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LAN Technologies and Protocols
Local Area Network concepts focus on layer 1–3 technologies such as network cabling types, physical and logical network topologies, Ethernet, and others.
Ethernet
Ethernet is a dominant local area networking technology that transmits network data via frames. It originally used a physical bus topology, but later added support for physical star. Ethernet describes Layer 1 issues such as physical medium and Layer 2 issues such as frames. Ethernet is baseband (one channel), so it must address issues such as collisions, where two nodes attempt to transmit data simultaneously.
Ethernet has evolved from 10-megabit buses that used “thinnet” or “thicknet” coaxial cable. The star-based physical layer uses Twisted Pair cables that range in speed from 10 megabits to 1000 megabits and beyond. A summary of these types is listed in Table 5.7.
Table 5.7 Types of Ethernet
| Name | Type | Speed | Max. Distance |
|---|---|---|---|
| 10Base2 | Thinnet | Bus 10 megabits | 185 meters |
| 10Base5 | Thicknet | Bus 10 megabits | 500 meters |
| 10BaseT | Star | 10 megabits | 100 meters |
| 100BaseT | Star | 100 megabits | 100 meters |
| 1000BaseT | Star | 1000 megabits | 100 meters |
CSMA
Carrier Sense Multiple Access (CSMA) is designed to address collisions. Ethernet is baseband media, which is the equivalent of a “party line.” In the early days of phone service, many people did not have a dedicated phone line for their house: they shared a party line with their neighbors. A protocol emerged for using the shared phone line:
- Lift the receiver and listen to determine if the line is idle
- If the line is not idle, hang up and wait before trying again
- If the line is idle, dial
Ethernet CSMA works in the same fashion, but there is one state that has not been accounted for: two neighbors lift their receivers and listen to hear if the line is in use. Hearing nothing, both dial simultaneously. Their calls “collide”: the integrity of their calls is ruined. CSMA is designed to address collisions.
Carrier Sense Multiple Access with Collision Detection (CSMA/CD) is used to immediately detect collisions within a network. It takes the following steps:
- Monitor the network to see if it is idle
- If the network is not idle, wait a random amount of time
- If the network is idle, transmit
- While transmitting, monitor the network
- If more electricity is received than sent, another station must also be sending (a) Send Jam signal to tell all nodes to stop transmitting (b) Wait a random amount of time before retransmitting
CSMA/CD is used for systems that can send and receive simultaneously, such as wired Ethernet. CSMA/CA (Collision Avoidance) is used for systems such as 802.11 wireless that cannot send and receive simultaneously. CSMA/CA relies on receiving an acknowledgment from the receiving station: if no acknowledgment is received, there must have been a collision, and the node will wait and retransmit. CSMA/CD is superior to CSMA/CA because collision detection detects a collision almost immediately.