Detailed description of OSI Layer-7 Model

1.1Network layering

This course mainly introduces two different layers:OSILayered Model andCiscoLayer-3 model.
Advantages of layering:
1. Divide complex networks into layers that are easier to manage.
2. When a layer is changed, other layers are not affected, which makes the applicationProgramDevelopers can perform specific design and development.
3. In today's network environment, no manufacturer can provide a complete set of solutions and all devices, and define a standard interface in a multi-vendor environment, that is"Plug-and-play".

1.2 OSIMain functions of the layer-7 model and devices working at different layers

I. UnderstandingOSIRelated models

Why?OSI? The most important reason is:OSIThe layer-7 model is the best tool to describe the content and functions behind the implementation of network protocols. To learn the network structure, network principles, and network devices, you must start from.OSIOpen Network Interconnection (Open Systems Interconnection)
OSIHistory and status quo:International Standardization Organization(I s o)CreatedO s IModel, and1 9 8 4The annual release provides suppliers with a network model so that their products can coordinate work on the network.O s IThe reference model provides a hierarchical analysis tool to understand the interconnection technology and the basis for current and future network development.

Ii. ExploitationOSIBenefits and concepts of hierarchy

1So that people can easily explore and understand many details of the Protocol.
2Standardized interfaces between different layers allow different products to provide only one part of the functions of each layer (for example, one or three layers of routers), or only provide part of the protocol function. (For exampleWin95InMicrosoft TCP/IP)
3, Create a better integrated environment.
4, Reduce complexity and allow easier programming change or rapid evaluation.
5, UseHeadersAndTrailersTroubleshooting.

A lower layer provides services for a higher layer.

III,OSILayer-7 functions and protocols and data formats

OSI Layers Function Protocol, data format, or device

ApplicationProvide communication services for Applications FTP,WWW browsers

Example:Word ProcessorTelnet,NFS,SMTP

Gateways,MailAnd so on

PresentationThe main role is to define the data formatTiff, GIF, and JPEG

Such as binary orASCIITransmission ASCII, MPEG, Midi

Himl

SessionDefine how to start, control, and end RPC, SQL, NFS,

SessionConversationsFor exampleATMMachine NetBIOS names

Two-way transmission of Transaction Processing Appletalk ASP

TransportThe fourth layer includes whether to provideTCP, UDP, and SPX

Protocol for error recovery

For exampleTCP →MinPacket →

IP → TCPCombinedSegment

NetworkDefine the end-to-end transmission of the package IP,IPX

It also defines Appletalk DDP

SetPacketSplit into smallerPacketVro

Data LinkSpecify transmission from a specific link or media FrameRelay

Data, defined to be transmitted through different linksHDLc, PPP

Example:802,3,802,2DefinitionEthernet1eee802, 3/802, 2

How to work,HDLC → point-to-pointFddl, ATM

Wan LinkNic, bridge, and switch

PhysicalPhysical Characteristics of physical media E 1a /T 1a ,232

Commector, pin, electrical currentE 1a /T 1a -449

Eneoding.Example:RJ45DefinitionWires/pinsV.35, V.24

EthernetAnd802.3DefinitionWires/RJ45, Ethernet

Pins1, 2, 3, 6802.3, 802.5 FDDI

4. Communication at the same layer of different computers

HostAHostB

ApplicationApplication

PresentationPresentation

SessionSession

TransportTransport

NetworkNetworkNetwork

Data LinkData LinkData Link

PhysicalPhysicalPhysical

Router (with only three layers of Functions)

V,Data encapsulationData encapsulation and data packet name

Data formatOSILayer Data Packet name

1Data Application Layer Data

2TCP DataTransport Layer Segment

3Ip tcp DataNetwork LayerPacket

4Lh ip tcp Data ltData Link Layer Frame

500100101011110110Physical Layer Bits

Vi. connection-oriented and non-connection-oriented protocols

Connection-orientedVS Connectionless

Connection-orientedError reability (reliability)LLC type2,TCP,SPX,X.25

Connection-orientedPre-established pathingX.25,FrameRelay,ATM

ConnectionlessSimply send data, not used IPX,UDP

Start of error recovery or path Creation LLC type 1

DifferencesError DetectionAndError Recovery

Error Detection: UseFCsTo detect transmission errors.

Error recovery: Retransmission due to data loss

Three steps for error recovery:

1, Use the initialization stream to create a connection protocol.

2, Define a connectionHeaders, SuchTCPHeader.

3, The sender needs to confirm that the data has been sent.

