent delays during data transfers.
• Jitter is the variation in latency - the delay between when a signal is transmitted and
when it is received.
• It can be caused by a number of factors including network congestion, collisions, and
signal interference.
• Jitter is a problem if different packets of data encounter different delays and the
application using the data at the receiver site is time-sensitive (audio and video data, for
example).
• If the delay for the first packet is 20 ms, for the second is 45 ms, and for the third is 40
ms, then the real-time application that uses the packets endures jitter.

Physical Layer: Transmission Media
• In lower level, a network can consist of two or more computers directly connected by
some physical medium, such as coaxial, twisted pair or an optical cable.
• Physical medium is called as links.
• It defined as a medium that can carry information from a source to a destination.

• Transmission media is classified into two categories,
• Guided (Wired)
• Unguided (Wireless)

Guided Media
• Provide a conduit from one device to another device.
– Twisted Pair
– Coaxial
– Fiber optic
• Coaxial and twisted pair cables accept and transport signal in the form of electrical
signals.
• Fiber optic cable accepts and transport signals in the form of light.

1. Guided Media –Twisted Pair
• Consist of two conductors (metallic), each with its own plastic insulation.
– One wire is used to carry the signals to the receiver.
– Another one for ground reference.
• Interference and cross talk may affect both the wires and make unwanted noise.
• Twisting the cable makes it probable that both wires are equally affected by external
influences (noise or crosstalk).
• The receiver calculates the difference between the two, receives no unwanted signals.
The unwanted signals are mostly canceled out.
• The number of twists per unit of length (e.g., inch) has some effect on the quality of
the cable.

1. Guided Media –Twisted Pair Types
• Unshielded Twisted Pair (UTP): Most commonly used in communication.
• Shielded Twisted Pair (STP):
– Has a metal foil or braided mesh covering that encases each pair of insulated
conductors.
– Metal casing improves the quality of cable by preventing the penetration of noise or
crosstalk.
– It is bulkier and more expensive.
Unshielded Twisted Pair Shielded Twisted Pair

• Unshielded Twisted Pair (UTP) Categories
Category Specification
Data Rate
(Mbps)
Use
1 UTP used in telephone < 0.1 Telephone
2 UTP originally used in T lines 2 T-1 lines
3 Improved CAT 2 used in LANs 10 LANs
4 Improved CAT 3 used in Token Ring networks 20 LANs
5
Cable wire is normally 24 AWG with a jacket and outside
sheath
100 LANs
5E
Extension of category 5 (Minimize the crosstalk and
electromagnetic interference)
125 LANs
6 The cable must be tested at a 200-Mbps data rate. 200 LANs
7
Sometimes called SSTP (shielded screen twisted-pair).
Each pair is individually wrapped in a helical metallic foil
followed by a metallic foil shield in addition to the outside
sheath. The shield decreases the effect of crosstalk and
increases the data rate.
600 LANs

• Unshielded Twisted Pair (UTP) Connector
– The most common UTP connector is RJ45 (RJ stands for registered jack).
– The RJ45 is a keyed connector, meaning the connector can be inserted in only one
way.

• Performance
– To measure the performance of twisted-pair cable is to compare attenuation versus
frequency and distance.
– With increasing frequency, the attenuation, measured in decibels per kilometer
(dB/km), sharply increases with frequencies above 100 kHz.
• Applications
– Telephone Network
– Local Area Network (10BaseT, 100BaseT)

2. Guided Media – Coaxial Cable (or coax)
• Carries signals of higher frequencies than twisted pair.
• Coaxial has central core conductor of solid copper wire enclosed in an insulating sheath.
• 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.

• Coaxial Cable Category
– Coaxial cables are categorized by their Radio Government (RG) ratings.
– Each RG number denotes a unique set of physical specifications, including the
wire gauge of the inner conductor, the thickness and type of the inner insulator, the
construction of the shield, and the size and type of the outer casing.
Category Impedance Use
RG-59 75 Ω Cable TV
RG-58 50 Ω Thin Ethernet
RG-11 50 Ω Thick Ethernet

• Coaxial Cable Connectors
– Bayonet Neill-Concelman (BNC) connector used to connect coaxial cables.
– Three popular types of BNC connectors:
• BNC connector: To connect the end of the cable to a device, such as a TV set.
• BNC T connector: Used in Ethernet networks to branch out to a connection to
a computer or other device.
• BNC terminator: Used at the end of the cable to prevent the reflection of the
signal.

