Mesh Metro Ethernet Backhaul for WiFi/WiMax and Cellular Applications

Erik Bock – Chief Technology Officer, Dragonwave

1 Backhaul Challenges Associated with Wi-Fi and Wi-Max

Access networks are proliferating new deployments by service providers as well as through municipal build outs. Wi-Fi/Wi-Max attractiveness is rooted in its ability to deliver high-speed access connectivity rapidly and more cost effectively.

With the availability of IEEE 802.11a/g technology, access rates available from single network access points (APs) are now commonly 54 Mb/s. Although this is the maximum, aggregate over-air bit rate, it is not uncommon for these access points to deliver 10 to 15 Mb/s of full duplex user traffic.

When considering the use of Wi-Fi and Wi-Max technology in the construction of access hotspots, one often-overlooked facet of the network implementation is the backhaul segment. Poor backhaul network design can contribute to:

1. Reduced access bandwidth

· Sharing access spectrum with in-band backhaul

· Poor backhaul throughput from T1/E1/DSL

2. Poor application/service performance

· High network latency and/or delay variability

· Poor sustainable data throughput

3. Reduced network reliability and availability

· Unlicensed wireless backhaul outages due to interference

· Inability to construct resilient, self-healing backhaul topologies

4. Increased susceptibility to interference

· Unlicensed backhaul outages

5. Poor network financial performance

· High lease cost of fiber/TDM circuits (when used)

6. Delayed time-to-market

· Delays in deployment of fiber (when used)

Service providers must address the backhaul connectivity question much sooner in the network design process to achieve a robust solution which allows the full capability of Wi-Fi and Wi-Max access technology to be realized. This paper explores the backhaul technology options available (wire line and wireless) and further explains the different wireless network topologies currently available.

1.1 Backhaul Technology Options

1.1.1 Leased Lines

Low Capacity services can be delivered with T1s, whereas higher capacity services may require T3/E3/Optics

· Upfront non-recurring costs

· Recurring Leasing costs

· Capacity and Scalability Limitation (E1/T1)

· Line availability (or time to deploy if unavailable)

o ROW Issues

o Building entrance consideration

o Potential construction delays

1.1.2 In-Band Wireless

In-band wireless backhaul may use Multi-Point or Point-to-Point techniques to provide low-speed backhaul links, on the order of 10 Mb/s or less. The central drawback with this solution is that the same spectrum that is used for access bandwidth has to be shared with the backhaul segment. This "sharing" inherently reduces access capacity.. Some additional drawbacks with these solutions are:

· Equipment expense for supporting high capacity point to point

o Requires dedicated base stations and multiple concatenated CPE's

o Use sector of spectrum

· As IP services increase so will bandwidth demands

o Limit the # of CPE's per base station

o Result in network re-designs to support traffic densities

1.1.3 Out-of-Band Unlicensed Wireless

Out-of-band wireless backhaul usually employs licensed Multi-Point or Point-to-Point techniques. The reason for this is that the unlicensed spectrum is typically employed for the access layer (2.4 and 5.8 GHz). While this backhaul solution is traditionally quick to deploy (no license required) and a low cost alternative, there is no assurance or guarantee's associated with availability. Other drawbacks associated with this solution:

· Extremely prone to interference

o No limit on device usages

· Indeterminate availability – no ability to guarantee SLA's

· High latency and jitter preventing high value IP services

· Limited bandwidth capacities

An exception to this is the 24 GHz ISM unlicensed spectrum in which high capacity Point-to-Point backhaul links can be operated without stealing capacity from the access layer.

1.1.4 Out-of-Band Licensed Wireless

An additional attraction of licensed wireless backhaul is the fact that it delivers interference-free operation, which means the backhaul links will perform properly and with the expected throughput.

In addition, out-of-band solutions typically provide high backhaul bandwidths of up to 500 Mbps enabling multiple hotspots to be connected in a mesh, without sacrificing capacity.

1.1.5   Conclusion

In order to provide the capacity and economics required for bandwidth intensive WiFi and WiMax deployments, licensed wireless backhaul is required.  In addition, service providers need to be able to control service levels, which they can only provide using licensed spectrum.

1.2 Licensed Wireless Backhaul Network Topology Comparison

1.2.1 Multi-Point

Multi-point solutions for Wi-Fi/WiMax backhaul may include LMDS, Wi-Max or other similar system. These solutions all employ shared RF carrier operation in which a number of end-sites are multiplexed onto shared RF carrier(s) (referred to as TDMA). The result is that a limited amount of bandwidth is generally available to any one subtended end-site. The result is that this technology is generally targeted at lower speed applications, i.e. < 10 Mb/s per end-site. These solutions also typically have high latency and jitter and lack a redundancy solution preventing voice and video services from being offered.

1.2.2 Point-to-Point

Point-to-Point technology can be used to implement high speed backhaul links and is generally the choice where high bandwidth and low latency are concerns. Point-to-Point links can be implemented in single links, daisy-chained or deployed in "virtual" multi-point arrangements referred to as "hub & spoke" multipoint. In the latter, the advantage over conventional multi-point technology is the tremendous potential for bandwidth scalability of any of the end-site links without affecting the attributes of the links to the remaining end-sites. In addition, each link can be hardened with redundancy for higher availability, and can provide enhanced services through ultra-low latency.

