Designing City Wide Voice over IP over Wi-Fi Cellular Networks

(Read Why Structured Mesh Networks out perform other mesh architectures)

City-wide Structured Mesh^TM for Carrier Class VoIP

   

Introduction

In the coming years, the driving force for city-wide wireless mesh will be VoIP as both Internet Service Providers and progressive Cellular Carriers seek to supplement existing cellular network services. 

Cellular currently has superior coverage and roaming today but users will opt to lighten the burden of their cellular bill by using VoIP whenever convenient. Cellular carriers will begin to offer city-wide VoIP solutions in order to re-capture business being siphoned away from cellular by VoIP over WiFi. 

To ensure even distribution of bandwidth over large areas requires a mesh networking approach - but all mesh networks are not created equal.  As we explain below VoIP over WiFi when serviced by a 3-radio Structured Mesh backhaul offers cost advantage of 100x over cellular. Other Mesh solutions cannot.

The Challenge of VoIP over WiFi

Moving packets through a WiFi network uses significant overhead. Combine this with the relatively small packets for VoIP that must be sent at regular intervals, (e.g. every 20 ms)  and the overall system becomes relatively inefficient – regardless of what network architecture is employed. 

With 64Kbps VOIP traffic, the frame size is 160 bytes.  Unfortunately, there are 40 bytes of IP overhead and 30 bytes for Mac layer overhead, for a total of 230 bytes per frame. Compare this with the typical data packet (2300 bytes per frame) and you get a sense of what’s happening. If there were no contention at each hop, the 802.11a backhaul could only carry a maximum of 11,162Kb as opposed to the theoretical maximum of 24.2Kb that is possible for data (effective TCP rate). As contention is added in the backhaul, this rate is reduced - and much more so with conventional mesh than with Structured Mesh.

Deploying VoIP for 5,000 Subscribers per Square Mile

Let’s look at a wireless mesh solution to cover a number of square miles of city. For this analysis, let’s assume initially that only VoIP conversations are being supported. Of course, when data transfers are mixed into the analysis, the number of simultaneous VoIP conversations may be reduced.

Let’s assume that there are 5,000 subscribers per square mile, and that during the peak hour of the day, each makes one call of 5 min duration. This produces a peak traffic level of 0.083 Erlangs per subscriber, resulting in an average of 416 simultaneous conversations per square mile during the peak hour.

With mesh nodes deployed on a ¼ mile grid, there will be 25 nodes covering the first square mile. As the number of square miles increases, this number converges on an average of 16 per square mile. The 802.11b service radios on each node can support 29 simultaneous conversations with a G.711 64Kb voice codec and 60 conversations with a G.729 8Kb codec. 

Can Structured Mesh Deliver?

The question is: can the 802.11a Structured Mesh backhaul support the required packet transfer rate?  
The current 3-radio Structured Mesh module  supports up to 695 conversations per Internet connection. 

If we assume one DS3 connection per square mile and 64Kbps VOIP calls, then the 3-Radio module supports 416 simultaneous conversations needed for the 5,000 subscribers and in fact, leaves additional bandwidth available for significant data traffic. If the G.729 codec were used (8Kbps), 3-radio modules support up to 960 simultaneous conversations per square mile - essentially supporting a subscriber density of 10,000 per square mile given the same usage parameters. Future upgrades for the 3-radio Structured Mesh module will increase these numbers by around 4X.

MeshDynamics 3-Radio Structured Mesh module supports heavy and concurrent VOIP traffic in the same manner as most cellular phone networks.

Cost Comparison vs. Cellular for the 3-Radio Module

These subscriber densities are certainly high given that VoIP deployments over WiFi are few and far between today. Instead of supporting one square mile per DS3 connection, an initial deployment may allocate one DS3 per 4 square miles instead. 

Based on these assumptions, the total cost/user for 3 years, including CapEx and OpEx for the current 3-radio module is $241 using the G.711 codec (64 Kbps) and $117 with the G.729 codec (8Kbps). 

For the Structured Mesh deployment scenario above, the overall cost-per-minute-per-user, including the cost for DS3 Internet connections, as well as installation, service, and maintenance costs, is .027 cents using the G.711 (64Kb) codec. If this is compared to the overall cost for cellular at 5 cents, the mesh solution beats cellular by 200x. 

Even if cellular are reduced to 2.5 cents, 3-Radio Structured Mesh is still 100 times more cost effective.

These numbers do not include the termination costs that are required to place a VoIP call to a conventional phone. Assuming all calls required termination charges and that these charges were 0.5 cents per minute, the advantage for citywide mesh VoIP is still 10x over cellular if cellular costs are 5 cents/min/user. 
  
Clearly, in spite of obvious limitations relative to cellular, neither cellular Carriers nor ISPs can afford to disregard the ability of VoIP over WiFi to offer a practical supplement to cellular minutes. With certainty, VoIP over WiFi will be used by most cellular users as VoIP phones and dual-mode phones proliferate.

VOIP performance on competing mesh products

With 29 users per hop, even the best of the conventional mesh architectures supplies only .0007 of the root bandwidth to a client 3 hops away. Since the maximum bandwidth for VoIP on any 802.11a backhaul is 11,162Kb (64Kb codec), a conventional mesh (1+1) would have only 7.8Kb available to the farthest clients. To support the required 64Kb VoIP rate, a conventional mesh could only have 4 clients connecting at 3 hops. There is no choice but to add expensive Internet backhaul (DS3) connections much more often, and run the conventional (1+1) mesh at 1 or 2 hops. 

Even worse results are found with the original 1-radio mesh architecture, still used by even some well known mesh network vendors. Here, only 8 VoIP clients can be supported at 2 hops. In contrast to these capabilities, 3-radio Structured Mesh can support 29 VoIP users per hop at up to 6 hops, allowing initial city-wide installations to be connected to the internet with even fewer expensive DS3 connections.

