Hail

A hail is a form of subspace communication between starships, starbases, and other ports of call. It has been in use since at least the mid-22nd century and can be digital (computerized messages), audio, or visual.

Most hails are visual, although routine operations can be handled by the computer or communications officer without requiring the commanding officer's involvement. In addition, hails may be audio-only. The Earth-Romulan War was conducted entirely over audio channels, as neither side had the technology to send visual images to the other.

A hail, whether audio, visual, or computerized, involves two-way communications. A hail is sent by one party, and must be responded to by the other to initiate communications. When a recipient is unwilling to accept a hail, a one-way wide-band subspace transmission can be initiated by "opening a channel," although it is customary to answer all hails. Such a message is usually not directed at any specific target, and there is no guarantee that it will not be eavesdropped upon if it is earmarked for a specific recipient. In fact, there is no guarantee that it will be received at all. However, it can be reasonably assumed that any race that posesses the technology to receive these messages would probably view or listen to them out of curiosity, if nothing else. If needed, the receiving party can escalate the communication into a full two-way conversation by starting a return transmission.

Frequencies
There are many subspace frequencies over which hails are sent. Most of the major powers, such as the United Federation of Planets and the Dominion, have their own frequencies and protocols for sending messages such as distress calls, but there are also general frequencies. 

When contacting a starship or space station the normal command was "open hailing frequencies". ( et al.)

Many types of atmospheric disturbance and certain elements or compounds will prevent hailing signals from reaching their destination. Engineers can often find creative ways to solve this problem, but some conditions are impossible to penetrate.

Jamming of hailing frequencies is a common technique used for purposes such as disrupting enemy communication in battle and preventing vessels from emitting a distress call. Most jamming techniques do not prevent the jamming party from sending out its own hails. Of particular note is the Breen method of sending out a rotating electromagnetic pulse. 

Non-contact hail
Hailing does not require direct communication with the other party; messages relating to routine procedure or an unimportant matter can be handled via computer signals. For example, when Jem'Hadar attack ships request ketracel-white from Dominion installations, the procedure is conducted entirely through computerized hails. It is more common for a computerized message to precede visual contact, however. 

When dealing with a vessel or location whose hailing protocols or capabilities are unknown, a general message can be sent over wide-band subspace. Additionally, wide-band subspace can be a technique for contacting anyone in the vicinity during dire situations.

Standard hail
The most common type of hail is through audio or visual contact. The recipient must first accept the transmission in order to initiate such contact.

Making visual contact is normally standard, but it is not required. An audio link can be established when either a visual link is not possible or one does not wish to be seen. Some foreign entities are also incapable of a visual link and thus communicate over audio channels.

The more common method involves a two-dimensional display showing the party on the other end. The area displaying the image is known as the viewscreen. One can pause the hail, which briefly stops the transmission, as well as mute the other side when necessary.

Standard hail normally takes place between two parties, although three-way hails are possible and have been used on occasion. 

Holo-communications
Holo-communicators were a new type of communications device used briefly by the United Federation of Planets circa 2374. They utilized holographic technology to create a life-size version of the person on the other end of the hail. As Captain Benjamin Sisko said to Captain Sanders when he first used a holo-communicator, "It looks like you're on my bridge!"

Although holo-communicators had the potential to revolutionize hailing as we know it, this was not to be. It is possible that holo-communicators were too costly or incompatible with normal hailing devices; for whatever reason, they never saw wide use.

Holo-filters
When making visual contact, a person can often use a holo-filter to mask their appearance. However, if the transmitting ship is scanned at close range, sensors could alert the recipients to the person's true identity. 

Background information

 * Hails have been used in all incarnations of Star Trek. The exact point at which they were developed has never been stated.


 * Like subspace itself, the nature and specifics of hailing technology remain largely unexplored.


 * In truth, the holo-communicator was a piece of "Treknology" that never caught on. It was seen in several DS9 episodes and never seen again.