Ethernet evolves in the metro

Communications News,  Oct, 2001  by Sam Halabi

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The metro area network (MAN) is undergoing a steady transformation, progressing from time division multiplexing (TDM)-based infrastructure--such as synchronous optical network/synchronous digital hierarchy (SONET/SDH) and asynchronous transfer mode (ATM), built to support relatively low-level voice services--to optical Gigabit Ethernet, carrying heavy streams of data. The rise in more widely distributed IP traffic has created a wealth of unpredictability, as any PC or server can become an end point, generating a multitude of network requests.

In response to these challenges, hardware-based terabit routers, forming the core of the Internet's backbone, offered much-needed relief in performance over traditional software routers. In the LAN, the IEEE's 802.3z Gigabit Ethernet ousted competing technologies, such as ATM, as the networking alternative of choice--with its ability to eliminate traffic bottlenecks, while improving scalability and reducing complexity.

Gigabit Ethernet's victory in the LAN leaves one final challenge: delivering bandwidth to "the first mile," an area representing the connection from the Internet point of presence (PoP) to the end-user's doorstep. Meeting the demand for bandwidth in the first mile is forcing a revolutionary change in technology--managing content, delivering services and handling network traffic flow. Scaling the network to deliver such services requires processing the flows at the network edge and not at the source of the content/service.

Representing the next wave in connectivity techniques to open up the network traffic bottlenecks in the first mile is today's optical MAN. Optical technology brings major advantages over the legacy copper medium by providing the ability for extended distances and, in collaboration with wavelength technology, the ability to deliver expanded bandwidth capacities that were unheard of even a couple of years ago. Traditional MANs, however, have been around since the late 1980s. Even during those days, they relied on an existing fiber infrastructure, but were concentrated within business districts only.

EVOLUTION OF THE MAN

The predominant characteristic of these first-generation MANs was that they relied on proprietary TDM technologies with sophisticated bandwidth management, which provided soft and on-the-fly capabilities, and the ability to connect a wide variety of interfaces, protocols and speeds. SONET/SDH-based infrastructures were later introduced and primarily deployed to handle voice traffic.

In the late 1990s, some service providers deployed ATM in the MAN because of its multiservice focus--the quality-of-service capabilities of ATM were touted to be the perfect remedy for carrying data, voice (using circuit emulation over ATM) and video.

The rapidly increasing volume of data traffic in metro networks is challenging the capacity limits of existing transport infrastructures that are based on circuit-oriented technologies like SONET/SDH and ATM. Packet-based transport technology, a natural fit with the now-ubiquitous IP protocol, is considered by many to be the likely alternative for scaling metro networks to meet traffic demands.

Ethernet has quickly become the leading technology for packet transport in the metro. As an alternative to circuit-oriented transport technologies like SONET and ATM, optical Gigabit Ethernet technology is a good match for carrying Internet protocol data traffic because of its capability to support fiber spans of more than 70 kilometers.

Since nearly all IP packets begin and end their trips across the Internet as ethernet frames, carrying data in a consistent packet format, from start to finish throughout the entire transport path, eliminates the need for additional layers of protocol and synchronization that result in extra costs and complexities. In addition to efficient handling of IP packets, ethernet has the advantages of familiarity, simplicity and low cost.

SONET'S DISADVANTAGES

The disadvantages of using SONET/ SDH in the metro include:

* Fixed circuits. SONET/SDH provisions point-to-point circuits between ring nodes. Each circuit is allocated a fixed amount of bandwidth that is wasted when not used. That fixed allocation puts a limit on the maximum burst traffic data transfer rate between end points. This is a disadvantage for data traffic, which is inherently bursty.

* Waste of bandwidth for meshing. Provisioning the circuits necessary to create a logical mesh over a SONET ring is not only difficult, but also results in an inefficient use of ring bandwidth. As the amount of data traffic that stays within metro networks is increasing, a fully meshed network that is easy to deploy, maintain and upgrade is becoming an important requirement.

* Multieast traffic. On a SONET/SDH ring, multicast traffic requires each source to allocate a separate circuit for each destination. A separate copy of the packet is sent to each destination. The result is multiple copies of multicast packets traveling around the ring, wasting bandwidth.