Computer Networks
Introduction to Computer Networks
Lecture 1A
Instructor: T. Dlamini (B.Eng, EECS, IE)
Department of Electrical and Electronic Engineering,
University of ESWATINI, Kwaluseni, ESWATINI
tldlamini@uniswa.sz
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Outline
1 Introduction
2 Network Basics
3 Cables and Connection
4 Network Topology
5 Performance Metrics
6 Switching in Networks
7 Network Types
8 The Internet
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Outline
1 Introduction
2 Network Basics
3 Cables and Connection
4 Network Topology
5 Performance Metrics
6 Switching in Networks
7 Network Types
8 The Internet
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Open System Interconnection Model
The general problem of communication network between cooperating dissimilar hosts
situated on interconnected, but diverse, networks was studied by committees under
the sponsorship of the International Organization for Standardization (ISO)
- Their work resulted in the open system interconnection reference model (OSI
model or OSIRM)
OSI model: it is designed to guide the development of open systems so that they can
communicate with each other
- Open systems are defined by the parameters of the interfaces between their
functional blocks
- Note: an equipment from one vendor that implements a function will work
with equipment from another vendor the implements the next function
The OSI model does not define the equipment, only states what must exist at their
interfaces
- It is the designer’s problem to create equipment that satisfies network
requirements
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Open System Interconnection Model (cont.)
The model divides the actions of each host into seven independent activities that
are performed in sequence.
Figure 1: OSI model of data communication between cooperating systems
Layer 5-7: for conditioning or restoring the user’s data; layer 1-4: implement data
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OSI model versus TCP/IP Protocol Suite (Preview)
Figure 2: OSI model vs. TCP/IP
OSI model acts as a reference model and is not implemented in Internet because of
its late invention. Current model being used is the TCP/IP model
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Outline
1 Introduction
2 Network Basics
3 Cables and Connection
4 Network Topology
5 Performance Metrics
6 Switching in Networks
7 Network Types
8 The Internet
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Network Basics
Figure 3: Network infrastructure basics.
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Outline
1 Introduction
2 Network Basics
3 Cables and Connection
4 Network Topology
5 Performance Metrics
6 Switching in Networks
7 Network Types
8 The Internet
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Cables and Connection
Network connections
Figure 4: Cables layout.
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Cables and Connection (cont.)
Rollover cable: sometimes called hyperterminal cable
Figure 5: The rollover cable.
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Cables and Connection (cont.)
To specify when to use the crossover cable or straight-through cable, we should
remember the following:
- Group 1: Router, Host, Server
- Group 2: Hub, Switch
- One device in group 1 + one device in group 2: use straight-through cable
- Two devices in the same group: use a crossover cable
Figure 6: The network setup.
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Outline
1 Introduction
2 Network Basics
3 Cables and Connection
4 Network Topology
5 Performance Metrics
6 Switching in Networks
7 Network Types
8 The Internet
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Network Topology
A network is the interconnection of a set of devices capable of communication
- In this definition, a device can be a host such as a large computer, desktop,
laptop, workstation, cellular phone, or security system
- A device in this definition can also be a connecting device such as a router or
switch, a modem that changes the form of data, and so on
Network criteria
- Performance - Throughput, delay
- Reliability - the frequency of failure
- Security - the data protection from unauthorized access
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Network Topology (cont.)
Types of connection
- Point-to-point connection: a dedicated link between two devices
✓ The entire capacity of the link is reserved for transmission between those
two devices
✓ Use an actual length of wire or cable to connect the two ends, but other
options, such as microwave or satellite links
- Multipoint (also called multidrop) connection: more than two specific devices
share a single link
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Network Topology (cont.)
Mesh topology (e.g., public telephone network)
- Pros: Provides redundant paths between devices
- Pros: The network can be expanded without disruptions to current uses
- Cons: requires more cable than the other LAN topologies
- Cons: complicated implementation
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Network Topology (cont.)
Star topology (e.g., high-speed LAN)
- Adv: easily expanded without disruption to the network
- Adv: cable failure affects only a single user
- Adv: easy to troubleshoot and isolate problems
- Cons: requires more cable
- Cons: a central connection device allows for a single point of failure
- Cons: more difficult to implement
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Network Topology (cont.)
