This document provides an overview of computer networks and the Internet. It describes the nuts and bolts view of the basic components including end systems, communication links, routers, packets, and protocols. It also describes the service oriented view in which the Internet provides distributed applications with either a connection oriented reliable service or a connectionless unreliable service. Finally, it discusses network cores including circuit switching which reserves bandwidth for a call, and packet switching which transmits data in packets and allows statistical multiplexing and sharing of bandwidth.

1. Lecture#12
Computer Communications
& Networks CS-576

2. Network Edge
 An edge device provides an entry point into the enterprise.
The devices which make up a network and provide access
to it. For example routers, routing switches, multiplexers,
WAN & MAN.

3. What is Internet
 A Nuts and Bolts Description
 End systems
 Communication Links, Bandwidth
 Routers, Packet
 ISPs
 Protocols, TCP/IP
 Internet Standards, RFCs
 A service Description
 Distributed Applications
 Connection Oriented Reliable Service
 Connectionless Unreliable Service
 What is a protocol?

4. Internet
 Two ways to describe the Internet
 Nuts and Bolts View
 The basic hardware and software components
 Service Oriented View
 The networking infrastructure that provides services to
distributed applications

5. Nuts and Bolt View
 Hosts or End Systems
 Computing Devices such as PCs, PDAs (Personal Digital Assistants),
TVs, servers, mobile computers, automobiles, etc. connected to the
Internet are called hosts or end systems.
 Communication links
 End systems are connected together by communication links.
 Communication links are made up of different types of media, including
twisted pair, coaxial cable, fiber optics, and radio spectrum.
 Bandwidth
 Different links can transmit data at different rates.
 The link transmission rate is often called the bandwidth (i.e, the width
of the band) of the link which is measured in bits per second (bps).

6. Cont.
 Routers
 End systems are not directly connected to each other via a single
communication link. They are indirectly connected to each through
intermediate switching devices known as routers.
 A router receives chunk of information from one of its incoming
communication link and forwards it to one of its outgoing
communication link.
 Packets
 The chunk of information is called packet.
 Route or Path
 The path that the packet takes from the sending end system, through
a series of communication links and routers, to the receiving end
system is known as a route or path.
 Packet switching
 It allows multiple communicating end systems to share a path, or
parts of path at the same time.

7. Cont.
 Internet Service Providers (ISPs)
 End systems access the Internet through the Internet Service
Providers (ISPs).
 The different ISPs provide a variety of different types of network
access to the end systems, including 56Kbps dial up modem access,
cable modem or DSL, high speed LAN access, and wireless access.
 Protocols
 End systems, routers, and other pieces of the Internet, run protocols
that control the sending and receiving of information within the
Internet.
 TCP (Transmission Control Protocol) and IP (Internet protocol) are
two of the most important protocols in the Internet.

8. Cont.
 Internet Standards
 At the technical and development level, the Internet is made possible
through creation, testing, and implementation of Internet Standards.
 These standards are developed by Internet Engineering Task Force
(IETF).
 RFCs
 The IETF standards documents are called RFCs (Request for
comments).
 RFCs started out as general request for comments (hence the name)
to resolve architecture problems of the Internet.
 They define protocols such as TCP, IP, HTTP, SMTP.

9. Network Core
 Switched Networks:
 Circuit Switched Networks
 Packet Switched Networks.

10. Service Oriented View
 Distributed Applications
 The Internet allows distributed applications running on its end
systems to exchange data with each other.
 These applications include remote login, electronic mail, web
surfing, instant messaging, audio and video streaming, Internet
telephony, distributed games, peer-to-peer (P2P) file sharing, and
much more.
 Communication Services
 Connection oriented reliable service
 Connectionless unreliable service

11. Cont.
 Internet Provides two services to its distributed
applications:
 Connection Oriented Reliable Service
 It guarantees that data transmitted from a sender to a receiver will
eventually be delivered to the receiver in order and in its entirety.
 Connectionless Unreliable Service
 It does not make any guarantees about eventual delivery.
 Distributed applications makes use of one or the other (but not
both) of these two services.

