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."
Summary
- 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.