Ethernet is the future of city connectivity

As prices fall, Ethernet will dominate city-wide networks.

As prices fall, Ethernet will dominate city-wide networks.

Networks stretching across cities around the world are undergoing a fundamental change which could crown Ethernet as the monarch of metropolitan area networks and ring the death-knell for current technologies.

With the UK's broadband roll-out hamstrung by arguments about BT unbundling the local loop, Europe has been the main testing ground for metropolitan Ethernet networks. Sweden, Italy and Austria have already established networks in several cities.

Neil Rickard, research director at analyst firm Gartner, said, "Metro Ethernet in the UK is still in its early stages and, although a few companies are using metropolitan Ethernet networks, they tend to be the biggest companies in the larger cities." This is a result, he said, of BT's stronghold on fibre networks and high costs. Although the costs seem reasonable on BTs website, this only applies when sites on the network are within a few kilometres of the same exchange.

High costs

Costing is complex but, as a rule of thumb, initial connection costs about £1,000 per end, or £2,000 to make a connection between two sites. Both sites must be within 5km of the exchange. Annual rental is £4,000 per end but if the network jumps from one exchange to another the costs mount. At £2 per metre, a 2km hop adds £4,000 to the bill. Corporates are generally the only organisations that can afford to connect from one side of a city to the other.

"I think this will change and there will be a slow trickle down as metro Ethernet gains more extensive use," Rickard said. "Although the links are expensive, the spread of Ethernet makes the system as easy to manage as a large campus network, meaning one administration policy and one support team. And the fact that Ethernet is already a mass-market technology means the hardware is cheaper."

Ethernet already offers data rates of 10gbps, but maximum bandwidth of up to 100gbps is promised in the next few years. However, it is the granularity offered by the technology that appeals, particularly at the low end of the market, where speeds of 100mbps may be sufficient. Currently, bandwidth is bought in fixed steps, but Ethernet technology means the bandwidth can be whatever the customer requires.

Another advantage of Ethernet is the easy convergence of voice, data and video, opening the way to cost savings by placing three loads on a single pipeline.

Until now, metropolitan area networks have been based on Asynchronous Transfer Mode because of its bandwidth. Underpinning this data link layer is the Synchronous Digital Hierarchy transport layer or, mainly in the US, the Synchronous Optical Network (Sonet).

The problem has always been that the local area networks that feed a metropolitan area network use Ethernet. This must somehow be translated into an ATM format using specialist switches.

ATM and Ethernet are quite different in structure. ATM uses frames of a fixed size at 53bytes, but Ethernet uses packets of variable length. In metropolitan area networks, Ethernet traffic has to be broken down into 48byte chunks with a 5byte header in order to be carried across the ATM network. After transportation, these packets are reconstructed, which adds to the latency, or delay, in moving traffic from source to destination.

Service levels

In Ethernet-based metropolitan networks the framing process is not required, reducing the complexity and latency in the system. Connection speed is just one element included in service level agreements, which are a must on a shared network where users will be running mission-critical applications. Ethernet SLAs are usually "best-effort" at most. Although Ethernet is good at handling point-to-point connections or Lan broadcasts, it has several shortcomings compared to SHD.

There is no way to reserve bandwidth for essential services, recovery from network failure is slow and scalability is limited. This means that anyone who hogs more than their allotted bandwidth will wreck any agreed SLAs negotiated with other users. Also, if there is a failure, SDH can recover from a network fibre cut in milliseconds, but Ethernet's rapid spanning tree technology takes one second for reconfiguration.

This is now improved by the use of multiprotocol label switching (MPLS). This is where a virtual Lan is used to constrict usage within agreed limits, allowing the service provider to offer guarantees on fine-grain, end-to-end network performance. The recovery is equal to SDH by allowing recovery paths to complement the primary path. If the primary path is interrupted by a fibre cut or router failure, the recovery path takes over.

This Ethernet/MPLS structure is the basis for companies such as 51¡, which rolled out metropolitan network loops in London's business districts in December 2002.

Paul Girvin, sales and marketing manager at 51¡, said the company used fibre that was already laid alongside LE Group's power cables. "Because the cable already passes within a few metres of potential customers, the reduced dig to connect the customer to the network means our costs are cut."

51¡ does not provide services, just the connection. Applications come from its partners, which act as service providers and compete for business. There are currently 16 partners, including Thus, Neos and Omnetica, but 51¡ plans to increase this to 30 by the end of the year.

Although these companies stand to do well, they must tackle established technologies and ensure smooth migration paths for users that have invested substantial sums in legacy ATM.

Cisco is supporting this through its Internetwork Operating System, which is adding handling for ATM, SDH, Sonet and Frame Relay. This means that hardware, such as the Cisco 7600 routers, can be software upgraded to handle new Ethernet and legacy protocols.

Marc Musgrove, Cisco's corporate relations manager for metro technology, believes costs will come down. He said Cisco is talking to cable TV companies who already have fibre in the ground. Cisco is arguing the case for local authorities to jump on board to provide community metropolitan Ethernet networks for teleworking.

Rickard said, "The writing is on the wall for ATM, and MPLS over Ethernet will be the final nail in the coffin. Once new competitors to BT appear, it may only be two years before metropolitan Ethernet networks take over."


  • Ethernet, the ubiquitous Lan standard, is increasingly being used over longer distances in metropolitan area networks

  • Because Ethernet is already prevalent inside corporate firewalls, the gateway equipment needed to translate data into carrier standards such as ATM and Sonet is not required

  • Bandwidth provision is more granular than with ATM and Sonet and integration of voice and data is smoothed by the use of IP.

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