Ethernet to the subscriber
By CHRIS SETTY, World Wide Packets--By focusing on effective data transport instead of attempting to solve traffic management issues within the core technology, Ethernet offers a flexible, cost-effective and scalable solution for voice, video, and data services.
By CHRIS SETTY
World Wide Packets
Ethernet is poised to dominate the first mile networks that will deliver voice, video, and data services to both residential and business customers. Its superiority in the access network space is due to a combination of attributes--technology, management, cost, and transportable services.
Flexible data transport
Unlike other transport technologies, Ethernet has remained pure and true to it's original intent: simple, effective data transport. While ATM tried to integrate quality of service (QoS) and traffic management features into the core technology, the Institute of Electrical and Electronics Engineers' (IEEE) 802.3 committees working on the Gigabit Ethernet standard opted to keep Ethernet flexible so other factions can augment it via proprietary and standards-based device features.
This is not to say that QoS and traffic management are unnecessary. It isn't the need for these features that is disputed here; it is the method of implementation. Why complicate Ethernet as a technology with built-in QoS attempts when packets can merely be marked in such a way to be handled by other standard classification methods, such as differentiated services (DiffServ), type of service(ToS), and 802.1d priorities (formerly 802.1p)? Why establish a means to manage different traffic requirements (e.g. available bit rate, undefined bit rate, constant bit rate, variable bit rate) when this can be done satisfactorily based on standard Ethernet packet fields like 802.1q virtual local area network (VLAN) tags and DiffServ-byte/ToS markings? By focusing on effective data transport instead of attempting to solve management issues within the technology, Ethernet has been able to increase its bandwidth capabilities by an order of magnitude at a very rapid pace. Speeds have increased from 10 Mbits/sec, to 100 Mbits/sec, to 1 Gbit/sec, with 10 Gbits/sec currently in the standards process (IEEE 802.3ae).
Merely sending bits back and forth is not enough to successfully implement a first mile access network. Services must be provisioned, equipment must be managed, traffic must be counted and problems must be proactively detected and solved. Packages abound for simple network management protocol, capacity planning, alarm management, trouble ticket administration, statistics collection and every other conceivable management function. In addition to off-the-shelf products, vendor-specific tools allow access to proprietary settings and interfaces with various databases. This wealth of tools, packages, and technologies paves the way for operators to successfully implement access networks and services without being overrun by operations costs and complexities. Ethernet's superior manageability gives it the edge over every other technology available in the realm of the first mile.
Ethernet devices are extremely cost effective compared to equipment based on competing technologies. The Ethernet standards focus is resulting in a wide availability of interoperable devices, creating an affordable alternative to ATM-based systems such as asynchronous digital subscriber line (ADSL) and ATM passive optical networks. Additionally, network operators need to deploy access networks not only at a low per-subscriber capital expenditure, but also must take into account the cost of operating the network and its longevity. If a network can be deployed for 80% of the cost of a competing solution, but is rendered obsolete in half the time, the benefit of the initial capital expenditure "savings" is non-existent. With Ethernet, per-subscriber deployment costs are comparable to competing technologies, and its bandwidth capabilities allow for unprecedented long life.
Technology, management, and cost are certainly important factors in choosing an access network technology, but the services it can deliver determine the overall success of its deployment. Ethernet is obviously capable of high-speed Internet access, but that isn't the only service required in the first mile. Toll-quality voice and video services must also be transportable over the access network.
Ethernet's first advantage is its enormous bandwidth capacity. If you contrast the 100 Mbits/sec, or even 1 Gbit/sec of bandwidth per subscriber in an Ethernet-to-the-subscriber network with the meager bandwidth available in a cable or DSL deployment (< 8 Mbits/sec), that point is clear. But Ethernet's standards-based functionality and scalability are additional benefits that make the technology perfect for access networks. For instance, the 802.1q VLAN standard provides traffic separation that allows for differential treatment of applications, as well as security, billing and rate limiting. These functions, combined with huge bandwidth capacity, produce a solution that can carry data, voice, and video services, each with different levels of prioritization (see Figure).
