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HFC's wireless edge

Sun, 04/30/2006 - 8:00pm
Don Bishop, Chief Technology Officer, Founder and CEO, MediaCell

The cable industry is in need of clarity regarding wireless technology as it relates to a wide range of possible applications in cable operations, including premises networking, wireless drop, "hot spot" service coverage, fixed and mobile service integration via dual-mode handsets and mobile service offerings.

An in-depth exploration of technology trends and existing options leads to the conclusion that not only can cable operators immediately put this valuable technology to use without waiting for future versions to be standardized and commercialized, they can do so using a platform that is precisely optimized for integration into the existing hybrid fiber/coaxial (HFC) infrastructure at a level of functionality that fits virtually any current or future requirements.

The opportunity now at hand for cable operators rests in their ability to leverage their hybrid fiber/coax network as the core transport system for an intelligent wireless edge platform that can accommodate virtually any application. An intelligent wireless edge architecture allows operators to optimize the functionalities of Layers 2 and 3 to maximum advantage, thereby avoiding the limitations on QoS (quality of service), scaling and service flexibility that are inherent with architectures that depend exclusively on operations at Layer 2. Seen in this light, the cable industry's wireless migration strategy is altogether different from the strategies of other types of service providers and therefore does not need to become bogged down in issues that are cause for confusion, debate and delay elsewhere. In addition, there are many new revenue opportunities within both the consumer and commercial services sectors to be exploited through immediate deployment of fixed wireless technology.

The advantages of Layer 3/4 operations

Figure 1
Figure 1: Utilizing all network layers to maximum advantage
in a hierarchically structured architecture with Layer 2
at the core and Layers 3 and 4 at the edge.
The key to success is to utilize all network layers to maximum advantage in a hierarchically structured architecture that is anchored in the HFC network: In other words, Layer 2 in the core, and Layers 3 and 4 at the edge. To take full advantage of the wireless domain to support ubiquitous reach of service no matter where end users might be requires a wireless system exploiting the Layer 3 and 4 routing capabilities for which IP was designed. Relying solely on Layer 2 functionality ignores the advantages offered by IP.

This is an architecture that follows logically from the scaling limitations of operating exclusively in the Layer 2 domain. The "spanning tree" approach used in Layer 2 depends on linking of IP addresses to MAC addresses, which means all switches must keep track of which MAC addresses are associated with specific IP addresses as ever-increasing numbers of users are added to the tree. As the user base expands across the wide area, the system becomes overloaded and slows down. The solution to the spanning problem in Layer 2 applications is to use Layer 3 at the edge so that adding more users doesn't burden the Layer 2 switching system with keeping track of ever more MAC and IP address associations.

While support for wide-area scaling alone would be sufficient reason to implement a Layer 2 core/Layer 3 edge architecture, there are many other benefits to be derived from this approach, especially in conjunction with its use in the HFC environment. By leveraging DOCSIS Layer 2 capabilities in the core and Layer 3 at the edge, the cable operator sets the stage for use of fixed wireless for any multimedia application.

Comparing WiMAX and Wi-Fi

With a huge lead in market penetration, Wi-Fi technology today is much farther down the descending cost curve than is WiMAX technology, which is just getting out of the starting gate. Therefore, for the next several years, the Wi-Fi cost advantage will argue in that technology's favor as long as the applications requirements of cable operators can be met through extensions to 802.11. After all, the Wi-Fi pricing structure leverages a deployed base of over 400 million devices, whereas WiMAX currently has virtually no base. Or, to put it another way, for WiMAX to succeed, it must essentially obsolete that installed Wi-Fi base.

It's also important to keep in mind the fact that Layer 2 alone, whether it's via WiMAX or Wi-Fi, does not address the need to provide a flexible, scalable and quality-assured wireless access solution directly to the end user's device, whether in the home, the office or on-the-go. The only way to address these requirements is through the use of Layer 3 and 4 functionalities enabled by intelligent edge components at the point of interface between the HFC network and the airlink.

WiMAX is often considered a "faster" technology. However, upon closer examination it is clear that 802.11 has no speed deficit. The optimal raw data rate of WiMAX is approximately 72 Mbps over a 20 MHz channel, representing an improvement of 18 Mbps over 802.11g and a. Accounting for WiMAX overhead, the actual optimal payload throughput is in the 36 Mbps range, representing about a 33 percent improvement over 802.11a and g. However, WiMAX bits-per-Hertz capacity, at 3.6 bpH in terms of raw data rate, is well below the 5 or better bpH contemplated for 802.11n. On-going improvements mean that 802.11 is superior to WiMAX when it comes to speed.

Enhanced 802.11 equals speed, security and versatility

While 802.11 technology was originally designed to operate as an Ethernet hub at Layer 2, there is no reason Layer 3/4 capabilities can't be added to 802.11 infrastructure. It's strictly a matter of assuring the routing intelligence is in place at the edge where the wireline network hands off the traffic to the radio network, as has been done in many fixed wireless applications outside the cable industry.

