Data Link Layer (osi 2) Flashcards
Name and describe the two sublayers of the data link layer
2a: logical link control
- error correction and detection
- flow control
- SAP management
2b: medium access control
- coordinates transmissions on the medium/channel
- decides how to deal with collisions
- secures reliable transmission process, so that little to no data is corrupted or goes missing
List and explain the services a link layer protocol offers
-framing:
encapsulating packets from upper layer into frames (using MAC address)
-error handling:
Detection and sometimes even correction of received bit stream
If data corrupt, ask for repeat
-contention resolving:
Medium access control, deciding when to transmit/receive so that no collisions occur
-flow control:
pacing between sender and receiver (avoid data overload)
all the methods learned in the lecture to implement the framing service
- counting (to define when frame begins and ends)
- sentinels: inserting flags to mark beginning and end of frames
- clock-based(SONET)
HLDC: High Level Data Link Control
- primary and secondary classes for messages and links
- commands only from primary
- responses from secondary
- unbalanced p2p : 1 prim. 1 second.
- unbalanced multi point: 1 primary mult. secondary
- balanced p2p: stations both primary and secondary
Message types:
- supervisory..error, flow control
- information
- unnumbered.. connection establishment/tear down
-can use gbn and selective ARQ
Frame:
Flagged, address, control field, checksum, SDU
Explain what unslotted ALOHA does
- Random Access MAC protocol
- error detection and recovery specification
- send data immediately
- if collision occurs wait random time and send again
-channel has to stay silent for at least 2 slots, otherwise collisions can occur
-probability of k packets in channel is poison distributed:
P(k)=(G^k*e^-G )/ k!
with G offered traffic
Throughput S = G * P(successful transmission)
Theoretical Max throughput 18,4 %
Explain what slotted ALOHA does
- MAC protocol (Cf unslotted Aloha)
Instead of sending frames right away, send in timeslots (synchronized clocks on all nodes)
-slots equal size (about frame time)
- frame arrives :
- > send in next slot
- if collision occurs:
- > send in next slot with probability p
Throughput S=GP(successful transmission) = Ge^-G
S,Max = 36,8%
P (next two slots free) = P(Slot free)*P(slot free)
Explain the basic CSMS mechanism
Carrier Sense Multiple Access
Avoid secure collision by listening to the channel
%If medium free:
Send
%if collision
Collision handling
%If medium not free:
Wait random time
Issues:
max distance to ensure that propagation delay does not obfuscate a blocked channel
if collision occurs, transmission time wasted
CSMA 1 persistent
CSMA type where frame is transmitted as soon as channel is free
CSMA non-persistent
If channel is blocked , wait random time and try again, otherwise send
CSMA p-persistent
If channel free, transmit. If not, wait until free and send with probability p. If not sent, start over after waiting a fixed time slot.
If blocked, wait random time and try again
Explain what CSMA with collision detection does differently than pure CSMA
If collision during transmission, send jamming signal and abort.
Back off of transmission for a bit of time and then start again.
Constraints: frames must take longer than 2t,prop at least for the jamming signal to return before transmission ends
Efficiency:
Two stations: roh,max = 1/(1+2a)
a is normalized delay bandwidth product = t,prop/t,frame
Ethernet
Uses CSMA CD
BinaryExponentialBackoff:
after collision number n, choose a number k between 0 and 2^n - 1 and wait 512*k bits before trying again
Ethernet frame: synchronization preamble, addresses, type, data, crc
Type indicates higher level protocol
Connectionless service
Unreliable
