Trunking Theory and Grade of Service

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Questions and Answers

What is the impact of micro cell zoning on channel assignment?

Channels must be reused across the entire cell.

Channels are assigned to specific base stations only.

Any base station channel may be assigned to any zone. (correct)

Handoff is required when moving from zone to zone.

What does achieving a larger frequency reuse factor indicate?

Reduced number of cells in a cluster.

Increased capacity of the system. (correct)

Increased co-channel interference.

Lower trunking efficiency.

Which advantage is NOT associated with micro cell zones?

No requirement for handoff.

Reduction of interference.

Increased control base station size. (correct)

Active channels are localized.

What is the primary function of repeaters in a communication system?

To extend coverage range. Signup and view all the answers

How does sectoring affect the signal-to-interference ratio (S/I)?

It improves the S/I ratio to 7.2 dB. Signup and view all the answers

What happens to blocked calls in the Erlang C model?

They wait until channels are available. Signup and view all the answers

Which method involves dividing cells into smaller units to increase capacity?

Cell splitting Signup and view all the answers

What is one effect of reducing a cell radius from R to R/2 in cell splitting?

Power reduction increases Signup and view all the answers

Which term refers to the probability of calls being delayed longer than a specific time in the Erlang C model?

Pr(delay > 0) Signup and view all the answers

What is the main benefit of cell sectoring?

It improves coverage by using directional antennas. Signup and view all the answers

In Erlang C, what is the average delay D expressed as?

$Pr[delay>0]H/(C-A)$ Signup and view all the answers

What is a significant consequence of implementing microcells?

Reduced transmission power Signup and view all the answers

What is the purpose of frequency borrowing in cellular networks?

To allow congested cells to take frequencies from adjacent cells. Signup and view all the answers

What does trunking refer to in wireless communications?

The channel is allocated on demand and recycled after usage Signup and view all the answers

What is meant by Grade of Service (GOS) in trunking theory?

The likelihood of a call being blocked or delayed Signup and view all the answers

How is traffic intensity represented in trunking theory?

As the product of call requests and call duration Signup and view all the answers

In trunked systems with blocked calls cleared, what characterizes the arrival of calls?

Calls follow a random arrival rate based on Poisson distribution Signup and view all the answers

What does the Erlang B formula calculate?

Probability of call blocking in a trunked system Signup and view all the answers

Which equation represents traffic intensity for a user in terms of call requests?

$A_u = \lambda H$ Signup and view all the answers

What happens when offered traffic exceeds the capacity of a trunked system?

Some calls will be blocked or rejected Signup and view all the answers

In the context of trunking, what does 'memory-less' refer to in an M/M/m/m system?

System does not retain information about past calls Signup and view all the answers

Study Notes

Trunking and Grade of Service

Trunking is a method of allocating channels on demand, and then recycling those channels after usage.
There is a trade-off between the number of channels and the probability of a call being blocked.
The grade of service represents the likelihood of a call being blocked or encountering a delay exceeding a threshold during peak usage periods.
The Erlang capacity refers to the percentage of lines or channels utilized over time.

Terms of Trunking Theory

Traffic Intensity: represents the amount of traffic generated by a user and is measured in Erlangs.
Offered Traffic: represents the total traffic intensity, encompassing all users.
Carried Traffic: represents the traffic intensity handled by the trunked system.

Trunking Theory

Each user contributes a traffic intensity of Au Erlangs, where Au = H * λ, H represents the average call duration and λ represents the average call request rate per unit time.
The total offered traffic intensity is calculated as A = U * Au, where U is the number of users.
The traffic intensity per channel is calculated as Ac = U * Au / C, where C is the number of channels.
The first type of trunked system is characterized by blocking calls: calls are immediately rejected if channels are unavailable, and no queuing or setup time is involved. This system can be modeled using the M/M/m/m queuing model, which assumes memory-less, Poisson arrivals, exponential service times, and finite channels.

Erlang B Formula

The Erlang B formula calculates the probability of blocking for the blocked calls cleared system.
The formula is as follows: Pr(blocking) = (A^C / C! ) / (∑_(k=0)^C (A^k/k!))

Erlang B Example

The text provides an example where the offered traffic intensity (A) is 10 Erl and the number of channels (C) is 10. The blocking probability is 0.08%.

Erlang C Formula

The Erlang C formula calculates the probability of a delay exceeding a threshold for blocked calls delayed systems, which means calls are placed in a queue until channels become available.
The formula is as follows: Pr(delay>0) = (A^C / (C-1)! ) / (A/C! * ∑_(k=0)^C (A^k/k!))

Examples

The text includes several examples demonstrating the application of the Erlang B and Erlang C formulas in different scenarios.

Approaches to Increasing Capacity

Several techniques enhance the capacity of a cellular network.
One technique is frequency borrowing, where congested cells can borrow frequencies from adjacent cells.
Cell splitting involves dividing a crowded cell into smaller cells, each equipped with its own base station.
Cell sectoring divides cells into sectors, each equipped with directional antennas.
Microcells involve deploying antennas on buildings, hills, and lamp posts.

Cell Splitting

Provides capacity enhancement by rescaling the system.
Reducing the cell radius from R to R/2 implies power reduction by 12 dB.

Sectoring

Replacing omni-directional antennas with directional antennas enhances system capacity by reducing co-channel interference.
Sectoring allows for fewer cells in a cluster, resulting in a larger frequency reuse factor and increased capacity.

Repeater

Repeater is a device that can extend the coverage range; it can be used with both directional antennas and distributed antenna systems.

Micro Cell Zone

Allows for flexible channel assignments, eliminating the need for handoffs and reducing interference.

Homework

The homework assignment involves answering various problems, some of which may be challenging.

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Description

This quiz covers the principles of trunking theory and its implications on the grade of service in communication systems. It explores key terms such as traffic intensity, offered traffic, and carried traffic, as well as the Erlang capacity. Test your understanding of how channels are allocated and the trade-offs involved during peak usage times.