quiz 3 networks

Multicast Ethernet Transmission

1 Sender inserts a 1 in LSb of the MSB1 of the Destination Address field in Ethernet frame header. The other 47 bits hold the rest of the 6 byte MAC address of "multicast group" Multicast group addresses are standardized by the IEEE. These addresses all have 01 as their first byte; see this Wikipedia page 2 Frame is sent after populating all header and body fields 3 All hosts on Ethernet can receive frame but a host accepts frame only if That host belongs to multicast group whose MAC address matches address in frame's Destination Address field, or That host is operating in promiscuous mode

Unicast Ethernet Transmission

1 Sender inserts destination's MAC address in Destination Address field in Ethernet frame header 2 Frame is sent after populating all header and body fields 3 All hosts on Ethernet can receive frame but a host accepts frame only if That receiver's MAC address matches address in frame's Destination Address field, or That host is operating in promiscuous mode

Stop and Wait protocol

1 Sender starts frame timer when sending frame F1 2 If Receiver's ACK to frame F1 arrives before frame timer expires, Sender concludes that F1 was successfully transmitted Therefore, Sender transmits the next frame, F2 3 If ACK to F1 does not arrive before frame timer expires, Sender assumes frame F1 was lost Therefore, Sender retransmits frame F1

Ethernet's Transmitter Algorithm

1 Sense for carrier; if busy, continue sensing 2 When channel is free, wait a further time period equal to the inter-frame gap (IFG) Now start transmitting and continue sensing for collisions 3 If collision is detected, send 32-bit jamming sequence and execute collision recovery algorithm 4 If transmission ends, and no collision detected, we are done

How can ARQ be implemented?

1 Stop-and-Wait protocol 2 Sliding Window protocol 3 Concurrent Logical channels

Cyclic Redundancy Check (CRC)

A mathematical algorithm that is executed on a data string by both the sender and the receiver of the data string. If the calculated CRC values match, the receiver can conclude that the data string was not corrupted during transmission. Refer to example in class for how to solve

Sliding window

A type of data window in which block sizes are variable. Window size is continually reevaluated during transmission, with the sender always attempting to send the largest window it can to speed throughput. Is more efficient than stop and wait protocol.

Chksum advatages and disadvantages

Advantage: Computationally efficient (fast) Disadvantages: Wasteful; requires large field size (many bytes) Weak protection e.g. 3 + 4 = 1 + 6 Implemented using 1's complement

Ethernet's Binary Exponential Backoff Algorithm

After collision, time is divided into discrete but unsynchronized slots of duration δ (= 51.2 µs for 10 Mbps Ethernet) .. rationale is slide 24 1 If this is the 1st failed attempt for this frame, then randomly wait for 0 or 1 slot duration before retransmission (i.e. a delay of either 0 µs or 51.2 µs) k x 51.2, k ∈ {0, 1} 2 If this is the 2nd failed attempt, transmit after randomly waiting for 0 1, 2, or 3 slot durations (i.e. a delay of 0 µs, 51.2 µs, 102.4 µs or 153.6 µs) k x 51.2, k ∈ {0, .., 3} 3 In general, after n failed attempts, the delay selected is one from 2n available slots

Broadcast Ethernet Transmission

Broadcast Steps: 1 Sender inserts all 1s in Destination Address field in Ethernet frame header 2 Frame is sent after populating all header and body fields 3 All hosts on Ethernet (i.e. same LAN) receive AND accept the frame

Multiple Acess Channels

Channel can be categorized as one of the foll: 1 Continuous (Asynchronous)-> There is no slot clock Transmission is asynchronous and can commence at any time 2 Slotted Time (Synchronous)-> Transmission occurs in equal-duration slots Transmitters and receivers synchronize at slot edges Frame transmissions initiated only at slot beginnings

Ethernet adressing

Ethernet address is a 6-byte address also called by these various names: Medium Access Control (MAC) address, or Network Interface Controller (NIC) address, or hardware address, or simply, machine address

Ethernet's Collision Recovery Algorithm

Ethernet uses a variation of the 1-persistent protocol: 1 Continue transmission until minimum packet time 2τ is reached to ensure that all receivers detect collision The value of 2τ is on a later slide 2 If (transmission attempts > max attempts allowed), abort transmission and report failure to upper layer 3 If (transmission attempts ≤ maximum attempts allowed), 1 Increment transmission attempts 2 Calculate backoff period using the binary exponential backoff algorithm .. see next slide 3 Wait for this (calculated) backoff period 4 Go to step 1 of transmitter algorithm on previous slide

What errors does 2-D parity not catch?

It may not catch even bit errors, but it will always catch 1-2 and odd bit errors.

How would a transmitter know a collision is its own?

It would have to transmit for the RTT

CSMA/CD

Means -> Carrier Sense Multiple Access with Collision Detection To minimize collisions: 1 Carrier Sensing: Before transmitting, listen for carrier signal on channel 2 If no carrier, then channel is idle; start transmitting 3 Collision Detection: While transmitting, listen for collisions 4 If collision detected, halt transmission, enter recovery mode

How is reliable transmission possible?

Reliable transmission is possible by means of - ARQ using timers and positive acknowledgements (ACKs) - NACKs (optionally), for performance enhancement

Sender and Receiver in chksum

Sender does the following: 1 Compute sum of all data words; this is the checksum 2 Transmit computed checksum along with original data, usually in DL trailer Receiver does the following: 1 Extract checksum from frame 2 Independently compute checksum on recvd data, and 3 Compare extracted checksum with its computed checksum ^If comparison fails, then reject frame Else, accept frame

Multiple Acess Channels transmitter bhvr

Transmitter behavior falls into one of following categories 1 -> 1-persistent CSMA 1 Before sending frame, first sense channel 2 If channel is busy, continue sensing till channel becomes idle 3 If channel is idle, transmit with probability 1 2 -> non-persistent CSMA 1 Before sending frame, first sense channel 2 If channel busy, stop sensing channel, wait random time period, and go to step 2.1, else go to step 2.3 3 If channel is idle, transmit with probability 1

Multiple Acess Channels transmitter bhvr (p-persistent)

p-persistent CSMA (for slotted time system) 1 Before sending frame, first sense channel 2 If channel busy, wait till next time slot and go to step 1 3 If channel is idle, transmit at beginning of time slot with probability p i.e. Defer transmission till next time slot with probability (1 − p) Repeat step 3 till frame is transmitted, or someone else starts transmitting 4 If someone else starts transmitting, treat this as a collision i.e. Wait random backoff time and go to step 3.1