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To: Glenn D. Rudolph who wrote (763)	12/16/1997 2:38:00 PM
From: Maverick	Respond to of 1629

GRF offers IP over SONET i/f Cell Taxes': A Necessary Evil? By Pankaj Chowdhry December 12, 1997 2:39 PM PST PC Week With the rise of IP throughout global networks, many high-volume links are trying to sidestep the "cell tax" that burdens ATM. But, as with most taxes, there is no way to completely avoid these costs. Most carriers use SONET (Synchronous Optical Network) as the OSI Layer 1 specification for data transmission over optical fibers in the public network, and ATM is then used as the OSI Layer 2 link. ATM's cell tax--the extra data that must be added to a packet to traverse an ATM link--can eat up more than 10 percent of ATM's raw bandwidth. IP over SONET is gaining popularity as a way to reduce the ATM cell tax and squeeze more bandwidth out of a link. IP over SONET removes ATM from the transmission picture, forgoing the traditional method of encapsulating IP packets into an ATM cell, which is then mapped into a SONET frame. Instead, the technology maps the IP packet straight into a SONET frame. The nominal data throughput rate of an OC-3 link is 155.52M bps. The first "tax" to be paid on this link is the management overhead inherent in SONET, which reduces the data rate to 149.76M bps. The major hit comes when ATM is placed on the link whose overhead reduces the data rate to 119M bps. When IP packets are placed within the ATM cells, the data rate is reduced even further, to 123M bps. Without ATM, IP frames are mapped straight into the STS-3c frames of a SONET link, which, when combined with the overhead of IP, provides a transmission rate of 135M bps. On expensive wide-area links, this 12M bps can mean a lot: Using ATM instead of IP over SONET is basically throwing away 10 T-1 lines' worth of data transfer time and money. However, not all networks can reap the benefits of IP over SONET. First, all data traversing the link must be IP. Because 155M bps of IP data is not easy to come by, this requirement is a major drawback of using IP over SONET. ATM can be more easily statistically multiplexed than IP over SONET, however, and ATM can be run on lower-speed links.

To: Glenn D. Rudolph who wrote (763)	12/16/1997 2:39:00 PM
From: Maverick	Respond to of 1629

GRF IP switch offer IP over SONET i/f, part II Virtually disconnected The cell tax is not the only reason for eliminating ATM from the wide-area equation, however. ATM is by definition a connection-oriented protocol, requiring virtual circuits to be set up for each flow. IP by its nature is connectionless and doesn't require multitudes of virtual circuits, as does ATM. Setting up these virtual circuits takes huge amounts of processing power and adds to the latency of ATM transmissions. There is also a lot of redundancy in running IP over ATM. The most common transport for IP is TCP, a control mechanism that has a provision for reassembling out-of-order packets. ATM also has this function, which proves useless when combined with TCP/IP. Another concern is the anemic 53-byte cell size in ATM. The minimum 64-byte IP data packet must be split into two ATM cells for transport. If either of these cells is dropped, the entire IP packet must be re-sent. The idea of removing ATM from a network transmission scheme might sound appealing, but the migration to IP over SONET is difficult, because the management infrastructure required for SONET is completely different from ATM's tools. The traditional data network can be traced from end to end with time-tested tools, through the different topologies of a switched Ethernet network to an ATM network running LAN emulation services. Once SONET is introduced, the landscape changes completely, and well-known tools no longer suffice. In the SONET world, such things as SNMP are nonexistent. Instead, most carriers use the TL-1 protocol, with a planned migration to the Common Management Information Service Element.

To: Glenn D. Rudolph who wrote (763)	12/16/1997 2:40:00 PM
From: Maverick	Respond to of 1629

GRF offers IP over SONET, part III Security implications must also be considered. ATM virtual channels can be set up to run multiple secure conversations on the same link. IP over SONET does not offer this capability, although similar services can be imitated by using advanced IP techniques. By far, the easiest places to implement IP over SONET are high-volume point-to-point configurations. High-traffic Web servers will also benefit from IP over SONET, as is evident by Internet Systems Inc.'s embrace of the technology. ISI provides Web hosting to high-volume sites such as Netscape Communications Corp. and Yahoo Inc. IP over SONET will become more pervasive as high-end routers become equipped with packet-over-SONET interfaces. These interfaces are offered in Cisco Systems Inc.'s recently released GSR 12000 routers and in Ascend Communications Inc.'s GRF IP switches.