[#ca]: CA

This could refer to Certificate Authority. The abbreviation “CSMA/CA” uses these initials (to mean “collision avoidance”). “.ca” is the international domain name for the nation of Canada (and “.ca.” is the FQDN). States in The United States of America are often abbreviated with a two-letter code (perhaps thanks primarily to the United States Postal Service), and the state of California uses this same two letter abbreviation.


The term “cable” may refer to a single wire used to transmit information (e.g. a “television coax cable”), or a group of wires. For example, a telephone cord actually consists of multiple wires that are then wrapped up, by plastic insulation, into a larger bundle. With the Cat5 cabling (described by copper cabling categories), which has been commonly used with Ethernet, the most common term has been “cable” (rather than “cord” or “wire”).

The signal does not need to be electricity transmitted over copper. Another example of cabling is the use of a “fiber-optic cable” which is designed to allow light to pass through it.

The term has also been used to refer to companies that have used the infrastructure to provide television content over cabling which was dedicated for that purpose (of providing television content). Such television programming was often referred to as “cable TV”. When these companies also started providing Internet access, the Internet services became known as “cable Internet”. So, today, “cable” service tend to refer to one or more of those specific services.

[#canonnic]: canonical format NIC link-layer addresses

See EUI addresses: section on canonical form.

[#cem]: Career-Ending Move (CEM)

An action, generally referring to a mistake, which is so bad that it not only results in the perpetrator's current job being ended, but will make it very difficult for the perpetrator to ever be placed into a position of trust in the field. For example, not only is the employee fired, but everyone in the company (managers, HR, etc.) know that the only deserved reference is bad, and maybe the incident was even covered by Slashdot or some other national headlines.

This term is a bit of a joke: unfortunately many companies do not do their due diligence in checking history of people that are placed into trusted positions. This does result in there still being opportunities for people.

[#csma]: Carrier Sense Multi(ple) Access (“CSMA”)

This is most commonly used as part of a larger abbreviation: either CSMA/CA or CSMA/CD.

CSMA often initially sounds complicated, but it isn't too challenging to understand when broken down into pieces.


A “carrier signal” is simply a signal that does (intentionally) carry some information.

Newer text for “carrier sense”
“carrier sense” refers to devices checking whether media (such as an electrical wire, or the nearby airwaves) seems to be available. If the “carrier signal” is “up”, indicating that the media is being utilized, then it is not avaialble.
Older text for “carrier sense”
A device tries to check whether the media is being used to carry a signal. If it is actively being used, the device will wait.
Multi(ple) Access
Newer text

Most commonly: “Carrier Sense Multiple Access”. Although, (perhaps?) it might sometimes be referred to as “Carrier Sense Multi Access” or “Carrier Sense Multi-Access” Which, basically, means the same thing.

All this refers to is the idea that there may be multiple devices (such as computers) that may want to share the media (such as an electrical wire) to be able to communicate on the network. The “multiple access” simply refers to the idea that the network is, or may be, accessed by multiple devices. This may often mean that devices are needing to take turns, so that other devices may also communicate.

Multiple devices may use the media.
The CA of CSMA

“Collision Avoidance” refers to a process that tries quite hard to prevent collisions.

(Newer text)

CSMA/CA probably sounds better than CSMA/CD so far, but there is one notable detail: CSMA/CA is notably higher overhead. Typically, wired communications use CSMA/CD so as to avoid that rather unnecessary overhead. However, collisions may be quite painful for wireless devices, and wireless devices may be more likely to cause some accidental collisions. (The reason is simply because a centralized device, like a BSS, may experience interference from one laptop. Another laptop, located on the opposite side of the BSS, might be out of range of the first laptop, and may not notice the communication of the first laptop.) Because of the increased likelihood of more collisions that are more expensive (as measured by the amount of time needed to deal with problems), Wi-Fi uses CSMA/CA. The increased overhead of CSMA/CA is deemed likely to be less than the increased overhead caused by the collisions that hopefully CSMA/CA will avoid.

(The following is believed, based on memory of an old study... It may need review.) With Wi-Fi, the CSMA/CA process may often involve a single device, called a “BSS”, giving permission for other devices to communicate for a certain length of time.

(Older text)

Devices are a bit more paranoid about trying to avoid collisions. Rather than just transmit a lot when the media looks free, they may try to get permission from a centralized source (the BSS of a standard Wi-Fi connection). This has higher overhead than collision detection, but may make collisions even less likely. This overhead is deemed acceptable for wireless because collisions could occur easier, and may even have a higher cost when they do occur.


