Gig-E vs SONET

Gigabit Ethernet (Gig-E) is about to enter the long-haul data and voice network market with a compelling story that may prove as devastating to the competition, mainly Synchronous Optical Networking (SONET), as the original Ethernet was to Token Ring on the local area network (LAN) two decades ago.

Gig-E is the new darling of long-haul data transmission for one reason speed. Just as SONET was designed to handle the huge bandwidth of optical voice transmission, so can Gig-E manage the enormous capacity of optical cable on the data side. And since both are low-level transmission protocols, level 2 (Data Link) in the seven-layer OSI, both can support packet networking, a level 3 protocol that will continue to dominate long-haul data transmission and which is rapidly capturing an increasing share of the voice market. SONET, however, has never crossed from voice to data, chiefly due to its high price.

What makes Gig-E particularly compelling, says Jerald Murphy, Senior Vice President and Service Director for the Robert Frances Group, is its price tag. While there is variation among equipment suppliers, Gig-E can cost as little as a quarter of the price of an equivalent SONET system. Furthermore, it is much easier to add capacity to a Gig-E network than to SONET. The trade-off, however, is in transmission quality. Gig-E is more vulnerable to interruptions and signal quality problems.

Behind these differences are two different design philosophies. SONET was designed to guarantee high quality voice service, including a 50-millisecond recovery rate from a transmission line failure. To achieve those quality guarantees, it has a double-ring architecture, with one ring usually running in the opposite direction from the other. This gives it a physical fail-over -- if one ring fails the other can take over the entire transmission and the session is never interrupted.

In contrast, Ethernet was developed for data LANs at a time when short-range transmission quality had improved to the point that the Ethernet designers could presume that the data would get through, says Murphy. They eliminated the parity checking and redundancy features of earlier LAN protocols and left data quality assurance to higher levels in the OSI stack, creating a streamlined data transmission solution.

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