1.2.3 Constrained Mesh

In the constrained mesh case, the network nodes are linked (typically with point-to-point links) to a constrained number of other network nodes. Using high speed protocols such as Rapid-Spanning-Tree (RSTP), the nodes autonomously detect network failures and switch traffic through pre-determined backup routes. Since the routes are pre-defined, the fail-switch-over dead time/outage can be reduced to 50ms - 100 ms through co-ordination with the wireless layer. The controlled nature of the constrained mesh topology also allows the network designer to control the network path delays, making it a superior solution for delay-sensitive networks (i.e. those handling VoIP, VIDoIP, etc).

1.2.4   Conclusion

Wireless mesh allows service providers to meet the backhaul capacity and scale required, while delivering 99.999% availability with the required economics.  In addition, mesh enables the service provider to meet the strict latency and jitter requirements of high value voice and video services.

1.3 Examples of the Application of Wireless Mesh Technology to Metro Wi-Fi and Wi-Max Backhaul

Constrained mesh networks used for metro backhaul offer a number of attributes, which are key to a variety of types of access networks:

· High bandwidth

· High degree of bandwidth scalability

· Ease of adds/changes

· Rapid self healing with "hitless" fail-switch-over[1]

· High availability through path and equipment redundancy

· Economical

When considering the use of constrained meshes, these can be constructed using the following frequency/spectrum arrangements;

· Unlicensed point-to-point: i.e. 24 GHz ISM

· Site-licensed: i.e. 6, 11, 18 or 23 GHz

· Area licensed: i.e. 24 GHz DEMS, 28 GHz LMDS

When considering the licensing options, it can be seen that there is virtually no limit to spectrum availability...and thus to network capacity. Mesh sub-circuit bandwidths can be scaled from low initial bandwidths (i.e. 10 Mb/s) up to 400 Mb/s (full duplex, sustained). In addition, low latency (< 200 us per mesh hop) is mandatory in insuring that all service applications operating in the user access layer will operate properly.

Figure 1 employs distributed Wi-Fi access points located to create full metro coverage with each Access Point delivering access over a few square city blocks.  The mesh is constructed using 2 layers, the first layer creates mesh sub-circuits which connect the Wi-Fi Access Points. The second layer then meshes sub-circuit root nodes to metro PoP locations.

The Wi-Fi access points in a network like this may require 1Mb/s - 20 Mb/s of backhaul Clusters of 5 - 10 Access Points are grouped onto individual mesh subcircuits. The backhaul demand on these sub-circuits may easily reach 50 to 100 Mb/s full duplex or more.  In turn, meshing the sub-circuit root nodes together in clusters of ~ 5 may demand backhaul bandwidths of 100Mb/s - 400 Mb/s full duplex. If the backhaul bandwidth is not able to deliver the required sustained data rates, buffering[2] and packet discard will result. Although some "best-effort" services can cope with this, delay and packet-loss sensitive applications do not do well, nor to time-sensitive network routing functions such as cell-to-cell handoffs used in mobility-capable networks.

Figure 1: An Example of a Nano-Cellular Metropolitan Mesh Network

On a wider area basis, mobile voice networks are often considered as being candidates in doing "double-duty" as Wi-Fi or WiMax enabled data networks.   Figure 2 illustrates a backhaul network in which a number of mesh sub-circuits are used to backhaul cellular mobile BTS locations distributed throughout a large metro area. The mesh sub-circuits employ root nodes, which are PoPs on a metro fiber ring (shown bold purple). In this case the traffic on the backhaul network is a combination of high-speed data and cellular voice traffic (carried as TDM-over-Ethernet). This allows the operator to provide high bandwidth data services, mesh resiliency & scalability, forward-looking IP/Ethernet centricity, whilst not abandoning any voice revenue or stranding G1/G2 cellular BTS equipment, which requires T1-based backhaul.

Figure 2: Backhaul Network Using Mobile Voice BTS Locations Providing both Voice and Data

1.4 Summary

Ethernet constrained mesh wireless backhaul offers an optimal backhaul topology for Wi-Fi(and WiMax) access networks. Mesh technology cost-effectively provides;

· High bandwidth

· High degree of bandwidth scalability

· Ease of adds/changes

· Rapid self healing with "hitless" fail-switch-over[3]

· High availability

· Low latency

· Economical

These attributes make the mesh topology ideally suited for a variety of different types of metro access networks, including; municipal Wi-Fi networks, emergency preparedness networks, cellular mobile voice+data networks or WiMax multi-point metro access networks.

Author's Biography

Erik Boch is a co-founder of DragonWave Inc, and the Chief Technical Officer (CTO) and VP of Engineering. Erik has been involved in various aspects of microwave & millimetre wave subsystem, system and network design for more than 22 years. While at DragonWave, Erik has led the company in the development low-cost, broadband millimeter wave radios and associated wireless Metro Ethernet network solutions.

DragonWave Inc.

600-411 Legget Drive, Ottawa, Ontario, Canada, K2K 3C9

Tel: 613-599-9991    Fax: 613-599-4225

www.dragonwaveinc.com

---

[1] "Hitless" refers to the deadtime being "SONET-like", 50 - 100 ms, which enables applications to operate without connection loss or service disruption during a user session set-up or whilst the session is in process.

[2] Buffering translates into delay and delay variability.

[3] "Hitless" refers to the deadtime being "SONET-like", 50 - 100 ms, which enables applications to operate without connection loss or service disruption during a user session set-up or whilst the session is in process.