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 More efficient use of shared resource

Time-Division Multiplexing

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.

Frames Header and trailer indicates beginning and end of frame.

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: , parity bit 1  Odd parity - data: , parity bit 0

Point-to-Point Networks

LAN’s Reduce complexity and cost by sharing communication channel TDM Synchronization Effective because of locality of reference –Spatial –Temporal

Topologies Star, Ring, Bus

Ethernet 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 Identify destination on shared channel Hardware or physical address Included in frame

Ethernet Frame

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

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

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