Part I Flashcards
What is the purpose / benefits of the OSI model?
At each layer certain things happen to prepare data for the next layer. Lower layers are in hardware.
Purpose is to have a standard open model to overcome difficulties and inefficiencies of multiple networking models.
Standard interfaces between layers, standardisation of components.
What is the traffic equation?
A = ys
A = traffic in erlangs
y = mean call arrival rate (calls per unit time)
s = mean cal holding time
What is an Erlang?
A unit of communication traffic equals to a load whose calls, if placed end to end, will keep one path continuously occupied.
How is call holding time distributed?
Negative exponentially
What is a property of the call holding time?
Memoryless - the history of a call’s duration does not affect the likelihood of it ending at any given moment
What are some things that affect traffic variation?
Time of day
Holidays or other events
Month of the year
What are delay systems vs lost-calls-cleared systems?
Delay systems place people in a queue if they cannot be serviced immediately.
Lost calls cleared clears the call.
What is the Erlang B blocking probability formula?
E_N[A] = [N] = (A^N/N!) / (sum(v=0 to N) of A^v/v!
E_N[A] is the probability of an incoming call being blocked if there are N states (lines) and the average traffic is A Erlangs. (Equal to the probability of all lines being full)
What are the assumptions of the Erlang B blocking probability formula?
- Calls are independent and arrive at random
- Any call can connect to any free line. Lost-calls-cleared
- Caller population»_space; number of lines
- The rate of call arrivals when the network is in state i is λi (starting from I=0)
the rate of call departure when the network is in state i is μi - Only one event (departure or arrival) can occur at one time
What does statistical equilibrium mean for the Erlang B blocking probability?
The number of calls leaving the system equals the number of calls arriving
What is a full availability system?
Where there are S sources of traffic and N outlets and any free source can use any free outlet. We assume S»_space; N
What is time congestion?
The proportion of the busy hour for which the network is in state N. = [N].
What is call congestion?
The probability (B) that a call arrives to find the system fully occupied. B = [N] for the Erlang assumption (S»N). (call congestion = time congestion)
How can you generate a random variable with a negative exponential distribution?
wi = -Aln(1-ui)
where wi is a random variable with neg exp distribution
ui is a uniform random variable
A is the mean of the exponential pdf
What happens if you sum the probabilities of being in any state? What does this imply for state 0?
They sum to 1 because these are the only possibilities. E0(A) = 1 because if you have no calls in the system and a call comes in, the probability of blocking is 100%
