Metro-access LANs take off

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Thus recently launched its City Ethernet LAN connectivity solution in the UK (above), and so far Shell, Conoco and the University of the Highlands and Islands in Scotland have signed up to its network. The Thus solution is designed to be an alternative to the traditional routers and leased lines companies commonly use to link different LANs in the same metro area. City Ethernet is initially only available in the cities of London, Bristol, Birmingham, Manchester, Leeds, Edinburgh, Reading, Aberdeen and Glasgow, and can deliver transport speeds from 10Mbit/s to 1,000Mbit/s (Gigabit Ethernet). City Ethernet is designed for connection distances of up to 70km, and Thus sees SANs as a key application that can be serviced by its solution. City Ethernet uses fibre connections from a customer's LAN site to reachable points of presence on the Thus network, which in turn link to the other customer LAN or LANs.

Up to now, most LAN-to-LAN connectivity has been based on much older, often more expensive and less flexible leased line, Frame Relay and ATM technology. But the use of optical networking technology in the MAN now seems to be a lynchpin to ensure that corporate data needs are addressed in a more efficient and cost-effective way.

Technology drivers for the transition to optical MAN access are partly related to the introduction of the Gigabit Ethernet and more recently the 10 Gigabit Ethernet protocols. These make it even easier to transfer predominantly Ethernet-based LAN traffic directly onto optical fibre in the MAN, to enable the high-speed transport of data to almost anywhere.

In addition to new networking protocols, metro access for LAN connectivity has also been made easier with the introduction of new fibre ring infrastructure across Europe, aimed at both the public and private sector. Countries that are experiencing major new fibre ring builds to deliver easier fibre access in the metro area include Ireland, France, Austria, and the UK.

A number of operators are also promising improved quality of service to customers with the use of new transport protocol multi-protocol label switching (MPLS). It is claimed that MPLS can expand on the quality of service facilities commonly found in the established SDH networks that currently run the majority of MAN network areas. Connectivity over metro areas is now widely improved with the use of the optical dense wavelength division multiplexing (DWDM) protocol.

Richard Collins (fig.1), MPLS VPN product manager at pan-European network operator Interoute, says: "Service providers that deploy MPLS over DWDM are in a better position to pass on the benefits of optical fibre connections in the MAN and WAN than those operating over SDH networks."

Collins explains, "An important benefit for MPLS is its ability to encapsulate Ethernet, giving customers high-speed LAN to LAN connectivity. It enables service providers to offer a high-speed service with improved scalability for enterprise customers."

In addition, MPLS over DWDM connections in the WAN do not present additional management complexities, with enterprise customers being able to treat such connections in the same way as their current Ethernet LANs.

One of the first companies entering the optical Ethernet space was neosnetworks, which now claims 200 different customers. Marketing director Neil Fairbrother (fig.2) says the flexibility of Ethernet as a transport protocol helps it compete against traditional SDH networks, originally designed for voice not data. He says,."A customer can choose bandwidth increments as small as 1Mbit/s with Ethernet and as high as 1,000Mbit/s with Gigabit Ethernet, but with SDH there are only increments of 2Mbit/s and 155Mbit/s."

Neosnetworks allows customers to automatically change their bandwidth requirements via a web portal, which delivers those changes within 48 hours as part of an SLA. But similar changes via SDH networks can take many weeks, leaving the customer to either pay for bandwidth they no longer need when they want to reduce their bandwidth space, or left with too little over critical periods when they want to increase it.

Geoff Spencer, director, optical networks architecture at Nortel Networks, says that high end enterprise applications like storage are driving the demand for optical DWDM in the MAN, supported by hardware like Nortel's OPTera Metro 5000 routing system.

Such metro DWDM connectivity can be extended using an optical amplifier to enable the traditional 80km metro limit to be expanded to regional and national distances, to complete company-wide connectivity using protocols such as Gigabit Ethernet.

