Introduction to Networking
(Ref: An Introduction to Networking
Chp 1)
What is a Network?
A network consists of 2 or more computers connected together,
and they can communicate and share resources (e.g. information)
Why Networking?
• Sharing information — i.e. data communication
• Sharing hardware or software
• E.g. print document
• Centralize administration and support
• E.g. Internet-based, so everyone can access the same administrative or support
application from their PCs
What are the different kinds of Networks?
•Depending on one’s perspective, we can classify networks in
different ways
• Based on transmission media: Wired and Wireless
• Based on network size: LAN and WAN
• Based on topology (connectivity): Bus, Star, Ring
• Based on management method: Peer-to-peer and Client/Server
Transmission media
LAN (Local Area Network)
• "Is a group of computers and associated devices that share a
common communications line or wireless link and typically
share the resources of a single processor or server within a
small geographic area .
• Usually, the server has applications and data storage that are
shared in common by multiple computer users.
• A local area network may serve as few as two or three users
or many as thousands of users.
LAN
• Small network, short distance
• A room, a floor, a building
• Limited by no. of computers and distance covered
• Usually one kind of technology throughout the LAN
• Serves a department within an organization
• Examples:
• Network inside the Student Computer Room
• Network inside your home
• Wide Area Network (WAN)
• A network that uses long-range telecommunication links to connect 2
or more LANs/computers housed in different places far apart.
• Towns, states, countries
• Uses routers and usually point-to-point links
Your home
• Examples:
• Network of campus
• Internet
WAN
Student Computer Centre
USA
EXAMPLE OF A WAN
Topology ― 3 of the basic types
Bus Topology
Ring Topology
Star Topology
Hub
10
• Bus Topology
• All devices are connected to a central cable, called the bus or
backbone. Ethernet systems use a bus topology.
• Simple and low-cost
• Only one computer can send messages at a time
• Star Topology
• Each computer has a cable connected to a single point (central hub)
• More cabling, hence higher cost
• All signals transmission through the hub; if down, entire network
down
• Depending on the intelligence of hub, two or more computers may
send message at the same time
11
Bus Topology
How to construct a network
with Bus / Star Topology?
BNC T-Connector
Coaxial
cable
Star Topology
Network Card
12
• Ring Topology
• All devices are connected to one another in the shape of a closed loop,
so that each device is connected directly to two other devices, one on
either side of it.
T T
Ack
• Every computer serves as
T
a repeater to boost signals
T dat
T dat
• Typical way to send data:
a
a
• Token passing
T
T
• only the computer who
T Ack
T Ack
dat
gets the token can send
a
data
T
• Disadvantages
T Ack
• Difficult to add computers
• More expensive
• If one computer fails, whole network fails
13
Network management method
Peer-to-Peer Networks
• Peer-to-peer network is also called workgroup
• No hierarchy among computers  all are equal
• No administrator responsible for the network
Peer-to-peer
•Advantages of peer-to-peer networks:
• Low cost
• Simple to configure
• User has full accessibility of the computer
•Disadvantages of peer-to-peer networks:
• May have duplication in resources
• Difficult to uphold security policy
• Difficult to handle uneven loading
•Where peer-to-peer network is appropriate:
• 10 or less users
• No specialized services required
• Security is not an issue
• Only limited growth in the foreseeable future
Clients and Servers
• Network Clients (Workstation)
• Computers that request network resources or services
• Network Servers
• Computers that manage and provide network resources and services to
clients
• Usually have more processing power, memory and hard disk space than
clients
• Run Network Operating System that can manage not only data, but also
users, groups, security, and applications on the network
• Servers often have a more stringent requirement on its performance and
reliability
•Advantages of client/server networks
• Facilitate resource sharing – centrally administrate and control
• Facilitate system backup and improve fault tolerance
• Enhance security – only administrator can have access to Server
• Support more users – difficult to achieve with peer-to-peer networks
•Disadvantages of client/server networks
• High cost for Servers
• Need expert to configure the network
• Introduce a single point of failure to the system
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How does communication occur in a network?
