1. Lecture (Mar 23, 2000) H/W Assignment 3 posted on Web –Due Tuesday March 28, 2000 Review of Data packets LANS WANS

2. 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

3. 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

4. Three Common LAN Architectures Star Bus Ring

5. Star LAN Architecture Server User 1 User 2 User 3 User 4 User 5 All nodes (users) connect to central server All communication STOPS if the server fails All packets are sent in two hops

6. Bus LAN Architecture User 1User 2User 3User 4User 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

7. Ring LAN Architecture User 1 User 2 User 3 User 4 User 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

8. 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

9. 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

10. Routing / Packet Switching Source Destination 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

11. 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

12. 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”

13. 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

14. 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