Nortel's Spencer says the connectivity options available to the enterprise WAN manager are highly variable, dependent on the applications that need to be supported. Spencer says: "For packet switched multi-point connectivity, the WAN manager has a wide choice of available services from carriers.

"Frame Relay has, by volume, been the most popular choice, closely followed by ATM. Recently, IP VPNs have begun to gain universal acceptance and are becoming increasingly popular.

"Finally, Ethernet switched connectivity over MPLS infrastructure is seen as a promising approach for providing LAN-to-LAN connectivity for many applications requiring moderate to good quality of service options, including VoIP, web-casting and front office connectivity back to data centres."

Spencer points out that this type of network has, up until now, not generally been the most appropriate for the most demanding back-office storage networking requirements, where instead Fibre Channel over DWDM or SDH has emerged as the most suitable solution. However, this situation may change with the introduction of 10 Gigabit Ethernet as an approved standard, although it must be pointed out that the Fibre Channel industry is working on developing 10G Fibre Channel in an attempt to maintain its position in this market segment.

Companies wanting to take advantage of DWDM architecture to transport their data can be put off by the relatively high cost that the high-speed system attracts, but Santanu Das, metro director at Optical Fibre Systems, says there is an alternative in the form of coarse wave division multiplexing (CWDM).

CWDM supports a number of different transport methods, including Gigabit Ethernet, and can be used to connect LANs over distances of up to around 150km. Das explains that CWDM is cheaper because of its shorter reach compared to DWDM.

He is also keen to defend the continuing existence of SDH despite the emergence of MPLS when it comes to quality of service issues. He says: "MPLS is a higher layer protocol, while SDH is still the dominant transmission layer protocol. SDH can provide the quality of service on the transmission layer and can support developing quality of service protocols such as resilient packet ring."

Cisco recognises both the benefits and deficiencies of SDH, and this is why it is promoting its MPLS Bandwidth Protection technology to service providers who are providing data services over traditional SDH voice networks.

SDH-based service providers have always allocated a dedicated spare extra channel of bandwidth for each customer so that if the original link goes down, traffic can be routed over the spare redundant link. This time division multiplexing (TDM) system is a wasteful way of using fibre for business connectivity and is costly to the provider, and hence to the customer.

TDM however is a well-performing system though, with a fast recovery time, following a failure, of under 60ms. What MPLS Bandwidth Protection enables is the same failure protection offered by TDM but the opportunity for service providers to use their redundant channels to also send data. It does this by calculating exactly how many back-up tunnels are really needed by the network at any given time, in response to traffic volumes.

Such solutions illustrate that there is still plenty of life in the existing war-horse SDH network, which is just as well as operators currently can't afford to completely rebuild parts of their core network.

While the various transport protocols, new and old alike, seem to be getting on well together in metro areas, what customers should always consider is that service providers will inevitably rely on a mixed-bag of transport solutions to deliver their metro access needs. But the management and quality of service issues that arise as a result, is something that users should pin their suppliers down on, as they don't have complete control of their network once portions of it leave their buildings.


  • The nature of the IT traffic and its relevance to business continuity. Are the networks mission-critical (back office system for a bank or a retail e-commerce operation for instance), or are they just supporting day-to-day activities such as employee web access?
  • The quantity and profile of the traffic. Is it multi-gigabit per second or a few megabits per second, and is it highly variable according to the time of day?
  • The network quality of service requirements for the applications being used. E-mail and web browsing do not require special handling whilst voice over IP (VoIP) and real-time storage replication do.
  • The cost and availability of the services available from the network providers available. Will providers lease or sell fibres to the enterprise? Do they support managed storage or managed wavelength services? What is the relative cost of leased SDH circuits versus leased fibre or managed ATM services?
  • What are the size and geographic distribution of the enterprise sites to be connected and what is the optimum network topology to support the IT infrastructure? Does the enterprise have many small branch offices that need to be connected to HQ storage and application servers, or does the business consist of a few campus sites in a major city centre?
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