DCCT 3300
Computer Networks
LAN, MAN and WAN.
This course deals with WAN Protocol.
LAN:
Operates within a small geographical area, such as a building or a campus or an
organization site.
MAN:
 Metropolitan area Network
 Covers about 100 miles
 Connects multiple networks which are located in different location of a city
or town
 Links are generally owned by a service provider
WAN:
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Wide Area network
Used for long distance transmission
Covers a large area such as a country or a continent
May use leased lines from Telephone companies, A PSTN or Satellites for
communication link
Protocols used for WAN transmission are called WAN protocols such as
X.25, Frame Relay (FR), TCP/IP, ATM, ISDN Etc
WAN carrier technology is either T1 or T3 lines or SONET(FIBER)
INTERNET :
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The Internet is a collection of thousands of interconnected Networks located
all around the world that are connected by Routers or Gateways. It allows
private individuals as well as organizations such as Government agencies,
Schools, colleges, universities, corporations, libraries, research facilities,
hospitals etc to use Internet Resources
Each router has a routing table containing information about the Networks
that are connected to the Router
Several networks may be connected to one Router
The router accepts frames or packets from a Network and checks its routing
table to determine if the destination is in the Network connected to the
Router
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If the destination is in the Network connected to the Router, it transmits the
frames to that Network. Otherwise forwards the frame to the next gateway,
which performs the same operation, the process continues until the frame
reaches its destination.
This marvelous communication system came into being in 1969
Read chapter one of your text book to become familiar with the history of
Internet
You are already familiar with OSI layers. Consider the TCP/IP Layers
TCP/IP Layers
APPLICATION
7
APPLICATION
FTP, SMTP, RPC, SNMP, HTTP, DNS
6
PRESENTATION
TFTP, ETC
5
SESSION
4
TRANSPORT
3
NETWORK
2
DATA LINK
PHYSICAL
1
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TRANSPORT
TCP
UDP
NETWOTK IP
ICMP, ARP, RARP
Works with any data link control
protocol
Works with any Physical layer protocol
Herein lies the strength of the TCP/IP. It can interconnect any machine using
any data link or physical layer protocol. It can therefore transfer data
between dissimilar machines, manufactured any where in the world using a
wide variety of different physical and data link layer protocols.
We shall study all these layers and protocols as we progress through the
course.
MECHANISM OF DATA TRANSFER IN TCP/IP FIG P250 Ela
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Application generates data to be transmitted. This could be FTP, SMTP,
SNMP, HTTP Etc. Application resides within the computer.
Application Layer forwards this data to the Transport Layer ( TCP layer).
TCP Layer sticks its own Header.
Notice that TCP Layer uses two Protocols – TCP (Transmission Control
Protocol) or UDP ( User Datagram Protocol)
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TCP is a Connection oriented, reliable Protocol and it is used when reliable
delivery is required.
UDP is an unreliable , connection less protocol, used when reliability is not
an issue such as an AD that flashes on your computer screen every 2 seconds.
Since it will be transmitted again and again every 2 seconds there is no need
for extensive error checking and reliability.
This becomes a TCP Segment or a UDP Datagram
TCP layer then forwards this TCP Segment (or UDP Datagram) to the IP
LAYER. (NETWORK LAYER)
The IP Layer treats this entire TCP Segment (or UDP Datagram) as Data
and appends its own Header
IP layer is also a connection less, unreliable Protocol
Its main function is Routing of the IP Packet.
It does not guarantee delivery. It makes a best effort to deliver the IP packet
to its destination. Just like your Post office does not guarantee delivery of
your letter. It makes the best effort to deliver it. If reliable delivery is
required, the customer uses extra service such as express priority service or
Acknowledgement receipt
Similarly if the reliable delivery of Data is required, IP service must be
combined with TCP.
Thus, applications that do not require reliable service would not go through
TCP layer. Instead it will be passed on to the IP layer directly.
This makes TCP/IP a very efficient, cost effective and flexible protocol .
Since IP layer is unreliable, it works with 3 additional protocols – ICMP
(Internet Control Message protocol), ARP ( Address Resolution protocol)
and RARP ( Reverse Address Resolution Protocol).
ICMP generates error messages for the sending Station if something goes
wrong. The messages like “ Destination unreachable or host Unreachable or
time out etc are generated by ICMP to inform the source machines of
delivery problems. This is necessary because IP is unreliable.
ARP determines the MAC Address given the IP address of a machine and
RARP does the opposite (determines the IP address corresponding to a MAC
ADDRESS). Each of these protocol also stick their own Headers.
We shall study ICMP, ARP and RARP in more details later in the course.
The packet just generated is called an “IP Datagram)
The IP layer then forwards this “IP Datagram” to the Network Card.
Notice that Application, Transport and IP layers exist in software and reside
in the machine.
DATA LINK or MAC Layer exists in Hardware – in the Network Interface
Card.
The NIC then adds its own MAC Header ( Ethernet or Token RING or FDDI
header – Ethernet being the most popular).
The Packet at MAC level is called a “FRAME”
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This frame is then converted into Electrical Signal ( or optical signal
depending upon the physical medium being used) and transmitted on the
wire as binary bits.
The physical layer protocol may be RS-232, 422, 423, V.35, HSSI, SONET etc
RCVR :
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At the destination, a reverse process takes place.
Bits are received by the Card
The Card removes the MAC Header and passes the IP Datagram to the IP
layer.
IP Layer examines the IP header, removes it and forwards the remaining
part of Packet to the TCP layer.
TCP layer also examines the TCP header, strips it off and passes the pure
Data to the corresponding Application Layer.
This completes the transmission, Reception, processing, moving and delivery
of Data using TCP/IP protocol Suite.
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