CHAPTER IV
SIGNALING SYSTEM
I. SIGNALING SYSTEM
A. MEGACO
Megaco/H.248, the Media Gateway Control Protocol, is for control of elements in a physically decomposed multimedia gateway, which enables separation of call control from media conversion. The Media Gateway Control Protocol (Megaco) is a result of joint efforts of the IETF and the ITU-T Study Group 16. Therefore, the IETF defined Megaco is the same as ITU-T Recommendation H.248.
Megaco/H.248 addresses the relationship between the Media Gateway (MG), which converts circuit-switched voice to packet-based traffic, and the Media Gateway Controller (MGC, sometimes called a call agent or softswitch, which dictates the service logic of that traffic). Megaco/H.248 instructs an MG to connect streams coming from outside a packet or cell data network onto a packet or cell stream such as the Real-Time Transport Protocol (RTP). Megaco/H.248 is essentially quite similar to MGCP from an architectural standpoint and the controller-to-gateway relationship, but Megaco/H.248 supports a broader range of networks, such as ATM.
There are two basic components in Megaco/H.248: terminations and contexts. Terminations represent streams entering or leaving the MG (for example, analog telephone lines, RTP streams, or MP3 streams). Terminations have properties, such as the maximum size of a jitter buffer, which can be inspected and modified by the MGC.
Terminations may be placed into contexts, which are defined as when two or more termination streams are mixed and connected together. The normal, "active" context might have a physical termination (say, one DS0 in a DS3) and one ephemeral one (the RTP stream connecting the gateway to the network). Contexts are created and released by the MG under command of the MGC. A context is created by adding the first termination, and it is released by removing (subtracting) the last termination.
A termination may have more than one stream, and therefore a context may be a multi stream context. Audio, video, and data streams may exist in a context among several terminations.
Protocol Structure - Megaco/H.248 (Media Gateway Control Protocol)
All Megaco/H.248 messages are in the format of ASN.1 text messages. Megaco/H.248 uses a series of commands to manipulate terminations, contexts, events, and signals. The following is a list of the commands:
• Add. - The Add command adds a termination to a context. The Add command on the first Termination in a Context is used to create a Context.
• Modify - The Modify command modifies the properties, events and signals of a termination.
• Subtract - The Subtract command disconnects a Termination from its Context and returns statistics on the Termination's participation in the Context. The Subtract command on the last Termination in a Context deletes the Context.
• Move - The Move command atomically moves a Termination to another context.
• Audit Value - The Audit Value command returns the current state of properties, events, signals and statistics of Terminations.
• Audit Capabilities - The Audit Capabilities command returns all the possible values for Termination properties, events and signals allowed by the Media Gateway.
• Notify - The Notify command allows the Media Gateway to inform the Media Gateway Controller of the occurrence of events in the Media Gateway.
• Service Change - The Service Change Command allows the Media Gateway to notify the Media Gateway Controller that a Termination or group of Terminations is about to be taken out of service or has just been returned to service. Service Change is also used by the MG to announce its availability to an MGC (registration), and to notify the MGC of impending or completed restart of the MG. The MGC may announce a handover to the MG by sending it Service Change command. The MGC may also use Service Change to instruct the MG to take a Termination or group of Terminations in or out of service.
All of these commands are sent from the MGC to the MG, although Service Change can also be sent by the MG. The Notify command, with which the MG informs the MGC that one of the events the MGC was interested in has occurred, is sent by the MG to the MGC.
II. CALL PROGRESS TONES AND SIGNALS
Call progress tones and call progress signals are acknowledgement and status signals that ensure the processes necessary to set up to terminate a telephone call are completed in an orderly and timely manner. Call progress tones and signals can be sent from machines to machines, machines to people, and people to machines. The people are the subscriber, and the machines are the electronic switching systems in the telephone offices and the telephone sets. When a switching machine outputs a call progress tone to a subscriber, it must be audible and clearly identifiable.
