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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
• 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
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