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| Question | Answer |
| Internet vs Ethernet | Ethernet is a network protocol that controls how data (packets) is transmitted over a LAN. Internet is a world wide packet switching network. Since it is based on packets, and Ethernet delivers packets, the carrier for the Internet that most people encounter is Ethernet |
| What is channel partitioning protocol? | Each node gets a certain amount of time to send data Pro: eliminates collisions and is fair Con: Each of N nodes gets R/N time, even if only one wants to send |
| Random access protocol | A transmitting node transmits at the full rate of the channel, R bps. If collision -> retransmission. Example: Slotted Aloha |
| Difference between Slotted Aloha and Pure Aloha | -In PA, whenever a station has data to send it transmits it without waiting whereas, in SA the user waits till the next time slot beings to transmit the data. |
| Pure Aloha | - the stations transmit frames whenever they have data to send. - stations expects and ACK -If the frame is destroyed because of collision the station waits for a random amount of time and sends it again. |
| Slotted Aloha | • In slotted ALOHA, the time of the shared channel is divided into discrete intervals called slots. • The stations can send a frame only at the beginning of the slot and only one frame is sent in each slot. • In slotted ALOHA, there is still a possibility of collision if two stations try to send at the beginning of the same time slot as shown in fig. |
| CSMA - Carrier Sense Multiple Access | Random access protocol: When a frame is ready for transmission the medium is checked first. If the medium is idle, the frame is sent. Otherwise it waits |
| Exponential back-off algorithm | Calculates the time a node should wait to retransmit after a collision. You pick a number K from 2^N -1 where N is the number of collisions, with the same prob p. Multiply that by a number K. For ethernet you multiply K by 512 bit times, i.e K times the amount of time it takes to send 512 bits. Ex: if you have a speed of 10 Mbit/s and slot time 512 bit times, then the Time Interval is 51.2 microseconds |
| Taking turns protocol | In contrast to random access. Two examples: Polling protocol Token passing protocol |
| Polling protocol | One node is the master node. It polls each of the nodes in round-robin. It tells nodes when they can transmit and how much. Pro: eliminates collisions and empty slots Con: polling delay and if Master Node fails the whole system fails |
| Token passing protocol | A token is exchanged between nodes. Node hods token if it has frames to send, otherwise token is sent to the next. |
| When does Ethernet use CSMA/CD? | for shared broadcast channels |
| CSMA/CD | CSMA + collision detection, when collision is detected it stops sending and then waits for a random time interval before trying to resend the frame |
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