12. Connection Oriented Services
 Reliable Data Transfer
 Using acknowledgements & retransmissions
 Flow Control
 sender won’t overwhelm receiver
 Congestion Control
 senders “slow down sending rate” when network congested
 TCP
 Applications using TCP are:
 HTTP (Web), FTP (file transfer), Telnet (remote login), SMTP
(email)

13. Connectionless Services
 Unreliable Data Transfer
 no flow control
 no congestion control
 Fast
 connectionless
 UDP
 Applications using UDP are:
 multimedia, videoconferencing, DNS, Internet telephony

14. TCP vs UDP
 Reliable Protocol
 Connection Oriented
 Performs three ways
handshake
 Provision for error
detection and
retransmission
 Most applications use TCP
for reliable and guaranteed
transmission
 Unreliable Protocol
 Connectionless
 Much faster than TCP
 No acknowledgement
waits
 No proper sequencing of
data units
 Suitable for applications
where speed matters more
than reliability

15. Network Core: Circuit Switching
 End-to-end resources
reserved for “call”
 link bandwidth, switch
capacity
 dedicated resources: no
sharing
 circuit-like (guaranteed)
performance
 call setup required

16. Circuit Switching
 Switched circuits allow data connections that can be
initiated when needed and terminated when communication
is complete
 Circuit switched network - a network in which a dedicated
circuit is established between sender and receiver and all
data passes over this circuit.
 The telephone system is a common example.
 The connection is dedicated until one party or another
terminates the connection.

17. Graphical View

18. Cont.
 Dedicated communication path between two stations
 Three phases (Establish, Transfer, Disconnect)
 Inefficient (for data traffic)
 Channel capacity dedicated for duration of connection
 Much of the time a data connection is idle
 If no data, capacity wasted
 Set up (connection) takes time
 Once connected, transfer is transparent
 Circuit switching designed for voice
 Constant Data rate (Both ends must operate at the same rate)

19. Cont.
 Multiplexing in Circuit Switched Networks
 Multiplexing is a technique, in which a single
transmission medium is being shared among multiple
users.
 Types of Multiplexing
 Frequency Division Multiplexing FDM
 Time Division Multiplexing TDM

20. Network Core: Packet Switching
 A network in which data is transmitted in the form of
packets.
 Multiple users share network resources
 No dedicated bandwidth is allocated.
 No resources are reserved, resources used as needed.
 Each packet uses full link bandwidth.
 Good for bursty traffic, simpler, no call setup.
 Packets queued and transmitted as fast as possible.
 Packets are accepted even when network is busy, which
causes the delivery to slow down

21. Graphical View
 Sequence of A & B packets do not have fixed pattern i.e.
statistical multiplexing
A
B
C
10 Mb/s
Ethernet
1.5 Mb/s
D E
statistical multiplexing
queue of packets
waiting for output
link

22. Cont.
 Two broad classes of packet switched networks are:
 Datagram Networks
 Any network that forwards the packet according to the destination
address is called a datagram network
 The routers in the internet forwards packets according to host
destination addresses; hence the Internet is a datagram
network.
 Virtual Circuit Networks
 Any network that forwards the packet according to the virtual
circuit identifier is called a virtual circuit network
 Examples are X.25, Frame Relay, ATM technologies

23. Packet Switching: Datagram
 Datagram Approach:
 Each packet is treated independently
 No reference to packets that have gone before
 Each node chooses next node on path using destination
address
 Packets with same destination address may not follow
same route
 Packets may arrive out of sequence, may be lost
 It is up to receiver to re-order packets and recover from
lost packets
 No Call setup
 For an exchange of a few packets, datagram quicker
 It is a connectionless service.
 Analogy: driving, asking directions

24. Datagram

26. Packet Switching: Virtual Circuit
 Virtual circuit packet switched network create a logical
path through the subnet
 Call request and call accept packets establish a virtual
connection
 Virtual route remains fixed through the call.
 All packets from one connection follow this path.
 Each packet contains a virtual circuit identifier instead of
destination address to determines the next hop
 Not a dedicated path
 No routing decisions required for each packet

28. Network Taxonomy
Telecommunication
networks
Circuit-switched
networks
FDM TDM
Packet-switched
networks
Networks
with VCs
Datagram
Networks