Signaling In any telephone system, some form of signaling mechanism is required to set up and tear down calls.
Signaling Let us first see why we need a signaling system by a simple example.
Various Stages in a Basic Call setup1. On-Hook 2. Off-Hook 3. Digits Dial 4. Ring Tone Idle Dial tone5. Caller hears 6. Callee 7. Callee 8. Caller hearsRing Back Tone answers Hang up Disconnect Tone
Basic Call setup1. First the phone is Idle. This is called “on-hook”2. when we remove the handset from its bar, the Localtelephone Exchange receives this signal.This signal informs the Local Exchange that the userintends to make a call. This signal is called “Off-Hook”.The Local Exchange acknowledges this signal by ahumming sound called “Dial Tone”. On listening to the “Dial Tone”, the caller understandsthat the telephone is operational & he can place a call.
Basic Call setup3. On hearing the dial tone, the Caller dials the digits (called party number). These digits are now collected by the Telephone Exchange.Once the Exchange receives all the digits, it analyzes the calledparty number. It checks the following things: o It first validates the calling number. Does he allowed to originate a call. (May be his outgoing calls are barred) o If it is allowed, then it checks, if the dialed (called) number is valid. o If it is valid, then it checks if it is a local call or a long distant call.
Basic Call setupo Then it checks if the caller has sufficient balance to originate this type of call (local or national or ISD)o If he has balance, then it checks if the callee (called party) is idle. (May be he is busy in another call)o If the callee is Idle, then the call is presented to callee.
Basic Call setup4. The callee phone (called party) starts ringing.5. At the same time, Caller listens to Ring Back Tone (RBT).6. The Callee picks up the handset & answers the call.
Basic Call setup7. As soon as the Callee answers the call, the RBT played to calleris stopped & voice path is established with the callee.8. Callee hangs up the call after conversation. 7. As soon as the Callee hangs up the call, voice path between the two is terminated. The caller now listens to Disconnect Tone.
SS7 As you see, numerous signaling is involved between the users & the Telephone Exchange. In case of a Long distance call, these signals has to traverse through multiple Telephone Exchanges, transit nodes & in some case, they have to even cross multiple countries before it reaches its final destination. These Telephone Exchanges, intermediate signaling nodes & the telephones are manufactured by multiple vendors in multiple countries . Yet they should be in a position to understand this signaling. Thus every one across the globe has to follow a uniform signaling method. The standard that is achieved is named as SS7 (Signaling System
SS7 The term "Signaling" has a special meaning in the telecom world: It refers to the information associated with a call which will be exchanged between a Telephone & the Telephone Exchange in order to set up, route, monitor and terminate across circuit. Today most of the telecom protocols (PSTN, GSM, CDMA, UMTS, IN) are build upon SS7. SS7 is also referred to as CCS7 by AT&T, C7 in Europe, and SS#7 by ANSI
SS7 In any telephone system, some form of signaling mechanism is required to set up and tear down calls. Originally in those days, telephone systems used in-band signaling to carry signals. With in-band signaling, signals (which are used for call set- up) are carried in the same circuit as the voice path. In contrast, Out of band Signaling SS7 signaling uses a separate channel dedicated just for transferring signaling information, which can greatly increase overall efficiency of the telephone network.
What is circuit? When I talk about “same circuit” or different “circuit” what exactly are these circuits or channels or voice path??? In order to transfer a Signal or the user data (voice packets), we essentially need a transmission medium to carry them from the Caller’s telephone to the Telephone Exchange to the Callee’ Telephone. A very simple & common transmission medium used today is copper cable (twisted pair) or optical cable. SS7 Signaling standard has defined physical & the electrical properties for this underlying transmission medium. Two variants have been defined: “E1 & T1”
What is E1? T1 is used by North America. E1 is used in India & Europe. E1 has an overall bandwidth of 2048 kbps. It provides 32 channels each supporting a data rate of 64 kbps. 2048 / 32 =64 Kbps What does this mean. This mean Local Exchange shall be able to transmit 32 users information simultaneously. Well how is that possible??? Simple. Have you read about Time Division Multiplex. That’s the answer. For this reason, E1 channels are also called as TDM links
What is E1? These 32 time slots can be used to carry two types of information: o Signaling or User data (voice) If a particular time slot is used to carry signaling, then it is called as SS7 “Signaling Link”. Or if a particular time slot is used to carry voice then it is called as “Trunk” or “Circuit”. In SS7, you need to specify (configure on telephone exchange) if a particular time slot will be used for Signaling or User data (Voice). The Signaling Links are uniquely identified by a numbering system called “SLC” – Signaling Link Code. The trunks are uniquely identified by “CIC” – Circuit Identification Code
ISUP Let us now revisit our old example of a Basic Call. How ss7 handles this basic call?? It handles by a protocol called ISUP ISUP -- ISDN User Part.
ISUP – Basic call Dialed Digit Collection Analyze Digits & IAM – Initial Address Message IAM find the route Alerts Called Party ACM- Address Complete Message Play Ring Tone ACMRing Back Tone ANM ANM – Answer Message STOP RBT End to End voice cut throughPlay Tone on cal disconnect REL Call Disconnect REL – Release RLC RLC – Release Complete
ISUP Basic Call setup Local Exchange Analyze Digits & find the route When the caller dials the caller party number, these digits are collected by the Local Exchange. It then looks into its subscriber database & finds out that Callee is actually present in a different Local Exchange. In this case Local Exchange A (LE-A) sends ISUP IAM message to Local Exchange B (LE-B)
ISUP Basic Call setup This IAM message carries the following information: Calling Party Number Called Party Number Circuit that will used for the voice path communication LE-B on receiving the IAM, it checks for the called party number. If the called party number is valid & complete, it sends ACM to LE-A. ACM carries the following information: Called Party status (Idle or Busy or unknown) On receiving ACM, the trunk cut through is done between LE-A & LE-B & the Caller start receiving RBT.
ISUP - Basic Call setup When Callee answers the call, LE-B sends ANM (Answer Message) to LE-A. On receiving ANM, RBT is stopped to Caller & voice path is established between them.
ISUP Basic Call setup After the conversation, Callee disconnects the call. Then LE-B sends REL (RELEASE) message to LE-A. On receiving this message LE-B, releases the trunk used between them so that this can be used for next use who wants to make a call Then LE-A sends RLC (RELEASE COMPLETE) to LE-B. If the caller doesn’t hang up the phone, he will listen to the Disconnect Tone.
ISUP – Called party busy Let us see an another example where the Called Party is busy in another call.
ISUP – Called party busy Dialed Digit Collection Analyze Digits & IAM find the route Called party busy in another callPlay User busy RELAnnouncement RLC
ISUP – Called party busy In this case, LE-B sends RELEASE after it receives IAM. The REL will have cause code as User Busy
ISUP ISUP is part of the SS7 which is used in the establishment and tear down of telephone calls.