CCNA learning: OSI

ISO:  International Organization for Standardization

OSI: open system interconnection

7 Layer advantage:

1. Promote Standard Work, let different provider do develop;

2. Every layer is seperate to each other, divide the network into module, decrease the complex

3. Flexible, one layer change will not impact on others

4. Every layer has one port to communicate with other layers

7 Layer:

Application Layer         应用层：User Port, such as: Telnet, HTTP, FTP, SMTP

Presentation Layer       表示层：Encryption and Code Mode, such as: ASCII, JPEG, EBCDIC

Session Layer                会话层：Define the type of data stream, such as: Operation system

Transport Layer             传输层：Reliable or Unreliable Data Transfer Method, Error Correction, Such as: TCP, UDP, SPX

Network Layer               网络层：Choose the logical path, such as: IP, IPX

Data Link Layer             数据链路层：Combine the bit stream byte then, flame, MAC into physical address, Error detection, Such as: 802.3, 802.2

Physical Layer                物理层：Generate bit stream, Port type, such as: RJ45

all people seem to need data processing: A / P / S / T / N / D / P

please do not tell sales people anything: P / D / N / T / S / P / A

PDU: protocol data unit

Top Three layer: message

Transport layer: segment

Network layer: packet

Data-link layer: frame

Physical layer: bit

Transfer Process in 7 layers:

From bottem to top: Encapsulation封装

Example:

Top three layers: message

Transport layer:     TCP + message                                            segment

Network layer:       IP + TCP + message                                    packet

Data-link layer:     MAC + LLC + IP + TCP + message + FCS   frame

Physical layer:       0001010111101111                                     

From top to bottem: Unencapsulation

Physical Layer

Media type, Connect type, Signal type

Device: Hub,  Repeater(Enlarge signal)

Hub: （30% - 40% network bandwidth can be used by users）

all devices in same collision domain and broadcast domain,

all devices share same bandwidth

More devices, more collisions happened.

Collision: two points transfer data at the same time. the data met in physical device. both data destroy.

Collision domain: one range which allow share device;

Broadcast domain: the network range which broadcast frame can be transfered (always be controlled by router, router will not transfer broadcast signal)

ISO define collision: If collision happen, the source of the data which make collision will be baned for some time. 

After that, they will be allowed to send data again.

CSMA/CD:

Due to collisions, Ethernet network use CSMA/CD(carrier sense multiple access/collision detect) to solve the problem

Process:

Before device start to send out signal, it will check whether other devices are transfering signal.

If other devices are transfering signal, it will wait some time.

Otherwise, it will send out signal.

During sending out signal, it will also detect the network. If found other signal, it will send jam block signal.

Using CSMA/CD, the Ethernet is half duplex

Full duplex：two direction, each direction two lanes;

Half duplex：one direction, two lanes;

Simplex：one direction, one lane;

Data-link Layer

Source physical address;

Service Access Point;

Network Topo;

Frame Order;

Flow control;

Two sub layer:

MAC sub layer:

MAC: media access control

MAC Visit Control Method:

1. Competition: CSMA/CD; FCFS (first come, first service) --> reduce collision, still have

2. In turn: TOKEN                                                                  --> no collision

MAC Protocol:

802.3, 802.5, FDDI(fiber distributed data port)

LLC sub layer

LLC: logical link control

LLC Protocol: 802.2

SAP(Service Access Point) recognize the protocol of data from upper layer

Switcher and Bridge(software switcher)

（80% network bandwidth can be used by users）

Each device has own collision domain

All devices are in same broadcast domain

Network Layer

logical address;

define network path; IP/IPX

Router table: target network, port, distance

Router

broadcast control

multipoint message control

path optimization

devide the network 

flow control

logical address detection

WAN connection

Transport Layer

recognize upper application; 

establish port to port connection for application level;

flow control;

provide reliable(TCP) or unreliable(UDP) data transport method

Reliable data transport method

3-way handshake:

1. sending TCP SYN

2. sending TCP SYN ACK

3. sending TCP SYN

4. connected

Unreliable data transport method

connect without 3-way handshake

Differenct between devices

                                  HUB vs Bridge vs Switcher vs Router

Collision Domain      1            4                4                4

Broadcast Domain    1            1                1                4

Data Transferm Mode

Application Layer  --> Example: Get http://www.weibo.com

Presentation Layer -->Example: 0101101110..... (change the command into 01010...)

Session Layer         --> Example: Add session information

Transport Layer      --> Example: Add port of source computer and destination computer

Network Layer        --> Example: Add ip address of source computer and destination computer

Data Link Layer       --> Example: Add mac address of source computer and gateway computer