Bus topology (e.g., traditional ethernet LAN)
- Pros: cheap and easy to implement
- Pros: require less cable
- Pros: Does not use any specialized network equipment
- Cons: Network disruption when computers are added or removed
- Cons: a break in the cable will prevent all systems from accessing the network
- Cons: difficult to troubleshoot
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Network Topology (cont.)
Ring network (e.g., token ring)
- Pros: cable faults are easily located, making troubleshooting easier
- Pros: Ring networks are moderately easy to install
- Cons: Expansion to the network can cause network disruption
- A single break in the cable can disrupt the entire network
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Outline
1 Introduction
2 Network Basics
3 Cables and Connection
4 Network Topology
5 Performance Metrics
6 Switching in Networks
7 Network Types
8 The Internet
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Performance Metrics
Bandwidth
- Range of frequencies contained in a composite signal
- Measured in Hz or bits/second
Throughput
- Measure of how fast we can actually send data through a network
- Although, at first glance, bandwidth in bits per second and throughput seem
the same, however, they are different
- A link may have a bandwidth of β bps, but we can only send T bps through
the link with T < β
Latency (τ )
- 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 source
- It consists of the following: propagation time (α), transmission time (µ), queu-
ing time (ζ), and processing time (ϕ)
τ =α+µ+ζ+ϕ (1)
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Performance Metrics (cont.)
Bandwidth-delay product
- Is a measure of the number of bits a sender can transmit through the system
while waiting for an acknowledgment from the receiver
- Its important if we need to send data in bursts and wait for the
acknowledgement of each burst before sending the next one
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Performance Metrics (cont.)
Jitter
- Time difference in packet inter-arrival time to their destination
- It is a specific issue that normally exists in packet networks and this phenomenon
is usually not causing any communication problems
✓ TCP/IP is responsible for handling the impact caused by jitter
✓ Observable in VoIP
- We can say that this is a problem if different packets of data encounter
different delays and the application using the data at the receiving end is time
sensitive (e.g., audio streaming, video data)
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Outline
1 Introduction
2 Network Basics
3 Cables and Connection
4 Network Topology
5 Performance Metrics
6 Switching in Networks
7 Network Types
8 The Internet
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Switching in Networks
The 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
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
- The solution is switching: a switched network consists of a series of
interlinked nodes, called switches
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Switching in Networks (cont.)
The most common types of switched networks:
- Circuit-switched network
- Packet-switched network
- Message switched network
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Circuit Switching
Is a method of allowing data terminal equipment (DTE) to establish an immediate
full-duplex connection to another data network on a temporary basis
Once the connection is established, exclusive use of the channel and its available
bandwidth is provided by the switching network until the channel is relinquished by
one of the stations
A circuit-switched network consists of a set of switches connected by physical links
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Circuit Switching
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 Frequency Division
Multiplexing (FDM) or Time Division Multiplexing (TDM)
Advantage: the switched connection is dedicated for the entire communication session,
and no time buffering is necessary
Thus, it is highly efficient in cases when relatively high data transfer rates occur
throughout the session
Addressing information occurs only once during the call setup procedure by the net-
work
Problem: Arises with the circuit switched network when multiple stations require the
use of the switching facilities
The switch must support a wide range of transmission rates from all users
simultaneously
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Circuit Switching (cont.)
Example: Telephone network
- The actual communication in a circuit-switched network the following phases
✓ Connection setup
✓ Data transfer
✓ Connection teardown
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Circuit Switching (cont.)