• Performance
– Attenuation is much higher in coaxial cable than in twisted-pair cable.
– Coaxial cable has a much higher bandwidth, the signal weakens rapidly and
requires the frequent use of repeaters.
• Applications
– Cable TV
– Traditional Ethernet LANs

3. Guided Media – Fiber Optic Cable
• Made of glass or plastic and transmits signals in the form of light.
• 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.

• Propagation Mode
1. Single-Mode
• Uses step-index fiber and a highly focused source of light that limits beams to a
small range of angles, all close to the horizontal.
• It is manufactured with a much smaller diameter than that of multimode fiber.
• Propagation of different beams is almost identical, and delays are negligible.

3. Guided Media – Fiber Optic Cable – Propagation Mode
2. Multimode: Multiple beams from a light source move through the core in different paths.
2.1. Multimode step-index fiber
• The density of the core remains constant from the center to the edges.
• A beam of light moves through this constant density in a straight line until it reaches the
interface of the core and the cladding.
• The term step-index refers to the suddenness of this change, which contributes to the
distortion of the signal as it passes through the fiber.
2.2. Multimode graded-index fiber
• It decreases distortion of the signal through the cable.
• A graded-index fiber, is one with varying densities.
• Density is highest at the center of the core and decreases gradually to its lowest at the
edge.

• Fiber Sizes
– Optical fibers are defined by the ratio of the diameter of their core to the
diameter of their cladding, both expressed in micrometers.
Type Core (μm) Cladding (μm) Mode
50/125 50.0 125 Multimode, graded index
62.5/125 62.5 125 Multimode, graded index
100/125 100.0 125 Multimode, graded index
7/125 7.0 125 Single mode

• Cable Composition
– The outer jacket is made of either PVC or Teflon.
– Inside the jacket are Kevlar strands to strengthen the cable.
– Kevlar is a strong material used in the fabrication of bulletproof vests.
– Below the Kevlar is another plastic coating to cushion the fiber.
– The fiber is at the center of the cable, and it consists of cladding and core.

• Connectors
– Subscriber channel (SC) connector: Used for cable TV. It uses a push/pull locking
system.
– Straight-tip (ST) connector: Used for connecting cable to networking devices. It
uses a bayonet locking system and is more reliable than SC.
– MT-RJ : A connector that is the same size as RJ45.
SC Connector
ST Connector
LC Connector

• Performance
– Attenuation is flatter than in the case of twisted-pair cable and coaxial cable.
– Need fewer (actually one-tenth as many) repeaters when we use fiber-optic cable.
• Applications
– SONET Network
– Cable TV
– Telephone Network

• Advantages
– Higher Bandwidth
– Less Signal Attenuation
– Immunity to electromagnetic interference
– Resistance to corrosive materials
– Light weight
– Greater immunity to tapping
• Disadvantages
– Installation and maintenance
– Unidirectional light propagation
– More expensive

Unguided Media
• Unguided medium transport electromagnetic waves without using a physical conductor.
• This type of communication is often referred to as wireless communication.
• Signals are normally broadcast through free space and thus are available to anyone who
has a device capable of receiving them.
• Unguided signals can travel from the source to the destination in several ways:
– Ground propagation
– Sky propagation
– Line-of-sight propagation
Electromagnetic spectrum for wireless communication

Unguided Media – Propagation Methods
1. Ground Propagation
• Radio waves travel through the lowest portion of the atmosphere, hugging the earth.
• Low-frequency signals emanate in all directions from the transmitting antenna and
follow the curvature of the planet.
2. Sky Propagation
• Higher-frequency radio waves radiate upward into the ionosphere where they are
reflected back to earth.
• This type of transmission allows for greater distances with lower output power.
3. Line-of-sight Propagation
• Very high-frequency signals are transmitted in straight lines directly from antenna to
antenna.

