Network media conversion: an Allied Telesyn white paper

New developments in fibre connections have made this high capacity medium an even more attractive proposition in the networking...

New developments in fibre connections have made this high capacity medium an even more attractive proposition in the networking technology marketplace

Media conversion products are products for evolving networks. Simply put, they convert from one network media type ( defined by cable and connector types and bandwidth ( to another. By performing this transition, the media convertor makes it possible to extend legacy networks with the latest technology, instead of the alternative of being tied to what the network was started with or, even worse, tearing it out and starting over. Alternatively, convertors allow the use of less expensive, lower bandwidth desktop connections from a state of the art fibre optic backbone. The ability to mix media and speeds on a network is critical to achieving optimal price/performance goals. Media conversion devices further the flexibility and extensibility of the network by:

Facilitating upgrades to the network to better, faster, more secure technology ( as with fibre cabling without requiring a full network retrofit

Legacy copper cabling can be left in place, while fibre is used for additions and extensions to the network

Providing a means by which to extend network distances

Using a media convertor to integrate fibre allows the network to support the longer cable distances available through the use of fibre

Making it easier to add a host of new devices to the network ( including the newer high-end, high-bandwidth switches and hubs ( regardless of connector restrictions

Maximising efficiency and economy in new networks. The media convertor can help the network manager optimise capital for efficiency by enabling a high-bandwidth fibre optic backbone to feed copper to workgroups and desktops

Being flexible. Media convertors can be inserted almost anywhere in the network

To the network architect or the network manager, a pure media convertor is best described as two transceivers or MAUs (Media Attachment Units) that can pass data to and from each other, and a power supply. Each of the MAUs has a different, industry standard, connector to join the different media: one medium goes in, the other comes out. The connectors themselves are standardised to IEEE specifications and, as IEEE-compliant devices, they utilise standard data encoding rules and link tests. Media convertors facilitate the connection of a multitude of devices as a result of the flexibility they lend to the network. They support connections to and from switches, hubs, routers and even direct to servers. Currently, the most commonly used media convertors support twisted pair to fibre connections. Standard fibre connectors of today typically are classified as either ST (simple-twist) or SC (subscriber-channel). New technology in connectors is coming to market in the form of 'next generation' small form factor fibre connectors. These new fibre connectors are the MT-RJ, the VF-45 and the LC. The media convertors most widely used today, though, are designed for quick, reliable, cost-effective connections between:

10Mb/s twisted pair cable segment or device and 10Mb/s fibre optic, singlemode or multimode (10Base-T to 10Base-FL)

10Mb/s 10Base-T segment or device and 10Mb/s Ethernet coaxial cable (10Base-T to 10Base2)

100Mb/s twisted pair cable segment or device and 100Mb/s fibre optic, single-mode or multimode (100Base-TX to 100Base-FX)

Speed conversion

There is another choice emerging within media conversion itself, and that is the small, 2-port switch. Low-cost, two-port unmanaged switches upgrade, extend and relieve network congestion where it's needed, and cost-effectively. The switches not only provide a means for connecting new network media or speed, but they further improve performance by providing a means to segment networks into smaller, connected sub-nets. While convertors are simple pass-through devices, these small switches provide media conversion that is supplemented with security and store and forward capability. These switches' connector options follow the conventions of the media convertors described above, but in a more feature-rich tool. Sometimes the application is worthy of the extra features; sometimes the simple convertor is all that is needed.

The real world

Media convertors can be used in a variety of applications; they are used from the heart of the network, the server, to its end, the workstation or DTE.

The market

In addition to the general heterogeneous nature of network installations built over time, the increasing use of fibre optics in networking is one of the key drivers of growth in the media conversion market. The benefits of fibre are compelling: increased security, increased bandwidth and greater supported network distances. Distance specifications drive the argument for using fibre as the backbone medium ( in standard Ethernet and Fast Ethernet networks, fibre specifications prescribe maximum distance of 2000m versus the twisted pair limit of 110m. Gigabit Ethernet, a fibre-only specification, supports singlemode fibre in addition to the multimode fibre supported by its other Ethernet cousins. The same 2000m limit applies to multimode fibre, but singlemode fibre is supported for Gigabit Ethernet to a distance of 3000m.

Cost considerations drive the need to link other media to the fibre backbone ( having achieved the goals of greater distances, security and bandwidth in the backbone; network managers can often justify a step-down approach to the desktop for economy. A fibre-to-coaxial or fibre-to-twisted pair media convertor between the backbone and the desktop run makes it all possible.

