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Topic 5 Introduction of Network Engineering. Two fundamental issues. In network there are many ways to and from a node. So need the switch in the node for selecting the output way go to destination.
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Two fundamental issues • In network there are many ways to and from a node. So need the switch in the node for selecting the output way go to destination. • A lot people in network need the protocols to access, queuing, routing for sharing common system with efficiency.
Switching • There are two kind of Switching (or exchange): circuited switching and packets switching. The circuited switching maintains continuously a path for a dedicated call. The packet switching serves different calls in a path by mean of packets (no focus in this subject) • By hierarchy there are local switches, tandem switches and transit switches. In a local area the calls traverse no more than one switch. Other calls destined for subscribers outside local area traverse a tandem or transit switches to another local switch.
Junction network • The overflow traffic is routed via the junction tandem
National network • long distance or trunk network
Cross-point Switch • Cross-point is a matrix of switch. An electrical contact is made by a horizontal and vertical relay • n>m: Concentrator, n<m: Expander.
Step by step switch • In general, 1000 subscribers need switching matrix 1000 x 1000 =1000000 cross points • Using concentrator and expander need 2 x 10 x (100 x 10) + 100 x 100 = 30000 cross points
Common control switch • Construction by common control : 10(100x10)+ 10(10x10)+ 10(10x100)= 21000
Construction of 3 stage-switch • Number cross points: Nx=2kN+k(N/n)2 • More than one path between every input and output line
Design for non-blocking switch • In worse case n-1 input and n-1 output are occupied, for non blocking k ≥ n-1+n-1+1 • Replace to formula • Differentiating Nx respect to n for minimum • For example, N=1000, Nxopt=184000 whereas a single-stage switch require 1000000 cross-point
Time Division switching • Unlike switch for analog this inter- connect (permute) for each time slot period
Traffic intensity • Product of calling rate and holding time is traffic of call (measure by Erlan) • Example: C=3 time/hour, T=10 minut/time Traffic A=3.10 minut/60minut=0.5 Erlan • Carried traffic, offered traffic, lost calls (minus of them) • A call is lost because it meet congestion or blockage at switch. • To dimension (or size) a telephone exchange, have to know the traffic intensity of busy time.
Grade of Service (GOS) • Three handling of exchange for a loss call: • Lost calls held (LCH, immediately redial, North America) • Lost calls cleared (LCC, hang up and wait some time interval before redial, redial is new call, Europe) • Lost calls delayed (LCD, put the loss call in queue)
Poisson traffic model • The Poisson formula is used to predict the pro-bability that a call will be blocked (North America) • where: P=Poisson loss probability N=Number of trunks in full availability group A=Traffic offered to group in Erlangs
Example 1 • Design a remote PABX connected by a tieline that will be used for all inbound calls to that PABX which will have 780 active ends. Known 30mE of inbound traffic per active end, and GOS should be better than 0.002. How many trunks do need in the tie line route? • Answer: to carry 23.4E (780*0.03), I need 39 trunks, and the actual grade of service should be 0.00196.
Example 2 • A Service need less than 5% of customer calls will get a busy signal. • But there are 2 x E1 trunk route (60 channels) on call centre carrying 51.806E. Should the incoming route be sufficient to meet that Service • Answer: No, busy rate should be about 14.3%.
Erlang B traffic model • The Erlang B formula is used to predict the probability that a call will be blocked • where: B=Erlang B loss probability N=Number of trunks in full availability group A=Traffic offered to group in Erlangs
Example 1 • Design a remote PABX connected by a tieline that will be used for all inbound calls to that PABX which will have 780 active ends. Known 30mE of inbound traffic per active end, and GOS should be better than 0.002. How many trunks do I need in the tie line route? • Answer: to carry 23.4E (780*0.03), I need 38 trunks, and the actual grade of service should be 0.0014.
Example 2 • A Service need less than 5% of customer calls will get a busy signal. But there are 2 x E1 trunk route (60 channels) on my call centre carrying 51.806E. Should the incoming route be sufficient to meet my Service Level Objective. • Answer: Yes, busy rate should be about 3.1%.
Erlang C queue model • The Erlang C formula is used to predict the probability that a call will be delayed, and can be used to predict the probability that a call will be delayed more than a certain time.(FIFO system) • where: P(>0)=Probability of delay greater than zero N=Number of servers in full availability group A=Traffic offered to group in Erlangs
Summary • In Network there are circuited switching and packet switching. The packet switching need a buffer in addition • A size of circuited switching (number of trunks) based on Erlan formula, which relating to offered traffic and grade of service. The local switch have to be a non-blocking switch in local area. • Multi access (share system) to exchange depend on the method of handling lost calls. There are three handlings concerning to Erlan formulas • Multiplexing (share a link) can carried out in the time domain. There is the switch mixing time and space.