Unguided Media – Bands
• Electromagnetic spectrum defined as radio waves and microwaves is divided into eight
ranges, called bands.
Band Range Propagation Application
very low frequency (VLF) 3–30 kHz Ground Long-range radio navigation
low frequency (LF) 30–300 kHz Ground
Radio beacons and navigational
locators
middle frequency (MF) 300 kHz–3 MHz Sky AM radio
high frequency (HF) 3–30 MHz Sky Citizens band (CB), ship/aircraft
very high frequency (VHF) 30–300 MHz
Sky and
line-of-sight
VHF TV, FM radio
ultrahigh frequency (UHF) 300 MHz–3 GHz Line-of-sight
UHF TV, cellular phones,
paging, satellite
superhigh frequency (SF) 3–30 GHz Line-of-sight Satellite
extremely high frequency
(EHF)
30–300 GHz Line-of-sight Radar, satellite

Unguided Media
• Wireless transmission is divided into three broad groups: radio waves, microwaves,
and infrared waves.
1. Radio Waves
• Electromagnetic waves ranging in frequencies between 3 kHz and 1 GHz.
• Radio waves are omnidirectional.
• When an antenna transmits radio waves, they are propagated in all directions.
• A sending antenna sends waves that can be received by any receiving antenna.
Omnidirectional Antenna
• Radio waves use omnidirectional antennas that send out signals in all directions.
Applications
• Used for multicast communications, such as radio and
television, and paging systems.

Unguided Media
2. Microwaves
• Electromagnetic waves having frequencies between 1 and 300 GHz.
• Microwaves are unidirectional.
• When an antenna transmits microwaves, they can be narrowly focused. This means that
the sending and receiving antennas need to be aligned.
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.
Applications
• Used for unicast communication such as cellular
telephones, satellite networks, and wireless LANs.

Unguided Media
3. Infrared
• Electromagnetic waves having frequencies between 300 GHz to 400 THz.
• Infrared waves, having high frequencies, cannot penetrate walls.
• It prevents interference between one system and another.
Applications
• Used for short-range communication in a closed area using line-of-sight
propagation.

Switching
• In large networks, we need to allow one-to-one communication between any two nodes.
• In LANs this is achieved using one of three methods:
– Direct point-to-point connection (Mesh)
– Via central controller (Star)
– Connection to common bus in a multipoint configuration (Bus)
– The solution is Switching Network
Switched Network
• Consists of a series of interlinked nodes called switches.
• Switches are capable to create temporary connections between two or more devices.
• The end systems (communicating devices) are
labeled A, B, C, D, and so on, and the switches
are labeled I, II, III, IV, and V.

Switching
Three Methods of Switching

Switching: Circuit Switched Networks
• 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
• Each connection uses only one dedicated channel on each link
• Each link is normally divided into n channels by using FDM or TDM.
• The link can be permanent (leased line) or temporary (telephone)

• Three phases are need to communicate two parties or multiple parties in a
conference call. They are:
– Connection setup
– Data transfer
– Connection teardown
• Setup Phase: Creating dedicated channels between the switches.
• Data Transfer Phase: After the establishment of the dedicated circuit (channels), the
two parties can transfer data.
• Teardown Phase: When one of the parties needs to disconnect, a signal is sent to each
switch to release the resources.

• Circuit switching takes place at the physical layer.
• Data transferred between the two stations are not packetized. The data are a continuous
flow sent by the source station and received by the destination station.
• There is no addressing involved during data transfer. Of course, there is end-to-end
addressing used during the setup phase.
• In circuit switching, the resources need to be reserved during the setup phase; the
resources remain dedicated for the entire duration of data transfer until the teardown
phase.

Efficiency
• Circuit-switched networks are not so efficient as the resources are allocated during the
entire duration of the connection.
• These resources are unavailable to other connections.
Delay
• Circuit-switched network normally has low efficiency, the delay in this type of network
is minimal.
• During data transfer the data are not delayed at each switch; the resources are allocated
for the duration of the connection.

