1
Telephone Losses
The common telephone losses that is normally due to the following:
Net Loss
The ratio of the signal power at the input and output of the channel.
Insertion Loss
Ratio of the power delivered from a source to a load, to the power from that same source to that same load through a transducer. It is the loss
caused by the insertion of a gain element to a transmission channel.
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Telephone Losses
Return Loss
A measure of the match between two
impedances on either side of a junction point. Return Loss Formula (dB value)
where: Z1 and Z2 = complex impedances of the two halves of the circuit (Ω)
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Telephone Losses
Echo Return Loss (ERL)
The weighted power-average return loss at the reflection point covering the band of approximately 500 to 2500 Hz. It is the ratio of the power of a broadband incident signal to that of the correspondingly broadband reflected signal. Singing Return Loss
The as echo return loss, but over a considerably narrow band near an edge of the voice band, e.g. 200 to 500 Hz or 2500 to 3200 Hz.
Singing
It is the result of sustained oscillations due to positive feedback in telephone amplifiers or amplifying circuits.
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Telephone Losses
Other Losses:
Crosstalk
Unwanted interference picked-up by one
channel of an electronic communication system from the other channel.
Far-End
Measured on a channel at a receiving point near the sending point of the interfering channel.
Near-End
Measured on a channel at a receiving point near the receiving point of the interfering
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Telephone Losses
Other Losses:
Telephone Echo
Maybe classified as talker or listener echo.
Talker Echo
A signal returned to the talker after making one or more round trip between the talker and a
distant reflection point.
Listener Echo
A signal first returned down to the talker at a distant reflection point and then reflected again towards the listener.
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Telephone Losses
Other Losses:
Propagation Time
CCITT recommends the following limitations on mean, one-way propagation time when echo
source exist.
0 to 150 ms acceptable
150 to 400 ms, acceptable, provided that care is
exercised connection when the mean, one-way propagation time exceed about 300 ms
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Telephone Losses
Other Losses:
Propagation Time Delay
Due to the great distances involved between parties at the opposite end of the local loop.
For Analog Network:
For Digital Network:
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Telephone Losses
Propagation Time
Example:
Calculate the one-way propagation delay for an analog network and digital network which spans 5 km and 8 km respectively.
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Telephone Losses
Other Losses:
VIA NET LOSS (VNL)
A concept of transmission planning that permits a
relatively close approach to an overall zero transmission loss in the telephone network and maintains singing and echo
within specified and tolerable limits.
In short, VNL is a loss to be introduced to avoid “singing” phenomenon
Where:
L = One-way length of the trunk
Vp = Velocity of propagation Δt = One-way propagation
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Telephone Losses
Other Losses:
VIA NET LOSS FACTOR (VNLF) in dB/km
Overall Connection Loss (OCL)
Fixed-Loss Plan
It is based on a 0-dB loss on all-digital intertoll trunks. It specifies a 6-dB trunk loss betweem local exchanges
regardless of the connection mileage. Where:
D = path delay (ms)
Vp = Velocity of propagation N = number of trunks in
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Telephone Losses
Via Net Loss
Example:
A telephone signal takes 11.8 ms to reach its destination.
a. Calculate the via net loss required for an acceptable amount of echo.
b. Find VNLF if the distance covered is 20 miles. c. Find the OCL if there are 12 trunks in tandem.
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Telephone
Traffic
1. How many lines / switches do I need?
Why can’t I get through sometimes?
2. What happens to the call?
Erlang’s formula Blocked? Delayed? 3. Demand 4. Dimensioning 5. Grade of Service
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Traffic Engineering
it is the application of knowledge of statistics including queuing theory, the nature of traffic, their practical models, their measurements and simulations to make predictions and to plan
telecommunication networks.
TERMINOLOGIES
Concentration
The function associated with a switching network having fewer outlets than inlet terminals
Coordinate Switch
A rectangular array of cross-points in which one side of the crosspoint is multiplied in rows and the other side in columns
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Traffic Engineering
TERMINOLOGIES
Crosspoint
A two-state switching device containing one or more elements that a low transmission impedance in one state and a very high one in the other
Full availability
Property of a switch or switching network capable of providing a path from every inlet terminal to every outlet terminal
Internal Blocking
The inability to interconnect an idle inlet to an outlet
because all possible paths between them are already in use Number of crosspoint required = N(N-1)/2
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Traffic Engineering
TERMINOLOGIES
Busy Hour
The continuous one-hour period that, on consecutive days in the busy part of the year, contains the maximum average traffic intensity
Call
A discrete engagement or occupation of a traffic path Calling Rate
The average number of calls placed during the busy hour Occupancy
The traffic intensity per traffic path. One hundred percent occupancy implies all paths busy.