Error RecoveryProtocol examples and their features

FeatureTCPSPXLlc2

Are you sure you want to do this in both directions??YesYesYes

Confirm forwarding?YesYesYes

UseByteOrFrame/PacketCount? BytesPackets FrameS

Re-transmission of all, or only one, and then re-transmission Only one All retransmission All retransmission

VII. Flow Control (Flow Control)

The reason for Throttling is that the computer sending data is faster than the receiver, or faster than the intermediate device.

Without flow control, packets are discarded.

3Basic Methods:

1,Buffering Only the receiver uses enough buffer space to place inbound data until it is processed. To slow down the sender.

2,Congestion AvoidanceThe recipient notifies himBufferIt is full.

3,WindowingWindow means the sender does not need to confirmAcknowledgementMaximum data size that can be sent

Flow Control Method--Summary

Buffering(Alias)N/

Congestion AvoidanceStop/start,RnR, Source QuenchSDLC

Lapb, llc2

WindowingTCP. SPX. llc2

1.3 Basic Ethernet Knowledge

I,Data Link (Layer2) Functions

Layer 2 protocol:Ethernet,Token Ring,HDLC,FrameRelay,HDLCYesCiscoDefault vroData LinkProtocol (encapsulation)

Function 1:ArbitrationArbitration.

EthernetUseCSMA/CD. Carrier Sense Multiple Access/collision Defect

Token RingUseTokenToken.

HDLCNo arbitration required,HDLCYesPoint-to-point link,Full Duplex. data can be sent at the same time.

FrameRelay full duplex linkNo arbitration required

CommonFrameType

· Function2: LAN Addressing

Lan mac addressTerms and features:

Mac Media Access control.802.3 (Ethernet)And

802.5(Token Ring)

Ethernet address, NIC addressMedia Access control.802.3 (Ethernet)And

Lan address, Token Ring address802.5(Token Ring)

Card addressOrBurnet-in-addressBurned by suppliersLanCard inRomOrEEPROMAddress in

Locally administrated address: The configuration can replace the burning address

Unicast addressSingle point transfer address: A specificMacAddress

Broadcast addressBroadcast address: Target address to be received by all NICs

Multicast addressMulticast address: The same address of a specific set of NICs(It cannot be used for a ring.)

Function addressFeature address : Used to specify the reserved service to a specific device.

For exampleSource-routeBridgeRPSFeature address

HDLCNo address, because it is a point-to-point link

FrameRelay virtual circuits

Data-link connection identifiers dlci

Function 3:Error Detection

FrameCheck sequence (FCS)Frame verification sorting

Semantic ical redundancy check (CRC)Cyclic Redundancy check

802.2,SAP,Snap,HPLC, FrameRelay.

Ii. Full duplex and half duplex

10 BaseT full-duplexOperation: because it is impossible to conflict, no needLoopbackReceivePair

Features of Fast Ethernet:

①Fast100 Mbps

②Automatic Control: allows the device to determine the type of the device on the other end, whether it is half duplex or full duplex

10 BaseT half-duplexOperation: RequiredLoopbackTo this Nic to prevent both sending and receiving data.

1.4 Cisco L3 distributed model and equipment

This is a distributed model defined by Cisco.CiscoThe layer-3 distributed model helps us select the right Cisco device.

Access layer: directly controls the network connection to the desktop, most of which use the bottom switch, suchCisco1900Series,Cisco2900Series switches

Distribution layer: connects to access layer devices and provides access lists, packet filtering and sorting, security and network policies, routes, and other functions. Most of them use routers, suchCisco2500Series,Cisco2600Series,Cisco3600Series routers.

Core layer: as the backbone of the high bandwidth, high speed, high availability, low latency and use of a shorter convergence time routing protocol, most of the use of high-end switches, suchCisco4000Series,Cisco5000Series,Cisco8500Series switches.

1.5 Cable

Ethernet is a very mature technology. Many network cables are Ethernet standard, such100 basetxAnd so on.100The bandwidth is100 Mbit/s,WhileBaseIt refers to the baseband that uses digital signals to transmit information,TXThe cable type is5,6,7ClassUTPTwisted Pair wires. For more information about cable types, see the tutorial.

CommonUTPCable Type:

Straight-throughExpress CONNECT: usedPCConnection between the server and the hub and vswitch

CrossoverCrossover line: Used for the connection between hubs and vswitches.PCThis type of line is also required for machine interconnection.

rolover : connects the com port of the Pc host to the console port of the vswitch or vro, configure the vswitch or vro on the Super Terminal of the Pc machine .