• The term endpoints refers to the communicating devices.
• The network consists of nodes(the switches and endpoints) and links.
• The switches and links are called the network infrastructure.
• A switch receives bits in the form of data frames arriving over its links,
processes them, and forwards them over one or more other links.
• These frames are called packets.
Functions of network switches
• The fundamental functions performed by switches are to multiplex
and demultiplex data
• 1.Forwarding:When a data frame arrives at a switch, the switch needs
to process it, determine the correct outgoing link, and decide when to
send the frame on that link.
• 2.Routing: The process by which the switches in a network
collaboratively compute the network topology, is called
routing.(establishing the routes that data packets take on their way to
a particular destination to determine the best path)
• 3.Resource allocation: Switches allocate their resources—access to
the link and local memory—to the different communications that are
in progress.
How do switches forward data?
• There are two techniques which differ in the way the switches
forward data and allocate resources The first method, used by
networks like the telephone network, is called circuit switching.
• The second method, used by networks like the Internet, is called
packet switching.
Circuit switching
• Circuit switching is the older
technology
• Resources include link
bandwidth and switching
resources.
Establishing a circuit requires some set-up time before actual communication
commences. Even if one momentarily stops talking, the circuit is still in
operation. When the call is finished, link and switching resources are released
for use by other calls.
If insufficient resources are available to set up a call, the call is said to be
blocked
Packet switching
• A packet is a group of
bits consisting of header
bits and payload bits.
• The header contains the
source and destination
address, priority levels,
error check bits, and any
other information that is
needed.
The payload is the actual information (data) to be transported.
Many packet switching systems have a maximum packet size. Thus, larger
transmissions are split into many packets and then gathered at the receiver
• Circuit switching makes sense for a network where the workload is
relatively uniform, with all information transfers using the same
capacity, and where each transfer uses a constant bit rate.
• However, circuit switching tends to waste link capacity if the workload
has a variable bitrate, or if the frames arrive in bursts at a switch.
• But a large number of computer applications induce burst data
patterns.
• Another drawback of circuit switching shows up when the (N+1)
communication arrives at a switch whose relevant link already has the
maximum number(N)of communications going over it.
ADSL
• ADSL is a data communications technology that enables faster data
transmission over copper telephone lines
• ADSL is capable of providing up to 50 Mbps, and supports voice, video and data.
• ADSL is the #1 Broadband Choice in the World with over 60% market share
• ADSL is now available in every region of the world
What does ADSL mean
• Asymmetric - The data can flow faster in one direction than the other. Data
transmission has faster downstream to the subscriber than upstream
• Digital - No type of communication is transferred in an analog method. All data is
purely digital, and only at the end, modulated to be carried over the line.
• Subscriber Line - The data is carried over a single twisted pair copper loop to the
subscriber premises
How does ADSL work
• ADSL exploits the unused analogue bandwidth available in the
wires
PSTN
Upstream
4
25,875
Downstream
138
1104
KHz
ADSL Speed Comparison
Pure Fibre
Hybrid Fibre/Copper
FTTH
Enhanced
Copper
FTTx,
VDSL2,
ADSL2plus
ADSL
ISDN
Voice band
Modem
ADSL Range
• In general, the maximum range for DSL without a repeater is 5.5
km
• As distance decreases toward the telephone company office, the
data rate increases
Data Rate
Wire gauge Wire size
Distance
1.5 or 2 Mbps 24 AWG
0.5 mm
5.5 km
1.5 or 2 Mbps 26 AWG
0.4 mm
4.6 km
6.1 Mbps
24 AWG
1.5 or 2 Mbps 26 AWG
0.5 mm
0.4 mm
3.7 km
2.7
• For larger distances, you may be able to have DSL if your phone
company has extended the local loop with optical fiber cable
ADSL network components
• The ADSL modem at the customer premises(ATU-R)
• The modem of the central office (ATU-C)
• DSL access multiplexer (DSLAM)
• Broadband Access Server (BAS)
• Splitter - an electronic low pass filter that separates the
analogue voice or ISDN signal from ADSL data frequencies
ADSL Loop Architecture
DSL
Voice Switch
ISP
Central Office
Subscriber premises
How do we characterize the performance of a
network?!
