# polaris.umuc.eduitsTLMNtlmn645tlmn645cl3fa02.ppt

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```					   TLMN 645
Class 3, Fall 2002
Wireless Infrastructure
Quality of Service
•   1.
•   2.
•   3.
•   4.
•   5.
Analog vs. Digital - Signals
Analog

“noise”
Digital

Digital “Cliff” Effect
Quality
Analog

Distance from Transmitter
Multiple Access

2   3   ….   N
1
Number of channels needed?

How large should
N be?

2    3     ….      N
1
Erlang Definition
• Used for voice traffic.
• Represents the amount of traffic intensity
carried by a channel completely occupied
• Example: a channel occupied for thirty
minutes (30 minutes) during an hour is
passing .5 erlangs of traffic
TRAFFIC AMOUNT
For example, if a group of user made 30 calls in one hour,
and each call had an average call duration of 5 minutes, then
the number of Erlangs this represents is worked out as follows:
Minutes of traffic in the hour
= number of calls x duration
Minutes of traffic in the hour
= 30 x 5
Minutes of traffic in the hour
= 150
Hours of traffic in the hour
= 150 / 60
Hours of traffic in the hour
= 2.5
Traffic figure = 2.5 Erlangs
Trunking

Cell with N channels out
1
2
…N

We measure traffic intensity A as
A=  tm = Erlangs
where  average arrival rate (calls/hr)
tm =average message length time (hrs.)
Typical Message Lengths
• Phone call?
• Dispatch?
• Data?
Probability of Blocking

P( A, C ) 
 probability of blocking 
AC
C!
C

A
k
k!
k 0

C= number of channels needed
A=total traffic offered in Erlangs
Source:
Rappaport p.49
Blocking Probability
• Blocking probability, number of channels C
and total traffic offered are all interrelated.
• Specify two and the third is uniquely
determined.
Sample Problem
Source:
Rappaport p.49
FDMA

Source:IEC
TDMA

Source:IEC
TDMA

Source:IEC
CDMA
• multiple access
• transmission bandwidth much larger than
information bandwidth
• rf (radio frequency bandwidth) is
determined by a function other than the
information being sent
CDMA

CDMA

Direct Sequence   Frequency Hopping
Slow
Fast
OFDM
Single carrier
approach

Multi carrier
approach
OFDM
Data Multiple Access
• CSMA

• DSMA
Hidden Node
TLMN 645 Paper Description
Fall 2002
• Choose one of the wireless technologies
addressed in the TLMN 645 course.
Describe the technology from both the
implementers (i.e.. What does it do
technically) and user (what does it do for
the user) perspective.
Paper Description Slide 2
• Identify and describe two real case studies
where the technology was implemented.
Explain in detail how the implementations
improved (or failed) the business or
government organizational effectiveness.
• Explain the expected growth trend for the
technology through the year 2005. Include
specific information from the literature.

```
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