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Traffic Engineering
TERMINOLOGIES
Traffic Concentration
the average ratio of the traffic quantity during the busy hour to the traffic quantity during the day
Traffic Intensity/Traffic Load
the average number of calls present on a group of traffic paths over a period of time
Traffic Path
A channel, time slot, frequency band, line, trunk, switch, or circuit over which individual communications pass in
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Traffic Engineering
TERMINOLOGIES
Traffic Quantity
the aggregate engagement time or occupancy time of one or more traffic paths
Call Intensity
For many traffic-carrying elements, the number of calls making up the total traffic load is immaterial; the load
represented by two calls or ten minutes duration has the same impact as one call of twenty minutes duration
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Traffic Engineering
A systematic means of handling voice traffic in a telephone network.
Measurement of Telephone Traffic:
T
C
A
Where:
A = Traffic intensity in Erlang
C = Number of calls during a period of 1 hr.
T = Average holding time per hour, or per minute
S = Sum of all the holding time (min)
t = Observation period (1 hr or 60 min.)
t S
19 Unit of Traffic Intensity
The Erlang Unit
The international dimensionless unit for traffic intensity.
One Erlang is the traffic represented by an average of one circuit busy out of a group of
circuits over some period of time usually one hour or 60 minutes.
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Traffic Engineering
Other Units of Traffic Intensity
Call-Second (Cs) /Call-Minute (Cm) /Call-Hour (Ch) Units of traffic quantity representing occupation of a
circuit for a second, minute or hour. Century Call-Seconds (CCS)
Unit of traffic intensity equal to 1/36 of an Erlang. Also known as Hundred Call-seconds
Equated Busy-Hour Call (EBHC)
European unit of traffic intensity equal to 1/36 of an
Erlang. 1 EBHC is the average intensity in one or more traffic paths occupied in the busy-hour by one 2-min call or an
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Traffic Engineering
Other Units of Traffic Intensity
Traffic Unit (TU)
1 TU is the average intensity in one or more traffic paths carrying an aggregate traffic of 1 call-hour in 1 hour (the
busy hour unless otherwise specified) Relation between different traffic units
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Traffic Engineering
1 Erlang is equivalent to traffic intensity that keeps:
1 circuit busy 100% of the time, or 2 circuits busy 50% of the time, or 4 circuits busy 25% of the time, etc.
While 26 Erlangs is equivalent to traffic intensity that keeps:
26 circuits busy 100% of the time, or 52 circuits busy 50% of the time, or 104 circuits busy 25% of the time, etc.
1 ccs is volume of traffic equal to:
1 circuit busy for 100 seconds, or 2 circuits busy for 50 seconds, or 50 circuits busy for 2 seconds, or 100 circuits busy for 1 second, etc.
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Traffic Engineering
Call Rate (C)The number of times a route or traffic path is used per unit period, or, more properly defined, “the call intensity per traffic path during the busy hour”
Average number of calls initiated per unit time (e.g. attempts per hour)
also known as Arrival Rate
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Traffic Engineering
Traffic Volume (usually in ccs)V = n x T
where: V = volume of calls in time t n = # of calls in time period T T = mean holding time per call
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Traffic Engineering
Traffic Intensity (A)Traffic intensity or traffic flow is a total traffic volume divided by the duration of time.
I. For a single terminal
The traffic in Erlang is the average occupancy of the terminal while the traffic intensiuty or traffic flow is just the percentage of time the terminal is busy
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Traffic Engineering
Traffic Intensity (A)
II. For a group of circuits or terminal
a. The average number of circuits simultaneously busy within
a group.
b. The expected number of call arrivals per unit holding time. c. The number of circuits required to completely carry the
offered traffic if each circuit were operating at 100% occupancy.
Where: A = traffic intensity in Erlang C = calling rate
T = mean time holding time per call V = volume of calls n = # of calls in time period t
t = time period of observations µ = departure rate
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Exercises
Problem
Supposed that the average holding time is 2.5 min and the calling rate in a busy hour for a
particular day is 237. What is the value of traffic intensity in Cm and Ch?
Problem
If the total traffic handled by a telephone
exchange is equivalent to 7200 ccs. What is the total traffic handled in Erlangs?
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Exercises
Problem
Call established at 2 am between a central computer and a data terminal. Assuming a
continuous connection and data transferred at 34 kbps, what is the traffic if the call is terminated at 2:45 am?
Problem
A group of 20 subscribers generate 50 calls
with an average holding time of 3 minutes, what is the average traffic per subscriber?
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Traffic Engineering
Offered Traffic (To)Offered traffic is the traffic intensity that would occur if all traffic submitted to a group of circuits
could be processed.
