Indian Institute of Technology Kharagpur
Review of Network
Technologies
Prof. Indranil Sen Gupta
Dept. of Computer Science & Engg.
I.I.T. Kharagpur, INDIA
Lecture 2: Review of network technologies
On completion, the student will be able to:
1. Distinguish between a LAN and a WAN.
2. Explain the differences between circuit switching
and packet switching.
3. Explain the differences between virtual circuit and
datagram packet switching.
4. Explain the 7-layer OSI model.
5. Distinguish between the workings of hub, switch and
router.
6. Illustrate the typical architecture of an organizational
LAN.
1
Networking: Basic Concepts
• Computer Network
¾A communication system for
connecting computers / hosts
• Why?
¾Better connectivity
¾Better communication
¾Better sharing of resources
¾Bring people together
Types of Computer Networks
• Local Area Network (LAN)
¾Connects hosts within a relatively small
geographical area
Faster
ƒ Same room
ƒ Same building
Cheaper
ƒ Same campus
• Wide Area Network (WAN)
Slower
¾Hosts may be widely dispersed
Expensive
ƒ Across campuses
ƒ Across cities / countries/ continents
2
LAN and WAN: a Comparison
• LAN
• WAN
¾Typical speeds
ƒ 10 Mbps to
10 Gbps
¾Typical cost
ƒ 1 crore for a
hundred node
LAN (one-time
cost)
¾Typical speeds
ƒ 64 Kbps to
8 Mbps
¾Typical cost
ƒ 30 lakhs
(recurring
cost)
Circuit Switching
• A dedicated communication path is
required between two stations.
¾The path follows a fixed sequence of
intermediate links.
¾A logical channel gets defined on each
physical link.
ƒ Dedicated to the connection.
B
D
F
C
E
G
A
H
3
Circuit Switching (contd.)
• In circuit switching, three steps are
required for communication:
¾Connection establishment
ƒ Required before data transmission.
¾Data transfer
ƒ Can proceed at maximum speed.
¾Connection termination
ƒ Required after data transmission is over.
ƒ For deallocation of network resources.
Circuit Switching (contd.)
• Drawbacks:
¾Channel capacity gets dedicated during
the entire duration of communication.
ƒ Acceptable for voice communication.
ƒ Very inefficient for bursty traffic.
¾There is an initial delay.
ƒ For connection establishment.
4
Packet Switching
• Modern form of long-distance data
communication.
¾Network resources are not dedicated.
¾A link can be shared.
• The basic technology has evolved
over time.
¾Basic concept has remained the same.
¾Widely used for long-distance data
communication.
Packet Switching (contd.)
• Data are transmitted in short packets
(~ Kbytes).
¾A longer message is broken up into
smaller chunks.
¾The chunks are called packets.
¾Every packet contains a header.
ƒ Relevant information for routing, etc.
5
Packet Switching (contd.)
Message
H
H
H
PACKETS
Packet Switching (contd.)
• Packet switching is based on storeand-forward concept.
¾Each intermediate network node
receives a whole packet.
¾Decides the route.
¾Forwards the packet along the selected
route.
6
Packet Switching (contd.)
• Advantages:
¾Links can be shared; so link utilization
is better.
¾Suitable for computer-generated traffic.
ƒ Typically bursty in nature.
¾Buffering and data rate conversion can
be performed easily.
¾Some packets may be given priority
over others, if desired.
Packet Switching (contd.)
• How are the packets transmitted?
¾Two alternative approaches:
ƒ Virtual Circuits
ƒ Datagram
• The abstract network model:
B
D
F
C
E
G
A
H
7
Virtual Circuit Approach
• Similar in concept to circuit switching.
¾A route is established before packet
transmission starts.
¾All packets follow the same path.
¾The links comprising the path are not
dedicated.
ƒ Different from circuit switching in this
respect.
• Analogy:
¾Telephone system.
Virtual Circuit Approach (contd.)
• How it works?
¾Route is established a priori.
¾Packet forwarded from one node to the
next using store-and-forward scheme.
¾Only the virtual circuit number need to
be carried by a packet.
ƒ Each intermediate node maintains a
table.
ƒ Created during route establishment.
ƒ Used for packet forwarding.
¾No dynamic routing decision is taken by
the intermediate nodes.
8
Datagram Approach
• Basic concept:
¾No route is established beforehand.
¾Each packet is transmitted as an
independent entity.
¾Does not maintain any history.
• Analogy:
¾Postal system.
Datagram Approach (contd.)
• Every intermediate node has to take
routing decisions dynamically.
