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Networks. Common Xmit Media. Shared Media. Networks are a shared communication resource Only one user can access at a time. Packet and Circuit Switched. Circuit: Public Telephone Network Packet: Internet and US Mail Data divided into small units (packets) for transmission

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Common xmit media

Common Xmit Media

Shared media

Shared Media

  • Networks are a shared communication resource

  • Only one user can access at a time

Packet and circuit switched

Packet and Circuit Switched

  • Circuit: Public Telephone Network

  • Packet: Internet and US Mail

  • Data divided into small units (packets) for transmission

  • More efficient use of shared resource

Time division multiplexing

Time-Division Multiplexing

Packets and frames

Packets and Frames

  • Packet: A small, self-contained parcel of data sent across a computer network. Each packet contains a header that identifies the sender and recipient, and data to be delivered.

  • Frame: The form of a packet that the underlying hardware accepts and delivers.



  • Header and trailer indicates beginning and end of frame.

Transmission errors

Transmission Errors

  • Error Detection and Error Correction

  • Both require additional information beyond data, appended to end of frame

  • Many Schemes

  • Parity checking, Check Sum, CRC codes

  • Error Correcting Codes: Hamming, Reed-Solomon, etc...


  • Parity refers to the number of bits set to 1 in the data item

    • Even parity - an even number of bits are 1

    • Odd parity - an odd number of bits are 1

  • A parity bit is an extra bit transmitted with a data item,

  • chose to give the resulting bits even or odd parity

    • Even parity - data: 10010011, parity bit 1

    • Odd parity - data: 10010111, parity bit 0

Point to point networks

Point-to-Point Networks

Lan s


  • Reduce complexity and cost by sharing communication channel

  • TDM

  • Synchronization

  • Effective because of locality of reference

    • Spatial

    • Temporal



  • Star, Ring, Bus



  • Widely used LAN technology

  • Invented at Xerox PARC (Palo Alto Research Center) in 1970s

  • Defined in a standard by Xerox, Intel and Digital - DIX standard

  • Standard now managed by IEEE - defines formats, voltages, cable lengths, ...

  • Uses bus topology

  • Single coax cable - the ether

  • Multiple computers connect

  • One Ethernet cable is sometimes called a segment

  • Limited to 500 meters in length

  • Minimum separation between connections is 3 meters


  • Originally 3 Mbs

  • Then, 10 Mbs

  • Fast Ethernet, 100 Mbs

  • One computer transmits at a time

  • Signal is a modulated carrier which propagates from transmitter in both directions along length of segment

Hardware adresses

Hardware Adresses

  • Identify destination on shared channel

  • Hardware or physical address

  • Included in frame

Ethernet frame

Ethernet Frame



  • Packet Switch: Moves complete packets from one connection to another

Protocols and layering

Protocols and Layering

  • A rule that specifies format and meaning of messages that computers exchange

  • Protocols organized in layers, each of which solves specific aspect of communication problem.

Iso 7 layer model

ISO 7 Layer Model


  • Layer 7: Application

    • Application-specific protocols such as FTP and SMTP (electronic mail)

  • Layer 6: Presentation

    • Common formats for representation of data

  • Layer 5: Session

    • Management of sessions such as login to a remote computer

  • Layer 4: Transport

    • Reliable delivery of data between computers

  • Layer 3: Network

    • Address assignment and data delivery across a physical network

  • Layer 2: Data Link

    • Format of data in frames and delivery of frames through network interface

  • Layer 1: Physical

    • Basic network hardware - such as RS-232 or Ethernet

Nested headers

Nested Headers

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