The volume offered to a switch
Carried Traffic (Tc)
Carried traffic is the traffic intensity actually handled by the group of circuits.
The volume of traffic actually carried by a switch.
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Traffic Engineering
Lost Traffic (TL)Blocked traffic is that portion that cannot be processed by the group of circuits
The difference between the offered and carreid traffic.
Relation between To, Tc and TL
To = Tc + TL
TL = To x P(B)
TC = To x (1 - P(B))
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34 Blocked Calls and Lost Calls
Blocked Calls
Call attempts that were not able to progress because of the unavailability of circuit/s.
Lost Calls
Calls that had been blocked and were
terminated (removed from the queue) by the switch.
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Grade of Service (GoS) or Blocking Probability
The probability of meeting blockage during the busy hour of traffic.
The typical GoS is 0.01, meaning, an average of one call in 100 will be blocked or lost during a busy hour.
The ratio of the number of lost calls to the number of calls offered.
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Exercises
Problem
Calculate the trunk efficiency for a group of 26 trunks that offers 10 Erlangs of traffic and a
blocking probability of 0.2%.
Problem
If we know that there are 354 seizures (lines connected for service) and 6 blocked calls (lost
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Blocking Probability Models
The Erlang B
Blocked Calls Cleared or Lost Calls Cleared
The Erlang C
Blocked Calls Delayed or Lost Calls Delayed
The Poisson Model
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Lost Calls Cleared (Blocked Calls Cleared)
The LCC concept, which is use primarily in Europe or those countries accepting European practice, assumes that the user will hang up and wait some time interval before reattempting if the use hears the congestion signal on the first attempt. Such calls, it is assumed, disappear from the system.
The assumption that calls not immediately satisfied at the first attempt are cleared from the system and do not reappear during the period under consideration. Used in the Erlang B-Loss probability equation.
Where:
A = traffic intensity in Erlang N = # of trunks
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Exercises
Problem
Suppose we use 5 trunks, and the route offered 1.66 Erlangs of traffic. Calculate the grade of service required to implement this configuration.
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Lost Calls Delayed (Blocked Calls Wait)
The LCD concept assumes thatr the user is
automatically put in queue (a waiting line or pool). For
example, this is done when the operator is dialed. It is also done on most modern computer-controlled switching
systems, generally referred ti under the blanket term stored program control (SPC).
The assumption that calls not immediately satisfied at the first attempt are held in the system. Used in the Erlang C delay-probability equation.
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Lost Calls Held (Blocked Calls Held)
The LCH concept, which is the principal traffic formula used in North America, assumes that the telephone user will immediately reattempt the call on receipt of a congestion signal and will continue to redial. This concept further
assumes that such lost calls extend the average holding time is zero, and all the time is waiting time.
The assumption that calls not immediately satisfied at the first attempt are held in the system until served or
abandoned. Used in the Poisson (Molina) loss probability equation.
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Congestion
1. Time Congestion
Proportion of time a system is congested (all servers busy)
Probability of blocking from point of view of servers
2. Call Congestion
probability that arriving call is blocked
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%Overflow or Blocking
The measure of the rate at which subscriber’s call attempt failed or blocked.
%Overflow= ______# of calls rejected________ # of calls accepted+ # of calls rejected Occupancy or Utilization
The traffic intensity per traffic path. %Occupancy= ____Total traffic____ Total number of lines
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Offered traffic
The equivalent traffic offered to a group of switch.
Offered traffic= ____carried traffic______
(1 - %Overflow)
Carried traffic
The equivalent traffic carried by a group of trunk lines.
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Assignment
Problem
During the busy hour, the average calling rate per subscriber of a 2000 line telephone
exchange is 0.80. The holding time per call is 5 minutes. What is the total traffic handled by the telephone exchange in erlang during the busy hour?
Problem
On particular traffic relation, the calling rate is 461 (calls in a 1-hr period) and the average
call duration is 1.5 minutes during the busy hour. What is the traffic intensity in Erlangs? In ccs?
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Assignment
Problem
Company X has 10 employees, each placing an average of 20 minutes of loing-distance calls per day. The average call lasts 5 minutes. It has been determined that 20% of the calls are made during the busy hour. A total of 4 external phone lines are used to place the pool of calls. Calculate the traffic intensity in Erlang, during the busy
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Assignment
Problem
Suppose 100 data terminals are used to be connected to a computer by way of leased
circuits:
1st plan: the terminals are clustered into four
groups that use separate groups of shared circuits
2nd plan: Traffic from all terminals is
concentrated into one group of circuits
Determine the cluster traffic in both cases assuming each terminal is active 10% of the time.
