1. Mesh-Network VoIP Call Capacity
Scott Travis and Stan Komsky

2. Context and Problem
The prevalence of access points allows for many wireless communication needs, but VoIP suffers when the wireless channel becomes full and QoS cannot be sustained
VoIP technology is quickly becoming mainstream. Also, mesh-networks are being employed in areas where traditional networks would be more difficult to implement, such as large metropolitan areas
On a wireless network, VoIP call capacity is reduced significantly. Over a mesh-network, this capacity should be reduced even further

3. Context and Problem
Mesh networking allows for continuous connections and reconfiguration as data hops from one node to another until the destination is reached. They have advantages of being low cost, having robust coverage, and having the ability to natural scaling.
Mesh Networking uses multi-hop Ad Hoc network which can lower the QoS of VoIP calls.
Our project consists of determining the effect of increasing the number of VoIP calls on a 2 hop mesh network. We will determine the VoIP call capacity on this network and compare it to the call capacity of a traditional wireless network.

4. Work Outline Work Status Preliminary Research Completed
Traditional Test Bed Configuration
Traditional Test Bed Testing
Multi-hop Test Bed Configuration
Multi-hop Test Bed Testing
Result Analysis
Further Multi-hop Testing
To Be Done

5. Router/Wireless Acces Point
Work Done
Traditional Wireless Test Bed
1 hop
VoIP Client 1
VoIP Client 2
Router/Wireless Acces Point
Internet

6. Router/Wireless Access Point
Work Done
Multi-hop Mesh Network Wireless Test Bed
2 hops
VoIP Client 1
Ad-Hoc Laptop Router
Router/Wireless Access Point
VoIP Client 2
Internet

7. Work Done
Completed in both traditional and multi-hop mesh network testbeds:
Connection between machines achieved using SJPhone for Windows
iperf used to monitor connection quality
In the traditional network, iperf monitored Jitter, packet loss, average bandwidth
In the multi-hop mesh network, iperf was only able to reliably test jitter; average bandwidth and packet loss remained the same at 73.6Kb/s and 0%, respectively

8. Work Done
The multi-hop network used a laptop running Linux with two network cards acting as router between the second VoIP client and the wireless access point
Used the Linux IPTables “firewall” to intelligently forward packets between the two VoIP clients
Wifi_A
Wifi_B
IPTables

9. Work Done
Wifi_A
Wifi_B
IPTables
Using this setup, VoIP client 2 connected to the Linux laptop using an ad-hoc network, the Linux laptop connected to the AP using the AP’s wireless network, and VoIP client 1 connected to the AP via a standard Cat-5 cable
This setup also allows for scalability since any other VoIP clients could connect to the Linux laptop using the same ad-hoc network
Also, this setup guarantees the 2 hops we were looking to achieve to test our VoIP call capacity

10. Results Traditional Network Test Bed Measurements 2 3 4 5 6 Noticeable
Calls
Avg. Bandwidth
Avg. Jitter
Avg. Packet Loss 2
73.6K
3.287 ms
0.017% 3
73.5K
5.302 ms
0.009% 4
4.234 ms 0% 5
71.2K
6.037 ms
0.45% 6
68.2K ms
6.8%
Noticeable
QoS Loss
Declared Unusable

11. Results 2 Multi-hop Mesh Network Test Bed Measurements 3 4 5 6
Calls
Avg. Bandwidth
Avg. Jitter 2
73.6K
1.67 ms 3
73.5K
3.71 ms 4
5.89 ms 5
71.2K
10.77 ms 6
68.2K
16.89 ms
Noticeable
QoS Loss
Declared Unusable

12. Avg. Jitter Traditional
Results
Side By Side Comparison
Calls
Avg. Jitter Traditional
Avg. Jitter
Multi-hop 2
3.287 ms
1.67 ms 3
5.302 ms
3.71 ms 4
4.234 ms
5.89 ms 5
6.037 ms
10.77 ms 6 ms
16.89 ms

13. Results
Multi-hop networks, since they travel over multiple wireless hops, lose the QoS needed for a VoIP call at a faster rate
The chances of extra traffic across the path is also harder to calculate since there may be several nodes and wireless hops the VoIP packets traverse
However, since the multi-hop network may have multiple possible paths between any two destinations, using multi-path routing could increase useable bandwidth and available QoS

14. Results Traditional Wireless Network Wireless Mesh-Network Pros: Cons:
Ease of setup
Easier to provide QoS over a single known wireless hop
Cons:
Scalability
Reliability (if the AP goes down, the wireless cannot be used)
Wireless Mesh-Network
Ease of scalability
Reliability (If your AP goes down, there may be others in the area. Or, if an AP along the path goes down, several others may be available to get to your destination)
Original setup may be complicated
Trying to provide QoS across multiple wireless links can quickly degrade call capacity

15. Summary/Lessons Learned
Increasing the number hops increases the rate at which QoS is lost due to errors and delays in wireless transmissions
Multi-hop networks can be easily scaled, but the original setup can be much more complicated than traditional networks
VoIP across a wireless link is something that will likely become more widespread in the coming years. However, current technologies do not allow for an adequate number of VoIP calls in either a traditional or multi-hop wireless setup. More research is needed in this area on what exactly is causing the problems and how and where to address these issues.

16. Future Work
Testing could be expanding by comparing QoS with the number of hops in the network, we could also increase the bandwidth per call to see if that changes the results
Our multi-hop testbed is only one of several ways to setup a multi-hop network; testing different setups could show if there are differences in available QoS as each setup increases the number of hops