The datalink layer in broadcasting networks Flashcards
1
Q
Basic types of broadcasting networks
A
- Local area networks (shared wire (e.g. classic Ethernet) or shared wireless (e.g. Wi-Fi))
These have a very short delay. - Satellite networks (Wide area network). These have a very long delay (270 msec)
2
Q
Channel allocation problem
A
how to allocate a single broadcast channel among competing
users:
- Static Channel Allocation: Capacity of the channel is split among multiple competing users like FDM or TDM. Do not work well with bursty traffic
- Dynamic Channel Allocation: Gives the channel to a user when they need it. We use multiple access protocols
3
Q
Assumptions with multiple access protocols
A
- All hosts can transmit frames on the channel and all can receive from it
- Each frame sent is received by all hosts
- Hosts have to contend for the use of the channel
- If two hosts send more or less simultaneously, the two frames will collide. Both will be damaged (in data terms – electrically, the medium can cope)
4
Q
What are the assumptions that depend on the system
A
- hosts may/may not be able to detect collisions when they occur
- Timing of frame transmissions may be assumed continuous or slotted
- A network may have carrier sensing or not have it
5
Q
Contention protocol - Aloha
A
- The protocols used on satellite channels are called Aloha
- Developed at the University of Hawaii, using a ground-based radio system
- Two versions (pure ALOHA and Slotted ALOHA)
6
Q
Pure aloha
A
- Hosts transmit whenever they want. Colliding frames are destroyed
- If a frame was destroyed, the sender just waits a random amount of time and sends again
- Utilisation is bad under high load (18% utilisation)
7
Q
Slotted aloha
A
- Divide the time up into discrete intervals, each interval corresponding to one frame
(slotted time) - Hosts can only transmit at the start of each time slot
- This requires one special station to emit a “pip” at the start of each interval for all others
to synchronise (like a clock). This double the efficiency of Pure Aloha at 37%
8
Q
What is CSMA protocol
A
Carrier sense multiple access
9
Q
1- persistent protocol
A
- When a host has data to send, it first listens to the channel to see if anyone else is
transmitting - If the channel is busy, the host keeps listening until it becomes idle
- When the host detects an idle channel, it immediately transmits a complete frame
- If the frame is damaged, the host waits a random amount of time and starts all over
again
10
Q
non persistent protocol
A
- When a host has data to send, it first listens to the channel to see if anyone else is
transmitting - When the host detects an idle channel, it immediately transmits a complete frame
- When the sending host detects that the channel is busy, it does not continuously sense it
for the purpose of seizing it immediately upon detecting the end of the previous
transmission - Instead, it waits a random period of time and then repeats the algorithm
11
Q
p Persistent protocol
A
- When a host becomes ready to send, it senses the channel. If it is idle, it transmits with a
probability p (therefore the probability that it defers transmission until the next slot is q =
1 – p) - If that slot is also idle, it either transmits or defers again, with probabilities p and q
respectively - If a slot is busy, it waits a random amount of time and starts again
12
Q
Collision detection protocol
A
- With CSMA/CD, a host aborts its transmission as soon as it detects a collision, it waits a
random period of time, and then tries again - Quickly terminating damaged frames saves time and bandwidth
- This protocol is widely used on LANs including classic Ethernet
- A host can only detect a collision as it happens because
- A host uses highly special electronics that allows it to hear while it is transmitting
- Frame transmission time >= Round trip time
13
Q
Bit map protocol
A
- Following a frame transmission period, there is a short contention period which consists
exactly of N small slots for the N hosts on the channel - If a host j has a frame to send, it inserts a 1 into slot j of the contention period. After all N
slots have passed by, each host has complete knowledge of which hosts wish to transmit - At that point, they begin transmitting in numerical order
- Since everyone agrees on who goes next, there will never be any collisions
14
Q
Limited contention protocols
A
- Limited Contention Protocols combine the best properties of the contention and collision-
free protocols (uses contention at low loads to provide low delay, but uses a collision-free
technique at high loads to provide good channel efficiency)
15
Q
Adaptive tree walk protocol
A
- We divide the hosts up into groups. Only the members of
group j are permitted to compete for slot j. e.g. - In the first contention slot following a successful frame
transmission, slot 0, all hosts are permitted to acquire the
channel - If one of them does so, fine
- If there is collision, then during the next slot, slot 1, only
those hosts falling under node 2 in the tree (first sub-tree)
may compete - If one of them acquires the channel, the next slot is
reserved for those hosts under node 3 (the second sub-
tree), etc