In the DOCSIS environment, this means the intelligent edge becomes an extension of the media control that's applied from the CMTS (cable modem termination system). In this architecture, the DOCSIS role as a Layer 2 bridge, where all traffic is funneled to the CMTS, is supplemented by the routing capabilities of IP to ensure ubiquitous coverage in a nomadic environment.

Figure 2
Figure 2: Cable operators have a significant opportunity to leverage the
HFC network as the core transport system for an intelligent wireless
edge platform that can accommodate virtually any application.

At Layer 3, devices can register their presence wherever they are, thereby providing the information routers need to direct traffic to the end user. For this to happen in a DOCSIS environment, where traffic is guided to subscribers based on their MAC addresses, the intelligence at the wireless interface must be able to translate the DOCSIS Layer 2 MAC to Layer 3. Routing then takes over to transfer the traffic to the user's actual location, using RADIUS and LDAP for user authentication.

Security

Security, of course, is vital to deployment of commercial services, and here again the functionalities at the higher IP layers mean legacy 802.11 technology can be used to achieve unmatched security. For example, the intelligent wireless edge can support firewalls and provide termination support for VPNs. In the context of cable's commercial services agenda, it's important to note that the encrypted and authenticated network tunnels provided by VPNs afford a level of security in the wireless domain that is not achievable in standard Layer 2-based applications such as WiMAX.

Load management and QoS

There are important implications for filtering and managing traffic loads in the use of IP Layers 3 and 4, as well. When operating at the data link layer, there's no way to perform filtering using higher level mechanisms insofar as information about the packet read at Layer 2 is limited to data pertaining to the link layer protocol. In contrast, Layer 3 packet filtering has access to all Layer 4 protocol information, which, in the case of IP, allows filtering to be based on TCP/IP and the port number. Operating at Layer 3 adds bandwidth reservation to prioritization, allowing for commercial grade QoS. And, with properly configured software on an intelligent platform, network operators can exploit a wide variety of mechanisms to assure low jitter and low latency on a per-flow basis.

The Layer 3 intelligence at the point of interface with DOCSIS also has important implications for the scaling of user stations on a business LAN served through a single cable modem port. If there are numerous users on the LAN, their combined traffic over the cable modem connection serves to diminish the Internet access rate of any given user by a fraction proportionate to the number of users. Layer 3 traffic management techniques provide a means of setting a routing boundary at the interface with the cable modem, thereby assuring only fully shaped packets consume the external network bandwidth rather than leaving it to the CMTS to sort out the unwanted "chatter." This load management function has important implications for public area Wi-Fi service as well.

Full QoS support for integrated voice and data managed on a per-user basis over the local area allows cable operators to provide SMBs (small- and medium-sized businesses) commercial services that are on par with the offerings of enterprise-class services from incumbent and competitive local exchange carriers. For example, an integrated T-1 service supported by Layer 3 wireless access provides operators the flexibility to reconfigure capacity allocations to VoIP as new users are added. This includes the ability to add users at remote locations who are connected to the main office IP PBX over IP VPNs the operator might supply.

Availability of a cable-optimized solution

More than simply describing "nice to have" features, the criteria discussed above establish minimum requirements for an architecture that has been optimized for use in the cable plant. In essence, the outlined system bridges the massive investments cable operators have made in their networks to the massive investments by consumers in Wi-Fi equipment to create a platform tailored for multimedia services.

The optimized system can be further described as available in either strand- or pedestal-mountable formats, employing a UNIX-based 1-GHz processor operating at OSI Layers 1–4 to provide the full range of management, security, traffic shaping, packet prioritization and other QoS elements that go into enabling all the service applications cable operators need to support in the residential or commercial environments. And it does this through a DOCSIS-optimized interface that provides a seamless transfer of the Layer-2 DOCSIS-encapsulated traffic to pure IP Layer 3 mode. To facilitate maximum use of intelligence at the access point, HFC RF signals are converted to baseband before they are propagated over the wireless link. That way all Layer 3 and above processes can be applied onto a signal that requires no conversion for connection into networking and device components at the customer premises.

At the same time, because the transport layer is fully abstracted, the system can be deployed over other transport modes as well, allowing operators to use a variety of transport systems and to aggregate them together into one seamless wireless access array. In other words, the system works at Layers 1 and 2 to maintain the connections but manages per-user functions at Layers 3 and 4.

The optimized system employs an antenna technology known as spatial air link multiplexing, which uses multiple, omnidirectional antenna arrays to provide robust coverage with raw throughput for the service area of 212 Mbps. In terms of effective throughput, this translates to about 108–120 Mbps, which can be broken into per-user connections of 10 Mbps or higher. At any given instance, the connection between a given subscriber and the transceiver employs whichever air link is best on a packet-by-packet basis, ensuring that the most robust performance possible is sustained at all times.

E-mail: donbishop@mediacell.com

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