Lecture (Mar 23, 2000)

• H/W Assignment 3 posted on Web– Due Tuesday March 28, 2000

• Review of Data packets• LANS• WANS

Summary of Data Packets

• Computers communicate with other computers via packaging bits into data packets

• Packets can be variables sizes depending upon network dynamics and/or protocol specification

• Packets can arrive in arbitrary order– Tag field indexes packet

• Burst errors are typical in networks

Local Area Network (LAN)

• A LAN usually connects computers and peripheral devices, such as printers and large disk memories, within one location where the communication is over short distances, such as a laboratory, office, or factory.

• A LAN is typically referred to as an intranet

Three Common LAN Architectures

• Star• Bus• Ring

Star LAN Architecture

Server

User 1

User 2

User 3

User 4User 5

•All nodes (users) connect to central server•All communication STOPS if the server fails•All packets are sent in two hops

Bus LAN Architecture

User 1 User 2 User 3 User 4 User 5

•Data (files) are distributed over all users•Most common LAN architecture•Packets are arbitrated by BUS•If a given node fails, network still remains operational

BUS

Ring LAN Architecture

User 1

User 2

User 3

User 4User 5

•Packets flow around the loop in one direction “Traffic Circle”•In a ring containing N users (nodes), the average communication occurs in N/2 hops•If a link between two nodes is not functional, all communication stops

Ethernet

• Most common channel for transmitting packets on LANS– Ethernet (1 million bits per second)

– Fast Ethernet (100 million bits per second)

• Two cable types– Coaxial

– Twisted Pair

• Network Interface Cards (NICs) connect ethernet cable to a given computer

Wide Area Network (WAN)

• Connects devices across the city, state, or nation, where data communication occurs over long distances

• The most common WAN is the international network known as the Internet

Routing / Packet Switching

Source

DestinationRouter

Router

•Moving data packets through a WAN is called packet switching•A WAN is a dynamic environment that consist of switching computers called Routers

•Routers perform packet switching•There are many possible routes from source to destination

WAN Protocols

• A protocol is a method of data transfer– Defines the procedures and sequences of

communication

• Three Common Protocols– TCP/IP

– UPD/IP

– ATM

TCP/IP

• Stands for Transmission Control Protocol / Internet Protocol

• Guarantees the data is correct , no matter how long the transfer takes

• When a packet arrives, an acknowledgment (ACK) is sent to the sender

• An ACK is not sent to the sender if the receiver detects an error in the packet

• The sender waits for an ACK, if one is not received, the packet is sent out again.

• TCP/IP is used for “Surfing the Web”

UDP/IP

• Stands for Universal Datagram Protocol / Internet Protocol

• Transmits data quickly but does not retransmit erroneous packets

• Fastest/Shortest Route is chosen

• Error checking is performed; NO ACKS are sent and NO attempt to retransmit erroneous packets is made

• UDP/IP is used for transferring audio and video over the internet– Packet errors can be tolerated by Human Perceived Data

ATM

• Stands for Asynchronous Transfer Mode– Has absolutely nothing to do with Automated Teller

Machines

• Speed Increasing Features– All ATM packets are routed the same

– All packets are fixed size (53 bytes)

– Error checking is done only at the final destination

– No ACKS are sent, the receiver may request the packet to be sent again