Hardware

More modem basics

Despite the speed of DSL, it's unlikely that the 56-Kbps modem will become extinct soon, which means that it's time for a refresher course on modems. Learn about error detection, compression, and why synchronous communications are sometimes necessary.


In this lightning-fast, high-bandwidth millennium, the lowly modem is still chugging along in homes and offices around the world. Support pros beware: This aging technology is not yet obsolete. If you haven’t given modems much study time in the past, take a minute to learn about a few of the basics of this tried-and-true device.

In this article, I’ll discuss why modems sometimes need to use synchronous communications and how those synchronous communications work. Then I’ll discuss other concepts related to modems, such as baud rate, flow control, error detection, and compression.

Modem basics, part 1
In the first installment of this series, "Know your modem basics," I discussed how a modem converts digital signatures from a computer into analog signals that can be passed over a phone line and then converts those analog signals back into a digital signal that the computer on the other end can understand. I also discussed the concepts of synchronous and asynchronous communications and explained that during normal communications, modems typically use asynchronous communications.

Synchronous communications
The difference between synchronous communications and asynchronous communications is that synchronous communications are highly regulated and asynchronous communications aren’t. Although normal modem communications are usually asynchronous, there are times when synchronous communications are desirable.

Consider the case of a file download. If you download an executable file and even one bit in the file is incorrect, the file may not work at all. Therefore, when you download files, there must be a way to guarantee that the file has been received correctly.

Synchronous communications work by using a protocol to regulate the data. Both the sender and the receiver must be using the same protocol, or the process won’t work. Some examples of synchronous protocols are Xmodem, Ymodem, and Zmodem.

The actual synchronization method varies depending on which protocol is being used. Generally speaking though, the transmitting modem signals the start of a block with a start-of-text marker and finishes a block with an end-of-text marker. The end-of-text marker includes checksum information that’s been calculated by applying a formula to the data that was sent. When the receiver receives the block, the same formula can be applied to the data to see if the result equals the checksum value. If the values match, then the receiver sends an acknowledgment message to the sender. If the values don’t match, then the receiver sends a negative acknowledgment to the sender, at which time the sender will retransmit the block.

Baud rate
Baud rate is a term that you don’t hear too often anymore. In the past, a modem’s speed was associated with its baud rate. For example, a 300-baud modem moved data at 300 bits per second (bps). However, baud and bps are completely different. The baud rate is simply the cycle time that a modem uses as its carrier frequency. However, standard phone lines can only accommodate a baud rate of 2400.

The reason that a modem’s speed was conveyed through its baud rate in the past is that modems were only capable of transmitting one bit per cycle. Therefore, a 300-baud modem was actually moving 300 bps. However, the maximum baud rate is 2400 baud, and we all know that modems can transmit more than 2400 bps. The 2400-bps barrier was solved by designing modems that could move more than one bit per cycle. For example, a modem with a baud rate of 2400 that moves 4 bits per cycle is transmitting at 9600 bps.

Error detection
There are two different types of error detection. Error detection is normally associated with synchronous communications, as I described earlier. However, some modems perform hardware-based error detection. Hardware-based error detection is extremely fast and is invisible to the software. To take advantage of hardware-based error detection, both modems must support it.

Flow control
Modems rely on start bits and stop bits to determine the beginning and ending of a packet. Likewise, modems must use flow control to determine the start and end of a communication stream. Flow control is also sometimes referred to as handshaking, or XON/XOFF. XON/XOFF is an almost extinct, software-based flow control method. Today, flow control is almost always performed at the hardware level.

Compression
One final concept that you should be aware of is compression. Compression works similarly to zipping a file. The idea is that the modem compresses data before transmitting it, and the receiving modem decompresses the data before sending it to the computer’s serial port. By using compression, modems can transmit data much more quickly than they could without compression. Although compression was once proprietary to each brand of modem, today, there are standards for modem compression.

56-Kbps modems: Down, but not out?
Does your organization still widely use 56-Kbps modems? Do your mobile users all have high-speed network access, or do they still rely on a 56-Kbps connection? Post a comment to this article and let us know if the 56-Kbps modem is still alive and well in your organization.

 

Editor's Picks