Networks. Common Xmit Media Shared Media Networks are a shared communication resource Only one user can access at a time.

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: 10010011, parity bit 1

Odd parity - data: 10010111, 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