Ccna 2 Notes

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  • Topic: OSI model, Data Link Layer, Logical Link Control
  • Pages : 6 (1019 words )
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  • Published : January 5, 2011
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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

Chapter 2, WAN Protocols
Differentiate between types
Frame Relay
High performance
Physical & Data link levels
Designed for ISDN
Error correction is for higher levels
Connection oriented PVC's
Frame Relay VCis 2x DTE with packet switched network inbetween, eah VC is identified by a Data Link Connection Identifier (DCLI) ISDN - Integrated Services Digital Network
Physical, Data Link, Network, Transport Layers
set of comms protocols
HDLC - ISO standard
Data Link Layer, bit based for sync. serial links
High Level Data Link control
Cisco HDLC Won't communicate with any other vendor's
PPP
Async or Sync
Uses Link Control Protocol to build data link connections
Compression
CHAP / PAP authentication
Uses NCP to support IPX, IP, Appletalk, Decnet etc.
Frame Relay Terms
CO -...
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