Switching: Packet Switching
• Messages need to be divided into packets.
• Size of the packet is determined by the network and the governing protocol.
• No resource reservation, but allocated on demand.
• The allocation is done first come, first served based.
• When a switch receives a packets, the packet must wait if there are other packets being
processed, this lack of reservation may create delay.
1. Datagram Networks
• Each packet (called as datagrams) is treated independently of all others.
• All packets (or datagrams) belong to the same message may travel different paths to
reach their destination.

• Datagram Switching is done at the network layer.
• This approach can cause the datagrams of a transmission to arrive at their destination
out of order with different delays between the packets.
• Packets may also be lost or dropped because of a lack of resources.
• In most protocols, it is the responsibility of an upper-layer protocol to reorder the
datagrams or ask for lost datagrams before passing them on to the application.
• The datagram networks are referred to connectionless networks. There are no setup
or teardown phases.

1. Datagram Networks – Routing Table Mechanism
• Each 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.
• The destination address in the header of a packet in a
datagram network remains the same during the entire journey
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.

Efficiency
• Better than circuit-switched network.
• Resources are allocated only when there are packets to be transferred.
Delay
• There may be greater delay in a datagram network than in a virtual-circuit network.
• Each packet may experience a wait at a switch before it is forwarded. (No setup &
• Total delay = 3T + 3τ + w1 + w2 teardown phases)
• T: transmission times; τ: propagation delays; (w1 + w2) : waiting times.

2. Virtual-Circuit Networks
• It is a cross between a circuit-switched network and a datagram network.
Characteristics:
• There are setup and teardown phases in addition to the data transfer phase.
• Resources can be allocated during the setup phase, as in a circuit-switched network,
or on demand, as in a datagram network.
• Data are packetized and each packet carries an address in the header.
• Packets from a single message travel along the same path.
• It implemented in the data-link layer.

2. Virtual-Circuit Networks – Addressing
Global Addressing
• Source and destination needs unique addresses (used by the switches only to create a
Virtual-Circuit Identifier ) during the set up phase.
Local Addressing (Virtual-Circuit Identifier – VCI / Label)
• Actually used for data transfer.
• A small number used by a frame between two switches.
• When a frame arrives at a switch, it has a VCI; when it leaves, it has a different VCI.

2. Virtual-Circuit Networks – Three Phases
• Three phases in a virtual-circuit network: Setup, Data transfer, and Teardown.
• Setup Phase: The source and destination use their global addresses to help switches
make table entries for the connection.
• Teardown Phase: The source and destination inform the switches to delete the
corresponding entry.
• Data Transfer Phase: The two parties can transfer data.
.

2. Virtual-Circuit Networks - Tables
• To transfer a frame from a source to its destination, all switches need to have a table
entry for this virtual circuit.
• The table has four columns.
Switch and tables in a virtual-circuit network

Setup Phase – Setup Request
• The switch, in the setup phase acts as a packet switch; it has a routing table used to know
the output port number.

Setup Phase – Setup Acknowledgement
• When Destination B receives the up frame, and it is ready to receive frames from A, it
assign a VCI (In this case: 77). This VCI lets the destination know that the frames comes
from A not other sources.

Data-Transfer Phase
Teardown Phase
• Source A, after sending all frames to B, sends a special frame called a teardown request.
• Destination B responds with a teardown confirmation frame.
• All switches delete the corresponding entry from their tables.

Efficiency
• All packets belonging to the same source and destination travel the same path, but the packets
may arrive at the destination with different delays if resource allocation is on demand.
Delay
• There is a one-time delay for setup and a one-time delay for teardown.
• If resources are allocated during the setup phase, there is no wait time for individual packets.
• Total delay = 3T + 3τ + setup delay + teardown delay
• T: transmission times; τ: propagation delays

Switching: Message Switching
• Typically, a general purpose computer.
• Needs storage capacity to store the incoming messages (long).
• Message priorities due to store-and-forward technique.
• Traffic Congestion is reduced.

CS8591 Computer Networks - Unit I

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CS8591 Computer Networks - Unit I