• Link/Network Performance measures: throughput and delay
• Link Throughput –capacity potentially available for an application –
generally expressed in “bit per second”[bps] if the data has a size of
S bytes, and it takes T seconds to deliver the data, the throughput of
the data transfer is S/T bytes/second.
• The greater the throughput, the happier you will be with the network.
• The throughput of a data transfer is clearly upper-bounded by the
rate of the slowest link on the path between sender and receiver
• Our ability to optimize throughput depends on two factors:
• 1-the per-packet delay, sometimes called the per-packet
latency
• 2-the packet loss rate: if the sender sent St packets and the
receiver got Sr packets, then the packet loss rate is equal to
•
• or if the arrival rate of packets into the queue from the
sender is A packets per second and the departure rate from
the queue is D packets per second, then the packet loss rate
is equal to
• Network Delay :average time for a block of data to go from an
application on one system to corresponding application on another
system
• •4 types of delay contribute to overall delay:
Delay in PacketSwitched Networks
Processing Delay: time required to process a packet –to check for bit errors, to
determine output links, etc.
(a) at source prior to sending,
(b) at any intermediate router, and
(c) at destination prior to delivering to application
•on the order of 10-6·seconds or less –often negligible
Queueing Delay: time spent waiting in a queue at any point along the route
•depends on intensity and nature of traffic arriving at queue(s)
•on the order of 10-6·seconds to 10-3·seconds
• Propagation Delay: time for one bit to
propagate from source to destination at
propagation speed of the link
• depends on physical medium of the link
• on the order of 10-6·seconds
• negligible for two routers on the same LAN;
significant for two geostationary satellites
• Transmission Delay: time to send out /
absorb all of the packet bits
• also known as “store-and-forward” delay
•on the order of 10-6·seconds to 10-3·seconds
•negligible for transmission rates ≥10 Mbps;
significant for large packets sent over low-speed
links
Little’s Law
• A common method used by engineers to analyze network performance,
particularly delay and throughput, is queueing theory.
• Little’s law used widely in the performance evaluation of systems ranging
from communication networks to factory floors to manufacturing systems.
• For any stable queueing system, Little’s law relates the average arrival rate
of items (e.g., packets), λ, the average delay experienced by an item in the
queue, D, and the average number of items in the queue, N.
N=λ×D
• Note that if the queue is stable, then the departure rate is equal to the
arrival rate.
• Example.
• Suppose packets arrive at an average rate of 1000 packets per
second into a switch, and the rate of the outgoing link is larger
than this number. Suppose there are 50 packets in the queue on
average. What is the average queuing delay?
• Solution:
• N=50 and λ=1000
• The average queueing delay experienced by a packet is
D= 50/1000 seconds =50milliseconds.
The Open Systems Interconnection (OSI)
Model
• The OSI model is a tool for understanding data communications
between any two networked systems.
• It divides the communications processes into seven layers.
• The ISO (International Organization for Standardization) standard 7498-1
defined this model.
• This model allows all network elements to operate together, no matter
who created the protocols and what computer vendor supports them
• Each layer supports the layers above it and offers services to the layers
below it.
• A networking model offers a way to separate computer networking
functions into multiple layers.
• Such a model of layered functionality is also called a “protocol stack”
or “protocol suite”.
• Protocols, or rules, can do their work in either hardware or software
or, as with most protocol stacks, in a combination of the two. The
nature of these stacks is that the lower layers do their work in
hardware or firmware (software that runs on specific hardware chips)
while the higher layers work in software.
• The three lowest layers focus on passing traffic through the network to
an end system or end point.
• The top four layers come into play in the end system to complete the
process.
.
Benefits of the OSI model
• Helps users understand the big picture of networking
• Helps users understand how hardware and software elements function
together
• Makes troubleshooting easier by separating networks into manageable
pieces
• Defines terms that networking professionals can use to compare basic
functional relationships on different networks
• Helps users understand new technologies as they are developed