Network edge

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