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CHAPTER 18 The Network

18.1 The Network

Network types and access - Local Area Network (LAN) and Wide Area Network (WAN). LAN --> high speed connection between machines within a site. WAN --> geographically remote machines. Ethernet uses Carrier Sense Multiple-Access Collision Detection (CSMA/CD) to share a single transmission line. This means the system listens on the line prior to attempting to send a packet, any system can send at any time, and should a collision occur the system will sense this and retransmit after a random delay.

The International Standard Organization's Open System Interconnection (ISO/OSI) model is used by Sun and many other vendors as the network protocol. From the top down the layers are:
Network Protocol Layers



Applicationprovides network services, e.g. mail, ftp, telnet, NFS, YP, DNS, WWW
PresentationXDR (eXternal Data Representation); transformation services such as text compression, conversion between character codes (EBCDIC --> ASCII), etc. so that it can be recognized by other machines.
SessionRPC (Remote Procedure Call); enables programs to establish connections with each other via names rather than socket addresses; recovers from failed connections.
TransportTransportTCP (Transmission Control Protocol), UDP (User Datagram Protocol); TCP provides reliable communication between pairs of processes on the network, it establishes connections through "sockets" which are determined from the IP address and the port number; UDP provides a low overhead transmission service, but with less error checking.
IPNetworkIP (Internet Protocol); connects subnets to the Internet; handles fragmentation/recombination, routing and buffering; initiates and terminates connections between machines.


Data Linkdefines data frames; controls data encapsulation; detects and possibly corrects errors; determines how the line is to be shared by the multiple machines.
Physicalprovides an electrical connection, e.g. through coaxial cable, between machines; defines procedures for starting and ending sessions; transfers packets.

Unix System Administration - 8 AUG 1996
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