The growing use of Ethernet and Fast Ethernet switches in the network infrastructure is also a factor. Switches solve many of the problems that are common to larger networks, but the majority of Ethernet and Fast Ethernet switches on the market today are equipped with twisted pair connectors. Where the entire network is built of twisted pair cabling, the switches are easy to integrate ( just plug one in. However, for many new installations, network managers are looking to fibre optic for their cabling infrastructure because of its security features, bandwidth capabilities and its ability to span longer distances. For older installations that use BNC connectors, the same incompatibility arises. A simple fibre to twisted pair media convertor or a BNC to twisted pair connector can make these devices work on incompatible networks.

The future is now

Moving forward, the market is heating up with the introduction of three new, next-generation, small form-factor, fibre connectors. These new connectors are being promoted as the future device of choice for all network hardware vendors: the MT-RJ, the LC and the VF-45. These new connectors are an extremely positive addition to the technology base because they solve some of the common shortfalls of SC and ST connectors in the areas of port density and termination. Some hardware vendors have already stated their commitment to one or another of these connector types ( it is only a matter of time before any one of them becomes the de facto industry standard. These connectors are being put to use on various types of network hardware, including the latest in hubs and switches. The connectors represent a significant advance in that, because of their size, more ports can be placed in a given device. This higher port density has the potential to deliver cost savings to the consumer, so early adoption is highly attractive. Without an accepted standard, however, early adoption can be risky.

These new connectors were developed to resolve certain issues inherent in the current ST and SC fibre connectors (i.e., port density and termination issues), and as switches and hubs are starting to utilise more and more fibre, the issues became more and more critical. Prior to the introduction of the next generation small form factor connectors, a switch or hub could support no more than 12 fibre ports stacked one-high in a rack-mount unit. The new fibre connectors double the capacity, accommodating as many as 24 ports in the same size chassis as today's 12 port device. These new connectors have the added benefit of being easier to use and thus decreasing cable installation time.

In the time that precedes the emergence of a single standard, Allied Telesyn is committed to providing media conversion products for all three of the connector types so that network administrators and architects are able to continue to purchase network building and expansion products with confidence. Through its introduction of media conversion devices for all three of the "next generation" small form-factor connector types, Allied Telesyn is making it possible for consumers to proceed with purchases, taking advantage of the economies that come with the new small form-factor connectors that they use. Consumers can make these purchases without worrying about whether they have made the right choice from among the new small form-factor fibre connectors ( no matter what they choose, the network will retain flexibility and connectivity options without risk of early obsolescence as the de facto standard shakes out.

Link integrity status monitoring

Network managers need to know that the physical network infrastructure is intact and operational, whether it is fibre, twisted pair or coax. In the case of twisted pair, link integrity is usually tested by sending a signal past the transmitter electronics when the cable is not in use. If link integrity is not confirmed, the link is assumed to be lost. In the case of fibre optics, link integrity is monitored via the power of the light being received. If it drops beneath a certain threshold, link is lost. The loss of a link is normally indicated by a link LED somewhere on the network hardware device. If the link LED is on, the assumption is made that the cabling has a clean connection with the other side. If the link LED is off, the user knows there is a cable connection problem.

Link status can be checked either by observation of the link LED, or, as is more common in larger networks, through the use of network management software (i.e., SNMP).

Allied Telesyn offers a link test switch with each of its media convertors. This switch allows network managers to determine whether or not a clean link has been established. When the switch is in test mode, the network manager can determine if any corruption is present in the fibre link. With the presence of a clean connection established, the manager can set the link test switch to normal mode and the media convertor will work in normal fashion. Other products with SNMP software management can interrogate the manageable switches that are in use and display the link status of a segment, in either a graphical or a tabular format.

In addition, the SNMP manageable switch may also support the ability to be configured so that an alert is sent to the management workstation when link is lost. Allied Telesyn's MissingLink is an extremely useful tool for monitoring. MissingLink provides the host (router, hub, switch or server) attached to either end of a convertor critical information about the status of the other (remote) segment link. If either link fails, the convertor interacts with both hosts, making each instantly aware of the link fault. Either host can then execute a pre-programmed, redundant transmission path selection.

Rackmountable design

The option of mounting media convertors is a rackmount chassis is useful where multiple convertors are in use, or where they are anticipated in the future. All of the media conversion products that are available from Allied Telesyn can be rack mounted using the optional AT-MCR12 Media Convertor Rackmountable Chassis. The chassis holds 12 media convertors and/or two port switches, has an internal power supply and fits into a standard 19in network rack.

Compiled by Mike Burkitt

Copyright (c) 1998 Allied Telesyn International Corp

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