COP 4930 Computer Network Projects

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COP 4930Computer Network Projects

• Summer C 2004
• Prof. Roy B. Levow
• Lecture 2

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Packet Transmission

• Packet Concept
• Data divided into small blocks packets
• Packets sent individually
• Networks using packets are called
• Packet Network
• Pack Switching Network

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Packet Transmission.2

• Packets assist in
• Recovery from errors (smaller units)
• Sharing network (shorter holding time)
• Packet problems
• Do not guarantee fair sharing of network
• Do not guarantee quality of service
• Data rate, Delay, Jitter, etc.

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Packet Transmission Summary

• Allow the sender and receiver to coordinate
• Ensure fair, prompt access for all
• Take turns sending packets
• Small packet size limits time waiting for access

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Packets and Frames

• Packet refers to general concept
• Frame is specific form in which data is
  transmitted
• Simple frame format
• _______________________________
• SOH data EOT

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Transmission Errors

• Errors may occur during transmission of packets
• Receiver must be able to recognize when a
  received packet has errors
• Add error check code to end of packet
• Sender generates
• Receiver verifies by same calculation

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Error Check Codes

• Parity Bit
• Add bit by anding bits of packet
• Checksum
• Add blocks of data plus carry
• Cyclic Redundancy Check (CRC)
• Robust check based on higher math
• Divide frame value by polynomial modulo 2 and use
  remainder
• Appended to end of frame

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Ethernet Frames

• Ethernet is a family of widely used Local Area
  Network technologies
• Frame format
• Preamble marks staft of frame
• 48 bit (6 octet) addresses
• Each node has a unique address
• Some addresses for broadcast or multicast
• 46 to 1500 octets of payload (data)
• CRC-32