1. Figure Internet model

2. Underlying Technology

3. Local Area Networks
A local area network (LAN) is a data communication system that allows a number of independent devices to communicate directly with each other in a limited geographic area such as a single department, a single building, or a campus. A large organization may need several connected LANs.The most popular LANs are Ethernet and wireless LANs. We briefly review these technologies in this section.

4. Ethernet Traditional Ethernet
Traditional Ethernet had a 10-Mbps data rate
Access Method :CSMA/CD ( carrier sense multiple access with collision detection )

5. Figure CSMA/CD

6. 1- every station has an equal right to the media (multiple access)
2- every station with frame to send first listens to (sense )the medium .if there is no data on the medium , the station can start sending (carrier sense)
3- it may happen that two stations both sense the medium , find it idle , and start sending . In this case a collision occurs.
The protocol forces the station to continue to listen to the line after sending begun
If there is collision , all station sense collision
Each sending station sending a jam signal to destroy the data on the line
Each station waits a different random time , try again
random time prevent the simultaneous re-sending of data.

7. The factor are related in the CSMA/CD
1-minimum frame length (bit)
2- data transmission rate (bps)
3- collision domain (m). (max network distance )
The time the station needs to wait to be sure that there is no data on the line =min frame length/transmission rate
=the time required to send out the minimum frame length
Min frame length/ transmission rate is proportional to
Collision domain / propagation speed

8. In traditional Ethernet
Min frame length =520 bits
Transmission rate=10 Mbps
Propagation speed = speed of light
Collision domain = 2500 m

9. Figure 3.3 Ethernet layers

10. Layers The data link layer has two sub layers
1- logical link control (LLC)
2- media access control (MAC)
The LLC layer is responsible for :
flow and error control at data link layer
The MAC sub layer is responsible for operation of the CSMA/CD

11. Ethernet frame

13. Figure Ethernet frame

17. Figure 3.5 Ethernet implementations