“Collision Detection” refers to devices scanning for problems (namely electrical “reflections” when a signal gets reflected on a wire) and then proceeds to handle an existing collision. A collision is more likely with CD than with CA.

If two devices check a wired connection, and the wire appears to be unused, both devices may decide to start sending traffic. (This could, in theory, happen even after one device has started to use the wire, if the electrical signal hasn't yet travelled down the wire far enough for the second device to notice a problem). This is likely to lead to a “collision”. What happens (at least with Ethernet) is a jamming signal is then sent out, letting all devices know that recently received traffic has been, or at least might have been, corrupted due to a collision. The jamming signal gets broadcast on the wire for some time: enough time that an electrical signal could travel down the entire length of the wire in both directions. After that point, devices wait for a period of time. The term “backoff timer” is used to describe the period of time that all devices wait. Once that period of time has completed, each device uses a random (or, more likely, pseudo-random) number to determine when it may start trying to transmit again. (So, if the device has a large random number, more waiting may be required.) Hopefully that will be sufficient for one device to “win” the “race condition” so that devices don't again try to transmit at the same time. After waiting for the appropriate amount of time, the device will then try again, starting with the process of performing carrier sense. Hopefully that is sufficient so that there won't be another collision when data gets re-transmitted.

[#ccitt]: CCITT (Comité Consultatif International Téléphonique et Télégraphique)

Comité Consultatif International Téléphonique et Télégraphique was the (French) name of a committee that was in charge of some standards related to telephone communications. It may also have been known as the “International Telegraph and Telephone Consultative Committee”, but even still the name was internationally abbreviated by the initials of the French name: CCITT. The organization has since been renamed to be the “ITU Telecommunication Standardization Sector” (“ITU-T”). This name itself references the ITU. See: Wikipedia's page for CCITT (which redirects to a page about ITU-T).

[#cemabbr]: CEM
See: Career-Ending Move
[#chroot]: chroot
Short for “change root”, this concept is basically the same as a sandbox (or a jail).
[#cidrabbr]: CIDR
See: Classless Inter-Domain Routing (CIDR).
[#class]: class
[#classobj]: classes, as related to objects

Used as a computer term, the phrase class most often refers to its usage with object-oriented programming (“OOP”). In this sense, a class refers to the specifications/blueprints/details/guidelines about what an object may look like.

The following details will probably make more sense to computer programmers who are already familiar with OOP: A class will typically contain methods, which are references to functions. It may also contain properties, which are variables, and may also contain inheritance details. (A class with nothing but properties may commonly be called a class, although the term struct would often be even more appropriate. However, some languages might not have a reference to the term “struct”, and in such a case, the term “class” may be the term that is most commonly used when using that programming language.)

The term may also refer to a classication. A key example of this may be logging into a Unix (or perhaps, more specifically BSD?) where a user may be assigned to a login classification. Especially in that context, the term “class” is very often used instead of the word classification.

Of course, there are other meanings of the word class, such as a gathering to learn, or the character trait. Public IPv4 networks have been assigned to a “class” which once determined how many addresses were provided when an organization obtained a group of addresses. Unix users may belong to a “login class” (which may be referenced in the documentation related to the login.conf file).

[#cidr]: Classless Inter-Domain Routing (CIDR)

This refers to a process of supernetting. (See: supernet.) IETF BCP 122: Classless Inter-domain Routing (CIDR): The Internet Address Assignment and Aggregation Plan. (BCP 122 is, at the time of this writing, RFC 4632. RFC 4632 obsoleted RFC 1519, which obsoleted RFC 1338: “Supernetting: an Address Assignment and Aggregation Strategy”).

Just as the acts of subnetting and supernetting are similar (see: subnet and compare to supernet), this process is very similar to applying VLSM principles. In fact, VLSM is compared to CIDR so often that many people are not aware of any difference, and freely use the term CIDR when they are referring to VLSM. The two terms are extremely similar in concept: CIDR refers to combining smaller networks into a larger supernet, while VLSM is the process of splitting a larger network into smaller subnets. This is a minor distinction: Both processes require knowledge of what smaller subnets fit within a larger supernet. The only difference is the direction: CIDR is about going up to larger network sizes, while VLSM is about breaking them down into separate smaller subnets.

[#ckt]: “circuit” abbreviated (“CKT”)
Forum posting about the abbreviation for “circuit” being “CTK” identifies this as a standard which was “superseded” in 1998, and which has been used in the military. Even if the standard is now considered to be superseded, the term may be seen on some electrical equipment.
[#acrcli]: CLI

CLI stands for “command line interface”. (See: user interface basics: command line interface.) This is often used in environments that may have displays limited to text output. Some TUI (text user interface) programs, which can use text-mode to draw menus, may also often be used in such environments. One advantage to CLI is that it is often a relatively easy method to be implementing a lot of automation.

