Designing a Voice over IP Network
Chapter 9
Introduction
The design of any network involves striking a balance between three requirements.
Meeting the capacity needed to handle the projected demand (capacity)
Minimizing the capital and operational cost of the network (cost)
Ensuring high network reliability and availability (quality)
What is acceptable degree?
The Overall Approach
Understanding the expected traffic demand
Where traffic will come from and go to
What typical per-subscriber usage is expected
Establishing network design criteria
Build-ahead, voice-coding schemes, network technology…
Vendor and product selection
Network topology, connectivity and bandwidth requirements
Physical connectivity
Design Criteria [1/2]
Build-Ahead or Capacity Buffer
Avoiding the necessity for constant redesigning as traffic demand increases
Providing a buffer in case traffic demand increases faster than expected
Fundamental Technology Assumptions
H.323 vs. Softswitch
MGCP vs. MEGACO
Network-Level Redundancy
Design Criteria [2/2]
Voice Coder/Decoder (Codec) Selection Issues
Actual coder/decoder to use
Packetization interval
Silence suppression
Blocking Probability
A call will be blocked due to a lack of available channels.
QoS Protocol Considerations and Layer 2
Protocol Choices
Product and Vendor Selection
Generic VoIP Product Requirements
Node-Level Redundancy
Node Availability
Alarms and Statistics
Element Management
Traffic Forecasts
Voice Usage Forecast
(MoUs per subscriber per month) x (fraction
during work days) x (percentage in busy hour) / (work days per month)
E.g., 120x0.6x0.2/21=0.686 MoU/sub/busy hour
0.686/60=0.0114 Erlangs/sub/busy hour
Busy-hour call attempt (BHCA)
=Erlangs/MHT (average call length)
=0.0114x3600/300=0.137
A subscriber with 120 MoUs per month will make 0.137 calls each busy hour.
Traffic Distribution Forecast
Network Topology
How many network elements of a given type will be in each location
The bandwidth requirements between those
network elements and the outside world
MG Locations and PSTN Trunk Dimensioning
At least 1 MG in each of the 12 cities where the service is provided
To determine the size of the trunk groups to the PSTN
From Voice Usage Forecast, we know how much traffic we will send.
From Traffic Distribution Forecast, we know how
much traffic we will receive.
MGC Quantities and Placement
Assume that BHCA is the limiting factor.
A call passes between two MGs controlled
By the same MGC
By different MGCs
Determining the number and location of MGCs can be an iterative process.
1. An initial estimate of the number of MGCs
2. To allocate MGs to MGCs
3. To determine the total BHCA to be supported by each MGC
Calculating VoIP Bandwidth Requirements
The bandwidth required between MGs for VoIP traffic
The bandwidth required for a single call depends on the following factors.
Voice-coding scheme
Packetization interval
The use of silence suppression
Probability of excessive packet collision
Packet will be lost or delayed as a result of too many speakers talking at one time.
Peak in the Number of Simultaneous Speakers
Consider n speakers. If voice activity is 40 percent, then the probability of an individual user speaking at a given instant is 40 percent.
The probability that exactly x subscribers are speaking at a given time
Pa(x) = (n,x) px(1-p)n-x, where p=0.4
The probability that there are no more than x speakers at a time
Pb(x) = Pa(0)+Pa(1)+…+Pa(x)
To determine the value of x
Bandwidth Requirement
VoIP Bandwidth
Voice packet size + 40 octets (for IP, UDP and RTP) + WAN layer 2 overhead + MPLS overhead (if
applicable)
RTCP bandwidth should be limited to about 5% of the actual VoIP bandwidth.
Signaling and OA&M Bandwidth
Between MGC and MG
Between MGC and SG
Physical Connectivity
To determine how we will connect the different cities to provide the
bandwidth we need
Each city has an
alternative path to every other city to ensure the network does not fail.
