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High-Speed i-Mode Adopts TCP/IP

NTT DoCoMo Co, Inc of Japan will offer an i-Mode service via FOMA, a new service compliant with International Mobile Telecommunications-2000 (IMT-2000), scheduled to start in May 2001. This “high-speed i-Mode” will be able to handle large files, application program downloads and video distribution, opening up a new dimension in applications.

To make the new service possible, a portion of the communications protocol will be changed from the existing i-Mode system.

“We have always worked towards open cell phones,” says Takeshi Natsuno, executive director, Gateway Business Department at NTT DoCoMo. The firm has adopted technologies that are common to the Internet, like the hypertext markup language (HTML) for content description, and the Java program description language, promoting expanded participation by application developers. The new protocol change is also intended to facilitate the incorporation of new Internet technologies, and to invite participation by application developers.

The current i-Mode content description language and application layer protocol are compatible with the Internet, but the service does not use the core communications protocol group of the Internet, namely Transmission Control Protocol/Internet Protocol (TCP/IP). Instead, it uses its own Transfer Level (TL) Protocol. The goal was to reduce overhead caused by the communications protocol, boosting communications efficiency.

When FOMA enters service, it will offer a peak downlink (network-to-user) speed of 384 kbits/s, and a peak uplink speed of 64 kbits/s. This will make large-capacity data handling possible, and reduce the relative weight of the overhead, making it possible to switch to TCP/IP.

Modifying TCP

The transport layer will use a modified version of standard TCP, called wireless-profiled TCP. A number of modifications to TCP have been developed, and released on the Internet. NTT DoCoMo has collected the requisite functions from the modifications to TCP to create the wireless-profiled TCP.

There have been four key modifications, the first of which was the removal of a function that automatically controls the window size (data block received at one time) to suit network traffic. NTT DoCoMo recommends using a fixed window size of 64 kbytes.

In standard TCP, it is possible to send multiple data units at once to boost communications efficiency, and to send the next data when a response has been received. The window size for reception data changes in response to network conditions, based on the actual response time. In a wireless network environment, however, the response time could be considerable; the standard TCP window size could remain small, and the effective speed could remain low, even if the actual transmission path speed is high. For this reason, NTT DoCoMo recommends a fixed window size.

The second modification was to the TCP slow start function, which was changed to provide a larger window size so that the service can make full use of the high-speed transmission path from the initial packet.

Both changes were made on the assumption that the modified TCP would only be used on the firm’s network. Changing the window size on multi-network paths would cause considerable network congestion.

For the third modification, NTT DoCoMo simplified the data resend function in the event of a transmission error; and the last modification was to expand the data size to 1,500 bytes maximum, to boost datacom efficiency.

Proposed Standard

NTT DoCoMo has proposed wireless-profiled TCP as “A TCP profile for W-CDMA: 3G wireless packet service” to the Internet Engineering Task Force (IETF), an international standardization organization for Internet technology. The protocol stack, including wireless-profiled TCP, has been proposed to the WAP forum as “WAP-NG” (next-generation), to be officially announced in June 2001.

The WAP-NG proposal has already been endorsed not only by Japanese manufacturers, but also by Nokia, Ericsson, and Motorola, Inc, as well as by operating system developers Microsoft Corp and Sun Microsystems, Inc. There have been no announcements, however, as to whether or not this protocol will be adopted by these firms for use in any of their products.

by Takahiro Kikuchi

(April 2001 Issue, Nikkei Electronics Asia)

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