Efficiency
- It can be argued that circuit-switched networks are not as efficient as the other
two types of networks because resources are allocated during the entire duration
of the connection
- These resources are unavailable to other connections: In a telephone network,
people normally terminate the communication when they have finished their
conversation
Delay
- Although a circuit-switched network normally has low efficiency, the delay in
this type of network is minimal
- During data transfer the data is not delayed at each switch; the resources are
allocated for the duration of the connection
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Packet Switching
Here, the message is divided into discrete units and then routed independently to
their final destination
- Method of segmenting a user’s message into discrete, variable length units
called packets
- The size of the packet is determined by the network and the governing
protocol
By limiting the length of the packets, other users can effectively share the use of the
channel
Individual packets are appended with control information for routing, sequencing, and
error detection
- Packets are routed to various switching nodes throughout the network depend-
ing on the availability of the channel
- Individual packets may not necessarily take the same switching route as other
packets derived from the same message
- They may not arrive at the terminating switching node in the same order in
which they were transmitted
In a packet-switched network, there is no resource reservation; resource are allocated
on demand
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Packet Switching: 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
The datagram networks are sometimes referred to as connectionless networks
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Packet Switching - Datagram Network
Delay is a datagram network
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Packet Switching: Virtual Circuit Network
Refers to a cross between a circuit-switched network and a datagram network
It has some characteristics of both:
- Setup, data transfer, and teardown phases
- Resources can be allocated during the setup phases, or on demand
- Data are packetized and packet carries an address in the header
- All packets follow the same path established during the connection
- Normally implemented in the data-link layer
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Packet Switching: VCN Addressing
Global addressing - used only for creating a virtual-circuit identifier (VCI)
Virtual-Circuit Identifier (VCI) or the label
- The identifier that is actually used for data transfer
- A small number that has only switch scope; is used by a frame between switches
- When a frame arrives at a switch, it has a CI; when it leaves, it has a different
VCI
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Packet Switching: VCN Addressing (cont.)
Data transfer phase
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Packet Switching: VCN Addressing (cont.)
Source-to-destination data transfer in a virtual-circuit network
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Packet Switching: VCN Addressing (cont.)
Setup request in a virtual-circuit network
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Packet Switching: VCN Addressing (cont.)
Setup acknowledgement in a virtual-circuit network
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Packet Switching: VCN Addressing (cont.)
Delay in VCN
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Message Switching
Is a network switching technique in which data is routed in its entirety from the source
node to the destination node, one hope at a time
Process of temporary buffering or “storing” the message at the switching exchange
and “forwarding” it to the next switching exchange acting as a successor
- The technique is known as store and forward
During message routing, every intermediate switch in the network stores the whole
message
Since the immediate connection between DTEs is not necessary, the facilities can be
shared among several users on a per-message basis
Efficiency of the switching network is increased as a result of this technique
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Outline
1 Introduction
2 Network Basics
3 Cables and Connection
4 Network Topology
5 Performance Metrics
6 Switching in Networks
7 Network Types
8 The Internet
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Network Types
Local Area Network (LAN)
- Refers to the geometric pattern or configuration of intelligent devices and how
they are linked together for communications
- The intelligent devices on the network are referred to as nodes/stations
- It is usually privately owned and connects some host in a single office, building,
or campus
✓ Depending on the organization needs, a LAN can be as simple as two
PCs or a printer in someone’s office
- 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
- In design of a LAN, topology is considered to best suit a particular environment;
In other words, there are advantages and disadvantages to the various topologies
- Factors to consider: Message size, traffic volume, costs, bandwidth, reliability,
and simplicity
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Network Types (cont.)
Local Arean Network (LAN)
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Network Types (cont.)
Wide Area Network (WAN)
- A WAN has a wider geographical span, spanning a town, a state, a country, or
even the world
- It interconnects connecting devices such as switches, routers, or modems
- It is normally created and run by communication companies and/or leased by
an organization that uses it
A Point-to-Point WAN
- It is a network that connects two communicating devices through a
transmission media
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Network Types (cont.)
Switched Wide Area Network (WAN)
- Its a network with more than two ends: used in the backbone of global
communication
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Network Types (cont.)
Heterogeneous network
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Outline
1 Introduction
2 Network Basics
3 Cables and Connection
4 Network Topology
5 Performance Metrics
6 Switching in Networks
7 Network Types
8 The Internet
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The Internet
Composed of thousands of interconnected networks
- Backbones and provider networks are also called Internet Service Providers
(ISPs)
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Accessing the Internet
User must be physically connected to an ISP
- The connection is normally done through a point-to-point WAN
Using Telephone Network
- Dial-up service - add to the telephone line a modem that converts data to voice
- Digital Subscriber Line (DSL) - allows the line to be simultaneously for voice
and data communication
Using Cable Networks
Using Wireless Networks
Direct Connection to the Internet
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Assignment: Standards and Administration
Internet Standards
Internet Administration
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Thank you
Q&A
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