The Data Link layer also bars the responsibility of dictating the logical topology of a network, or the way the devices perceive the topology of the network. A logical topology differs from a physical topology in that the physical topology dictates the way the cables are laid out, whereas the logical topology dictates the way the information flows. The types of logical topologies are the same as the physical topologies except that the information flow specifies the type of logical topology in use.For example, in early days of Ethernet, networks were physically wired using a bus topology.Once twisted pair and fiber were introduced into the Ethernet Physical layer, hubs were designed to allow the network to be wired as a physical star while allowing the devices attached to the hub to continue to believe they were part of a bus, making this configuration a physical star/logical bus. Finally, the DLL can describe the method of media access.There are some media access methods, discussed below:
Contention: Every device "competes" with other devices for the opportunity to transmit, and each has an equal chance. If two devices transmit at the same time, a collision occurs and the devices try again.
Polling: A central device, called a controller, polls each devices in turn and asks if it has data to transmit. This type of media access virtually eliminates collisions.
Token Passing : This uses a special data packet called a token. When a device has the token, it can transmit. If it doesn't have the token, it can't transmit. This media access technology also eliminates collision problems.
Carrier Sense/Multiple Access with Collision Detection (CSMA/CD) : When a protocol that uses CSMA/CD has data to transmit, it first sense if a signal is already on the wire (a carrier), indicating that someone is transmitting currently. That's the "Carrier Sense" part. If no one is transmitting, it attempts a transmission. If someone else transmits at the exact same time, a collision occurs. Both senders "back off" and don't transmit until some random period of time has passed. That's the "Collision Detection" part. "Multiple Access" means that more than one station can be on the network at the same time.
Carrier Sense/Multiple Access with Collision Avoidance (CSMA/CA): It is identical to CAMA/CD, but instead of sending the whole data chunk and then listening to hear if it was transmitted, the sender transmits a request to send (RTS) packet and waits for a clear to send (CTS) before sending. When it receives the CTS, the sender sends the chunk.
Difference between CSMA/CD and CSMA/CA:
Say you want to cross a busy street and you want to use one of these protocols to cross it. If you are using CSMA/CD, you just cross the street. If you get hit, you go back to the curb and try again. If you're using CSMA/CA , you send your friend across. If he makes it, it's probably OK for you to go.
Contention: Every device "competes" with other devices for the opportunity to transmit, and each has an equal chance. If two devices transmit at the same time, a collision occurs and the devices try again.Polling: A central device, called a controller, polls each devices in turn and asks if it has data to transmit. This type of media access virtually eliminates collisions.
Token Passing : This uses a special data packet called a token. When a device has the token, it can transmit. If it doesn't have the token, it can't transmit. This media access technology also eliminates collision problems.
Carrier Sense/Multiple Access with Collision Detection (CSMA/CD) : When a protocol that uses CSMA/CD has data to transmit, it first sense if a signal is already on the wire (a carrier), indicating that someone is transmitting currently. That's the "Carrier Sense" part. If no one is transmitting, it attempts a transmission. If someone else transmits at the exact same time, a collision occurs. Both senders "back off" and don't transmit until some random period of time has passed. That's the "Collision Detection" part. "Multiple Access" means that more than one station can be on the network at the same time.Carrier Sense/Multiple Access with Collision Avoidance (CSMA/CA): It is identical to CAMA/CD, but instead of sending the whole data chunk and then listening to hear if it was transmitted, the sender transmits a request to send (RTS) packet and waits for a clear to send (CTS) before sending. When it receives the CTS, the sender sends the chunk.
Difference between CSMA/CD and CSMA/CA:
Say you want to cross a busy street and you want to use one of these protocols to cross it. If you are using CSMA/CD, you just cross the street. If you get hit, you go back to the curb and try again. If you're using CSMA/CA , you send your friend across. If he makes it, it's probably OK for you to go.
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