Ccna 2 Notes

CCNA Chapter 1

OSI 7 layer model
Application
File, Print, database, apps,web
Presentation
Encryption, compression, translations, ebdic etc.
Session
Dialog control - nfs, sql, rpc , Connection establishment, data transfer, connection release
Transport
End to end connection, Flow control, Data integrity, Retransmissions
Segments re-ordered before delivery, Make / tear down virtual circuits
Network
Routing , Map of network (logical)
Data Link
Framing
Ensures messages delivered to correct device
MAC - Media Access Control
Framing with header & trailer. contains MAC address of source & destination
Media Access - method of communicating with Physical medium
Logical Topology - eg Star shaped "ring" in TR error detection
LLC - Logical Link Control
Optional
Flow control with stop/start codes error correction
Destination Service Access Point (DSAP) and Source SAP allow upper-layers to work over different LANs by providing comms between Network & data/physical layers regardless of physical/logical actually used

Physical
Physical topology
DTE - Data terminal equipment = router etc.
DCE - Data circuit terminating equipment @ service provider
DTE connects through modem or Channel / data service unit (C/DSU)
Connection oriented / Connectionless
Connectionless
Not guarenteed delivery
IPX or UDP
IP. TCP adds flow control & reliability at Transport layer
Connection oriented
Uses a virtual cicuit
Still not guarenteed but has flow control, relability, error checking
SPX,TCP
Data link addresses / network addresses
Data link = hardware = mac address
Network address = logical = IP or IPX address = Network layer internetworks created with routers from logical addresses
Router places packet in frame with dest. mac address or next hop

Reasons for layered model
Divides complexity into sections
Uses standard interfaces to ease interoperability
Developers can change one layers features without others
It allows specialisation
Eases