移动网络流量工程 Traffic Engineering

z.B the input circuits != output circuits -> trunking effect
input1: 1E@95% -> 3Circuits x20
Probability of Blocking
output2: 20E@95% -> 25Circuits
Tr a
ffic
• the lost traffic Alost = A ⇥ p • the output traffic Aout = A
Alost = A ⇥ (1
p)
• For instance1
Req. traffic A 0.9 E 0.9 E 0.9 E 0.9 E 0.9 E
1 By using the online Erlang B calculator at
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Quality / Grade of Service
In telecommunication engineering, the quality of a carrier service is specified by two measures: the grade of service (GoS) and the quality of service (QoS), as per ITU-T Recommendation E.600/E.800.
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Circuit- vs. Packet-switched Eng.
• Packet-switched network. • available flow is limited by total, aggregated bandwidth
• Relevant question: how to avoid congestion ?
(and, therefore, processed later)
• The Erlang C Formula gives the probability that a resource reservation will
be delayed (but no blocked):
Probability of call delayed
/calculator/erlb/
Ressources m 1 2 3 4 5
drop prob. p 0.47 0.17 0.05 0.01 0.002
Alost 0.43 0.16 0.05 0.01 0.00
Aout 0.47 0.74 0.85 0.89 0.90
• Trunking efficiency, also called channel utilisation efficiency, is defined as:
E ciency =
A ⇥ 100 [%] m
• in practice, because Erlang’s hypotheses are not met in reality, efficiency is
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Extension : the Erlang C Formula
• In most cases, an access to a resources will not fail completely
• instead the reservation will be delayed • the caller is supposed to wait in an infinite queue for a limited amount of time
(Prim, Kruskall, Dijkstra)
• TelCo’s are vitally interested in traffic intensity
• dictates the amount of equipment they must supply • provides insight on achievable GoS (grade of service) • is a base to contracts and other legal/billable aspects
• Circuit-switched network. • circuit establishment is limited by total allocable circuits
• Relevant question: how to avoid call dropping ?
reserved dedicated resource z.B. voice communciation
•
is the average number of call attempts per unit time (units: [s
1
])
• H is the holding time, i.e., the average call duration (units [s]). • µ = 1/H can be interpreted as the the call release rate.
• Each application has a different QoS requirement (level of service) • Traffic classification of CoS may also depend on user agreement, operator
strategy, etc.
Class of Service
• Different telecommunications applications require different Qualities of
Service
• Voice over IP, full quality voice, GSM quality voice, etc. • IPTV, video on demand, etc. • Internet (best effort)
• The cellular system is designed for a GOS of 2%, i.e., 2 out of 100 calls will
be blocked due to channel occupancy during the busiest hour.
• Most systems (telephone, VoD) are designed for 0.5%
• Units are Erlang [E] which is adimensional
Basic algebra can be carried out with Erlang as u is releasing rate: thus if u is larger than \lamda -> could be problem
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Trunking efficiency
• The Poisson distributed arrivals (and departure) has the effect of achieving
better efficiency as the number of channels is increased.
The performance which a single circuit is designed or conditioned to provide, in terms of bandwidth, delay, or jitter.
Distinguish between GoS and QoS
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Traffic Intensity
1 A, T
Z
T 0
n ( t ) dt
• n(t ) is the instantaneous sessions at time t • A = H accounts for average occupancy during a specified period of time
limited by designers to 0.8 < Eff < 0.9.
when the Quantity of the network is large: z.B Q>100~200 then the efficiency plays the main role z.B. 500 communication -> 500/0.9 = lines
Grade of service
Probability of a call in a circuit group being blocked or delayed for more than a specified interval, expressed as a decimal fraction.
Quality of service
• Exploit the statistical behavior of users. • Each user is allocated a channel on a per call basis, and upon termination
of the call, the previously occupied channel is immediately returned to the pool of available channels.
1% GoS.
- nonlinear - strongly depending on available resource - normally, known A and GoS(target)(the prob) -> find m
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The Erlang B Formula
the upper bound of A: <- number of circuits
there is always over-head of the users z.B. 10 users -> >10 lines(res)