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 troubleshooting

5 conversion steps of data encapsulation

Each layer encapsulates layer above until full packet formed

Application produces data

Presentation encodes it

Session layer syncs the session with remote

Transport layer transports to remote host using: : Segment

Network layer to add routing info. : Packet / datagram

Data link layer frames packet : Frame

Physical puts the bits on the medium

Flow control /Buffering

Packets Q up in router buffers awaiting data link layer

Packets lost if buffer overflows

Source-Quench messages

Help prevent buffer overflow

Asks source to slow down

Receiving packets, gets too many so starts discarding.

Each time segment dropped send source quench message

source receives source-quench message and slows down sending until it no longer receives them then starts increasing speed until it gets them again.

Windowing

Destination ack's every n segments having agreed a window of n

Sourcesends 3 packets

Dest. received 3 packets, sends ack.

source recvs ack so sends 3 more segments

if source does not get ack it sends 3 packets again after a timeout, sending at a slower rate.

Internetworking functions of OSI model

Packets, frames, routing