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.