Although virtually any information can be encoded, most bar codes are used to encode an item identifier that, in and of itself, provides no useful information. Instead, the identifier contains a key to an entry in a database which contains the actual information (e.g. price, manufacturer, model, etc.) about the item. Thus, bar codes are seldom useful by themselves and typically have a companion database to provide the actual information about the item. When an item is scanned, the item identifier is used to look up the item in the database which returns the useful information about the item. While there are specialty types of codes that are different (including two-dimensional bar codes that look radically different), most bar codes look something like the following:
The bar code information is encoded as a series of varying width vertical lines (called bars) and spaces between the bars. Different combinations of bars and spaces represent different characters. Different bar code specifications (also called symbologies) use different combinations; thus, bar codes produced under different specifications will vary in how easy/hard they are to print and read, the space requirements for the code (often called the bar code density), and what information can be encoded. Most bar code implementations print the bar code information in text below the bar code; however, this text is for human reading only and is not readable by the scanner.
The bar code is read by a scanning process where a scanner shines a light on the bars. The light is absorbed by the dark bars, but is reflected by the spaces between the bars. A photocell detector converts the reflected light into an electrical signal. The waveform shape of this signal can then be used to decode the bar code.
Most bar code symbologies include extra space and extra characters in addition to the actual characters encoded in the bar. Some common uses for this extra information are:
- Quiet Zones: This is unused white space on either side of the bar code. It separates one bar code from another.
- Start and stop characters: These are special patterns of bars that begin and end each bar code. They are needed for encodings that allow a variable number of characters. (Some encodings like UPC are for a fixed number of characters.)
- Check digits: These are checksums that provide an accuracy check to be sure the bar code has been read properly. Typically, the numeric value of each character in the code is added to a sum which is divided by a fixed constant. The remainder (mod) of the number is then encoded.
| Bar code | Variable Length | Allowable Characters | Industries in use | |
| Older Bar codes | ||||
| Code 11 | Yes | 0-9 | AT&T pre 1990 | |
| Codabar | Yes | 0-9,$+.:/ | Blood Banks, Cotton, Transportation | |
| Plessey | Yes | 0-9,A-F | Shelf Labels | |
| MSI | Yes | 0-9 | Shelf Labels | |
| 2 of 5 | Yes | 0-9 | UPC Shipping Container | |
| UPC and EAN | No | 0-9 | Food/Discount Store Items | |
| Newer Bar codes | ||||
| Code 39 | Yes | 0-9,A-Z./+-%$Spc (2 character pairings for Full ASCII) | LOGMARS, HIBCC, AIAG, TCIF | |
| Code 128 | Yes | Full ASCII | UCC_128, EAN-128 | |
| Code 93 | Yes | 0-9,A-Z./+-%$Spc (2 character pairings for Full ASCII) | HIBCC Alternative, Canadian Postal Service | |
For this application (encoding the string "CSE370"), we require an encoding that provides for alphabetic characters as well as numeric characters and a variable length. We assume that we want to encode the string directly using ASCII character encoding, and not representing it in some other way. Since Code 128 provides for this, is widely used, and offers excellent density, we've concentrated on this code in the detailed discussions below, although we've used other encodings to contrast the differences.
