Which transfer protocol is best?

Just as determining which type of connection to use is affected by a variety of factors, so is selecting the best transfer protocol. The factors involved in selecting a transfer protocol include speed, cost, reliability, compatibility, availability and technical support.

If your business needs to regularly transfer data over standard phone lines, speed is important in keeping phone charges down. The speed of a transfer is controlled by the connection's speed and the protocol's capabilities. The more efficient a protocol is when communicating with the remote system, the faster the transfer. Some protocols use data compression or a "sliding window" to improve speed. A "sliding window" means the size of the packet can be increased or decreased, depending on the speed and reliability of the physical connection. XMODEM, YMODEM and older versions of Kermit don't support data compression and sliding windows. ZMODEM and newer versions of Kermit do support data compression and sliding windows. Commercial transfer protocols vary in their support of these features, but a few like WTERMCRC include both features.

If cost (other than phone charges) is a major factor, you'll probably choose a public-domain protocol. These protocols are available free or at a minimal charge, depending on the source. Commercial software can cost several hundred dollars because it usually includes many other features such as terminal emulation and scripting capabilities in addition to transfer protocols. Depending on your needs, these features may outweigh the additional cost. However, when selecting a protocol on price, don't underestimate the importance of your data. Corrupted or incorrect data costs money - your business requirements determine how much.

During a transfer, data can be corrupted by static and power surges. Reliability is essential. A data transfer that leaves data garbled and inaccurate is useless. While most protocols are reliable, from a statistical perspective commercially available protocols are more reliable. With a dependable physical connection, you can usually rely on a public-domain protocol. If your physical connection is unreliable, you should consider a commercial software product. If your connection is reliable like a short, direct serial line, and you want to transfer a text file, you may be able to forego a transfer protocol and send the file by "dumping" it right to the serial port. "Dumping" can be done using the DOS TYPE command or the UNIX cat command redirected to the serial port. This method is not recommended for regular data transfers or other types of connections. If you need consistent reliability, use of a commercial protocol is recommended.

As mentioned earlier, a data transfer requires the same protocol on each end of the connection. If you use ZMODEM on the local side, you must use ZMODEM on the remote side. Also, be aware that there are different versions of each protocol. Over time, public-domain protocols have been improved or modified. Therefore, it is possible to have an older version of a protocol that does not work with a newer version of the same protocol. A well-designed protocol will maintain compatibility with older versions.

Many times the remote system dictates which transfer protocol to use. For example, if you connect to an on-line information service via modem, your choice of protocol depends on what the service supports. Most on-line services support several transfer protocols including XMODEM, YMODEM and ZMODEM. Some even have their own proprietary protocol such as CompuServe B. When faced with several choices, speed and compatibility become determining factors. If you connect to multiple on-line services or bulletin board systems, a commercial package with support for many different protocols provides flexibility.

Consider the level of technical support available for the protocol you select. Most public-domain protocols have been developed by members of academia and receive minimal support. With these protocols, if you experience difficulties your options are limited to sending an e-mail message or asking a colleague for assistance. If your problem is "user-created," your e-mail message may not receive much consideration. Unlike public-domain protocols, commercial protocols are developed and maintained by companies which usually provide technical support by phone, fax and e-mail for all problems. The best companies provide technical support for minimal or no charge.