Wireless mesh networks built with Structured Mesh^TM scale to city wide deployments cost effectively and efficiently. Competing approaches do not because conventional mesh uses an inferior technology for the backhaul path that uses a single mesh radio, causing bandwidth degradation with each hop.

MeshDynamics multi-radio, multi-channel mesh backhaul does not suffer from bandwidth degradation. It forms our unique Structured Mesh - a tree-like structure that resembles a wired switch stack. All channels are dynamically and automatically allocated. This private, high performance backhaul enables us to supply orders of magnitude greater bandwidth than conventional 1-radio meshes. See Performance Analysis 

Structured Mesh^TM  can distribute more bandwidth, more evenly and over more hops (larger areas)

Managing Voice/Data Contention at the source
 

Communications functionality for data and voice behave quite differently. Part of this is the time-sensitive nature of voice and part is the enormous difference in packet efficiency between voice and data. New protocol enhancements such as 802.11e will give priority to voice packets, but the ability of large data packets to interfere with voice will not be completely eliminated. A (long) data packet may have waited its turn while voice packets went before it, but it’s length will still effect the timing of the next voice packet.

Dumb access points service all local clients, whether they are transferring voice or data. The mechanism that decides how bandwidth is apportioned between data and voice devices is CSMA/CA, which uses a random number based, collision-avoidance algorithm. Thus, if a number of data devices are requesting simultaneous transfers, they will interfere with voice traffic, thereby adding significant latency and jitter. Latency and jitter are enemies of VoIP, and this situation rapidly results in deteriorated call quality.

The only clean way to solve this problem completely is to separate voice from data – at the source – by using separate spectrum. The Structured Mesh software control layer eliminates interference by supporting separate service radios for voice and data thus preventing data devices from interfering with VoIP devices right at the source. MeshDynamics achieves this by recognizing VOIP phone traffic patterns vs. data traffic patterns and assigning voice and data devices to separate service radios.

Structured Mesh^TM  supports multiple service radios to manage voice and data traffic at the source.

Structured Mesh can support both WiFi and WiMAX VOIP phones. 

WiMAX is coming, and there are a diversity of opinions about how it will effect WiFi, VoIP and the cellular business. The primary benefit of WiMAX is range given its 20dB advantage over WiFi. In addition to long range point-to-multipoint last mile distribution systems, WiMAX will also be useful as a backhaul medium on mesh nodes for suburban and rural applications.

Structured Mesh has a radio and protocol agnostic architecture that supports WiFi and WiMAX for both backhaul and/or service radios. Structured Mesh modules can thus support both types of VOIP phones.

No one knows how WiMAX will progress or how widespread it will become. Nor do people know if WiMAX devices will outnumber WiFi devices or if WiMAX IP phones are the next wave beyond WiFi phones. But whatever the end result, the Structured Mesh approach of running the mesh control layer above the MAC level of the radio interfaces ensures interoperability between our existing and our future mesh products.

Mobility and Fast Routing

Clients roam and wish to experience smooth handoff during roaming. Future Structured Mesh solutions will include a scanning radio for enhanced client roaming functionality as well as mesh node mobility.
  
 MeshControl^TM software located on each node of a Structured Mesh operates as a distributed control system, using a field proven heart beat system. It also employs Store & Forward to assure no loss of data during handoffs 

The Structured Mesh Tree structure engenders a faster routing mechanism than ad hoc mesh - hence enterprise class wired network switch stacks use an efficient tree structure for routing. Ad hoc Mesh manages a large routing table - generally O(n2) - both system overhead and reaction time are higher.

Structured Mesh Tree structure engenders a faster routing mechanism than ad hoc mesh - hence enterprise class wired network switch stacks use an efficient tree structure for routing. Ad hoc Mesh manages a large routing table - generally O(n²) -  both system overhead and reaction time are higher.
 
Structured Mesh^TM  Tree topology results in efficient routing algorithms.

Full Duplex Voice Over IP (VOIP) support

The Ethernet is full duplex - transmit and receive occurs simultaneously over the LAN. In stark contrast, radios cannot send and receive at the same time. At any point in time each mesh backhaul radio is either sending or receiving but cannot do both simultaneously.

Structured Mesh is multiple radio based  by design and can support full duplex by splitting up each (logical) mesh backhaul radio into 2 (physical) radios, one for each direction. This enables simultaneous to and fro transmissions (full duplex). Additionally, the mesh backhaul bandwidth is doubled and latency per hop is effectively halved (because of reduced contention at each backhaul). 

Structured Mesh^TM  supports multiple radio interfaces - as a full duplex backhaul

Cost Effective Cellular Alternative

The convergence of voice and data over IP networks has resulted in ISPs being able to provide a more complete set of services (voice, video, data) for indoor environments. However, outdoor services are yet to benefit from this convergence - cell phones currently require a separate provider.
 
With the advent of low cost VOIP phones, Structured Mesh provides a cost effective cellular alternative for dense city wide deployments where subscribers with inexpensive VOIP WiFi phones can take advantage of VOIP indoors or outdoors. With efficient bandwidth distribution, Structured Mesh provides a cost effective alternative to cellular for meshed deployments of WiFi on a city wide scale.

More Mesh Network Information:

• Various Approaches of Mesh Networking
• Hybrid Mesh Networks
• Wireless ISP Mesh Networks
• Technical Overview of Mesh Networks
• Advantages of Structured Mesh Network Modules
• Mesh Network Management Platform
• Benefits of a Mesh Control Layer
• Performance Analysis of Mesh Networks
• Test Results of Mesh Networks Real World Testing
• Deploying Wi-Fi Cellular Networks for Voice over IP Services