Signaling can be broadly divided into two major categories: station signaling and interoffice signaling. Station signaling is the exchange of signaling message over local loops between stations and telephone company switching machines. On the other hand, interoffice signaling is the exchange of signaling message between switching machines. Signaling message can be subdivided further into one of four categories: alerting, supervising, controlling, addressing. Alerting signals indicate a request for service, such as going off hook or ringing the destination telephone. Supervising signals provide call status information, such as busy or ring-back signals. Controlling signals provide information in the form of announcement, such as number changed to another number, a number no longer in service and so on. Addressing signals provide the routing information, such as calling and called number.
III. SUPERVISORY SIGNALING
A. DUAL-TONE MULTIFREQUENCY (DTMF)
DTMF was originally called touch tone. DTMF is more efficient means than dial pulsing for transferring telephone numbers from subscriber’s location to the central office switching machine. DTMF is a simple two-of-eight encoding scheme where each digit is represented by linear addition of two frequencies. DTMF is strictly for signaling between a subscriber’s location and the nearest telephone office or message switching center.
The figure shown below are the four row by four column keypad matrix used with a DTMF keypad. As the figure show, the keypad is comprised of 16 keys and eight frequencies. The four vertical frequencies called the low group frequencies are 697 Hz, 770Hz, 852Hz, and 941Hz and the four horizontal frequencies called high group frequencies are 1209Hz, 1336Hz, 1477Hz, and 1633Hz. The digits 2 through 9 can also be used to represents 24 of the 26 letters Q and Z are omitted.
A. EQUIPMENT BUSY SIGNAL
The equipment busy signal is sometimes called a congestion tone or a no circuit available tone. The equipment busy signal is sent from the switching machine back to the calling station whenever the system cannot complete the call because of the equipment unavailability. This condition is called blocking and occurs whenever the system is overloaded and more calls are being placed than can be completed. The equipment busy signal uses the same two frequencies as the station busy signal, except the equipment busy signal is on for 0.2 seconds and off for 0.3 seconds. Because an equipment busy signal repeat at twice the rate as a station busy signal, an equipment busy is sometimes called a fast busy, and a station busy is sometimes called a slow busy. The telephone company refers to an equipment busy condition as a can’t complete.
B. RINGING
The ringing signal is sent from a central office to a subscriber whenever there is an incoming call. The purpose of the ringing signal is to ring the bell in the telephone set to alert the subscriber that there is an incoming call. If there is no bell in the telephone set, the ringing signal is used to trigger another audible mechanism, which is usually a tone oscillator circuit. The ringing signal is nominally a 20Hz , 90 Vrms signal that is on for 2 seconds and then off for 4 seconds. The ringing signal should not be confused with the actual ringing sound the bell makes. The audible ring produced by the bell was originally made as annoying as possible so that the called end would answer the telephone as soon as possible, thus tying up common usage telephone equipment in the central office for the minimum length of time.
C. RINGBACK SIGNAL
The ring back signal is sent back to the calling party at the same time the ringing signal is sent to the called party. However, the ring and the ring back signals are two distinctively different signals. The purpose of the ring back signal is to give some assurance to the calling party that the destination telephone number has been accepted, processed, and is being rung. The ring back signal is an audible combination of two tones at 440Hz and 480hz that are on for 2 seconds and then off for 4 seconds.
D. RECEIVER ON/OFF HOOK
When a telephone is on hook, it is not being used, and the circuit is in the idle state. The term on hook was derived in the early days of telephone when the telephone handset was literally placed on a hook. When the telephone set is on hook, the local loop is open, and there is no current flowing on the loop. An on hook signal is also used to terminate a call and initiate disconnect.
When the telephone set is taken off hook, a switch closes in the telephone that completes a dc path between the two wires of the local loop. The switch closure causes a dc current to flow on the loop. The switching machine in the central office detects the dc current and recognizes it as a receiver off hook condition. The receiver off hook condition is the first step to completing a telephone call. The switching machine well respond to the off hook condition by placing an audible dial tone on the loop. The off hook signal is also used at the destination end as an answer signal to indicate that the called party has answered the telephone. This is sometimes referred to as a ring trip because when the switching machine detects the off hook condition, it removes the ringing signal.
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