In an Ethernet access network model, Internet service can be securely delivered to individual subscribers on separate logical networks, each with its own data rate and priority if required. Subscribers can access the Internet from multiple devices instead of being limited to a single device, as they are with cable or DSL. Additionally, providers can offer two levels of service to a single home to address casual home use at one level, and telecommuting requirements at a higher bit rate and priority.
So how does this traffic get separated? The 802.1q VLAN standard is quickly emerging as the answer to the security issues that cable modems have brought to the fore. On an Ethernet access network, each subscriber can receive data services on a unique VLAN, thus privatizing any data sent and received. These VLANs are terminated at the Internet service provider (ISP) on a router interface, many of which can now support the full range of 4096 VLANs on each individual interface. The ISPs manage the IP interfaces, VLANs, and dynamic host configuration protocol models used for their subscription base. VLANs also provide a means to recognize frames that belong to a particular service level and to apply necessary rate limits and priorities as well as to count traffic for a particular subscriber.
Ethernet access networks use IP voice technology that is delivered on a VLAN. The VLAN separates voice traffic from data and video traffic and provides the marking by which the voice traffic can be given the utmost priority. On either end of the IP voice channel, a gateway terminates the connection - one is a gateway to the public switched telephone network, while the other is the gateway to the analog wiring and phones located in the home. There are a variety of protocols - media gateway control protocol, session initiation protocol, H.323 and Codecs G.711, G.723, G.729, etc - but all accomplish the same basic functionality, which is to compress the voice and encapsulate it in IP frames. There are also a variety of architectures from the service provider standpoint. Carriers can use voice-over-IP gateways, data link controls, and even IP-based Class 5 softswitches.
However voice traffic is deployed, prioritization is the key to maintaining the quality that subscribers are currently receiving from incumbent providers. Voice is a very low-bandwidth application, but it has little to no tolerance for latency and jitter. Ethernet's bandwidth coupled with standards-based traffic separation and prioritization can support thousands and thousands of simultaneous phone calls, over many providers, on a common infrastructure, without latency or jitter issues.
The majority of IP television deployments to date have been deployed over DSL networks. In these deployments broadcast digital video streams are received via satellite and encapsulated into IP, after some de-multiplexing is done to ensure that channels are streamed individually. Then the bit rate must be altered in order to fit it onto the bandwidth available. Typically, the bit rate is brought down to around 3 Mbits/sec. Remember this is one channel. No other services are available concurrently over the network because that one television channel uses all of the available bandwidth. Considering that the typical American household has 2.3 televisions each, the capacity to deliver only a single channel at a time over a DSL network is woefully inadequate. And these numbers are based on standard definition TV. High definition TV (HDTV) will consume 15-20 Mbits/sec of bandwidth for each channel delivered.
Ethernet again lends its superior bandwidth to the problem of video services delivery. Unlike other technologies that must convert signals to a low constant bit rate, which adversely affects quality, Ethernet can handle variable rate signals as received (typically ranging from 3-8 Mbits/sec). Secondly, the number of channels that can be sent down a 100-Mbit/sec broadband connection easily satisfies even the most teenager-laden households. Video is a bandwidth-intensive application that can handle some latency, but very minimal jitter. With Ethernet, video can be separated and prioritized in order to ensure QoS.
With an Ethernet broadband system subscribers can watch three HDTV channels, order a video-on-demand movie, talk on the phone, and surf the Web all at the same time, and still not tax the available bandwidth. This is an impossible feat for any other technology available today. The table details typical residential bandwidth consumption, which is easily accommodated by a 100-Mbit/sec Ethernet broadband connection.
With Ethernet, providers don't have to constrict their service portfolios because of technology limitations. Access networks that use Ethernet will also likely benefit from lower operating costs due to this technology's cost-effectiveness and superior manageability. Indeed, Ethernet promises to support future services without requiring expensive network reconfigurations because of its scalability.
Chris Setty is a product manager at World Wide Packets (Veradale, WA). For more information, visit the company's Web site at www.worldwidepackets.com.