¾Makes use of a routing table.
¾Every packet must contain source and
destination addresses.
• Problems:
¾Packets may be delivered out of order.
¾If a node crashes momentarily, all of its
queued packets are lost.
¾Duplicate packets may also be generated.
9
Datagram Approach (contd.)
• Advantages:
¾Faster than virtual circuit for smaller
number of packets.
ƒ No route establishment and termination.
¾More flexible.
¾Packets between two hosts may follow
different paths.
ƒ Can handle congestion/failed link.
Comparative Study
• Three types of delays must be
considered:
¾Propagation Delay
ƒ Time taken by a data signal to
propagate from one node to the next.
¾Transmission Time
ƒ Time taken to send out a packet by the
transmitter.
¾Processing Delay
ƒ Time taken by a node to “process” a
packet.
10
Circuit Switching
• After initial circuit establishment,
data bits sent continuously without
any delay.
Virtual Circuit Packet Switching
• Call request packet sent from source
to destination.
• Call accept packet returns back.
• Packets sent sequentially in a
pipelined fashion.
¾Store-and-forward approach.
11
Datagram Packet Switching
• No initial delay.
• The packets are sent out independently.
¾May follow different paths.
¾Follow store-and-forward approach.
Layered Network Architecture
• Open systems interconnection (OSI)
reference model.
¾Seven layer model.
¾Communication functions are partitioned
into a hierarchical set of layers.
• Objective:
¾Systematic approach to design.
¾Changes in one layer should not require
changes in other layers.
12
The 7-layer OSI Model
Application
Presentation
Host-to-host
Session
Transport
Network
Datalink
Point-to-point
Physical
Layer Functions
• Physical
¾Transmit raw bit stream over a physical
medium.
• Data Link
¾Reliable transfer of frames over a point-topoint link (flow control, error control).
• Network
¾Establishing, maintaining and terminating
connections.
¾Routes packets through point-to-point
links.
13
Layer Functions (contd.)
• Transport
¾End-to-end reliable data transfer, with
error recovery and flow control.
• Session
¾Manages sessions.
• Presentation
¾Provides data independence.
• Application
¾Interface point for user applications.
How Data Flows
S
A
B
D
14
Internetworking Devices
• Hub
¾Extends the span of a single LAN.
• Bridge / Layer-2 Switch
¾Connects two or more LANs together.
¾Works at data link layer level.
• Router / Layer-3 Switch
¾Connects any combination of LANs and
WANs.
¾Works at network layer level.
Campus Network Schematic
Departmental
Network
Departmental
Network
Backbone
Departmental
Network
Departmental
Network
Backbone: Fast Ethernet, FDDI, ATM, Gigabit
Ethernet
DN: Switched Ethernet, Bus-based Ethernet
15
Connecting a Computer to DN
Backbone
• Requires a
network interface
card (NIC).
• Connection
through
switch/hub.
• Typical scenario is
shown.
Ethernet Switch
Hub
Hub
DN
Connecting to Outside World
Radio
link
VSAT
Router
Leased
line
Modem
Telephone
line
Institute
Network
16
QUIZ QUESTIONS ON
LECTURE 2
17
Quiz Questions on Lecture 2
1. With respect to speed of data transfer,
which of LAN or WAN is faster?
2. What is the typical speed of a modern
Ethernet backbone LAN?
3. Why is circuit switching not suitable for
computer-to-computer traffic?
4. What are the three steps that are
required for data communication using
circuit switching?
Quiz Questions on Lecture 2
5. With respect to sharing of links, which of
circuit switching or packet switching is
more suitable?
6. Among virtual circuit and datagram,
which approach requires less
information in the packet header?
7. Which of virtual circuit and datagram
makes better utilization of the links?
8. Which of virtual circuit and datagram will
guarantee ordered delivery of packets in
the absence of any errors?
18
Quiz Questions on Lecture 2
9. Under what circumstances will the
datagram method of packet delivery
prove useful?
10. Five packets need to be sent from a host
A to another host B. Which of virtual
circuit or datagram would be faster?
11. For a 5 Kbytes packets sent over a 10
Mbps transmission link, what is the
transmission time of the packet?
Quiz Questions on Lecture 2
12. A 600-byte packet is sent over a 20 Kbps
point-to-point link whose propagation
delay is 10 msec. After how much delay
will the packet reach the destination?
13. Which layers in the OSI model are hostto-host layers?
14. What is the responsibility of the network
layer in the OSI model?
15. What is the responsibility of the data link
layer in the OSI model?
16. What is the basic difference between a
bridge and a router?
19