[#acrcms]: CMS

This may refer to:

Content Management System

A content management system may keep track of document revisions.

Customer/Client Management System

A customer management system may keep track of information about customers, such as the name, address, and phone number. If the customer is an organization (e.g. a business/company), further details may include information about with whom (which person, or department), at that company, positive interactions is typically made. These sorts of details can be used for improved customer service, keeping track of details for shipping information, and for marketing purposes.

[#collisn]: collision

When two devices try to communicate at the same time, but when this is not possible, there may be a “collision”. This often requires that some sort of activity is done to “clean up” the situation (e.g. Ethernet sends out a “jamming signal” to make sure any stray electrical signals are flushed off of the wire) and then the information may need to be re-sent (hopefully in a way that does not produce another collision). This may be handled by one of the forms of CSMA.

Prior to the dominance of Ethernet, proponents of Token Ring would often cite the fact that Token Ring does not have collisions. (Ethernet Collisions were a bigger problem back in the day. The problem was largely resolved when switches started to replace hubs.)

On a related note: an Ethernet switch on a point-to-point may have some superior ways of handling a collision. At least in this casae, the term “point-to-point” link is referring to when a switch is connected directly with a network device like a NIC on a computer, or a router, or even another switch, and not connected to something like a “hub” which might not support “full duplex”. A switch can often remember what data has been recently transmitted on an interface port that is using a point-to-point link, and the switch can interpret the reflections of a collsion, and understand what incoming data would have caused those reflections. Therefore, an Ethernet switch on a point-to-point link can often communicate in “full duplex” mode, so the results of collisions may really be a non-issue. There may be no impactful collisions when using a point-to-point connection on a switch.

[#colisndm]: collision domain

While the usage Ethernet hubs could easily allow some particularly damaging collisions, an Ethernet switch detects collisions and does not forward the problem onto other interface ports. Therefore, the problems of a collision become limited to only what is plugged (directly or, when Ethernet hubs get used as well, indirectly) to one interface port on the switch.

The switch, therefore, becomes a device that separates a network in a “collision domain”. A collision will only require collision handling to be performed by devices that are part of the same single “collision domain”.

[#connctix]: Connectix

Connectix is the name of a now-defunct company.

Connectix made software of a variety of software, often providing functionality for the Apple Macintosh platform before the functionality was provided by Apple. Virtual memory was available from Connectix years before Apple's solution. Other products which were memory related provided functionality that allowed more memory to be free (due to the use of compression), the simple yet potentially extremely impactful ability to access a larger amount of physical memory, and storing some data in memory for increased speed. Increased speed was another accomplishment achieved by some other Connectix software which users were using that software instead of whatever came with the operating system. Software compatibility was increased through the use of emulation offerings by Connectix. The company also released one hardware product, Quickcam, before that product line was sold to Logitech.

The company ended up having some difficulties selling the “Virtual Game Station” line because of a lawsuit which Sony filed against Connectix. Although Sony officially lost the lawsuit, the legal process prevented Connectix from selling VGS for much of the time when the Playstation 1 was a leading video game console (before the release of Sony's Playstation 2 system). Shortly after that lawsuit, Connectix sold Virtual Machine technology to Microsoft and Connectix discontinued all of the Macintosh products. The company is labeled as “Defunct” since “August 2003” by Wikipedia's page on Connectix which lists other products released by this innovative company that seems to have been brought to ruin by a bigger company, Sony. However, although Sony clearly objected to the Video Game Station product, there is no clear indication that Sony would have objected to the other products such as RAM Doubler. The fact that such software was discontinued, rather than being sold or made available for free, might have more to do with Microsoft's views of the Macintosh platform or software which isn't sold.)

(The transfer of software from Connectix to Microsoft took place shortly after Sony harmed Connectix via a lawsuit over Connextix Virtual Game Station, though being essentially forced to sell some of the business's assets and *then* closing up was a more profitable fate than the sad fate of the company known as “Bleem!”)

[#credenti]: Credentials

A reason why someone should be trusted. e.g., a password, or possession of an SSH key, or the ability to supply needed data when queried (such as biometric data when scanned). Perhaps even better is when multiple credentials are successfully used to satisfy the need for verifying authenticity.

[#csmaabbr]: CSMA
See: Carrier Sense Multi(ple) Access.