1 .CCS7 Signaling System
- Upon completion of this course, the trainees should be able to
- Understand the basic concepts in CCS7
- Understand the important composition of CCS7 signaling units
- Understand the basic signaling procedure of CCS7
1.1 Basic Concepts in CCS7
1.1.1 Part1 Signaling
Now let's take a look of the course arrangement: This course is divided into 3 sections.
Section 1 Basic Concepts in CCS7,
Section 2 Signaling Units and Section 3 Signaling Procedure.
In section 1, we will discuss two topics, Signaling and CCS7 Signaling Network. Now let's start from Part 1, Signaling.
Definition of signaling Generally there are two ways to define signaling. First, signaling refers to all the control signals used within or between communication equipment, whose function is to set up communication;
Or we can also define signaling as the signals except bearer information (voice information and non-voice information), which are used to control the action of the exchange.
Now the Classification of signaling:
We can classify signaling in different methods. Here, according to the working location, we can have subscriber line signaling and inter-office signaling. Subscriber line signaling refers to the signaling used between subscriber and the exchange, and inter-office signaling refers to the signaling used between exchanges. Here "office" has the same meaning as exchange, and "inter" means "between".
We can also classify signaling according to the used signaling channels. So we have CAS signaling and CCS signaling. First, let's take a look of CAS signaling. CAS refers to Channel associated signaling, it is composed of line signal and register signal. For CAS, the signaling channel is combined with the bear information channel or the two have fixed correspondence. For example, register signal is transferred on the channel which later on will be used to carry bear information, while line signal is transferred on TS 16 and its position has fixed correspondence with the bear information channel it represents.
CCS refers to Common channel signaling, in CCS, the signaling of a group of voice channels are transmitted on a common high speed data link in the form of time multiplexing.
Now the comparison between CAS signaling and CCS signaling. For CAS, it uses multi - frequency in - band signals, which means it uses 6 different frequency points within the bandwidth of voice and every time takes 2 out of these 6 to represent a signal. So the information volume is small, for each group of CAS signaling, we can only have 2-out-of-6, which equals 15 different signals. Secondly, CAS signaling uses mutual - control transmission and receiving mechanism, so the speed is very slow. The local exchange can not send the next signal until it receives a response from the opposite side. And the third, since the signaling is transferred on the same channel as used for voice, there is no signaling channel during conversation. So this is for CAS, for CCS, let's take a look. First, CCS adopts non - mutual - control mechanism, so the local exchange do not need to wait for the response so as to send the next signal, so the speed is very fast. Secondly, since for CCS, the signaling is transferred on a link which is independent of the voice channels, signaling could be transmitted during conversation. And thirdly, CCS is designed not only for telephone network, it has wide application area, and suitable for future development. So these are the advantages of CCS, but correspondingly, CCS also has some special requirements. The first one, it requires low error-code-ratio, so the demands for the quality of transmission and the quality of synchronization are very high. And secondly, since the signaling link is independent of the voice channels, even when the signaling messages could be transmitted between two exchange, we could not be sure of the status of the voice channels. So for CCS, it requires conduction check of the voice channels before we can use the channels for voice transmission. But fortunately, for CC08, we do not need to do conduction check every time before we use certain channel, because the DT board itself can automatically perform conduction check.
1.1.2 .CCS7 Signaling Network
So that's all for the description of signaling, now let's go to the second part of basic concepts, CCS7 signaling network.
Definition of CCS7 signaling network:
In CCS7, signaling links are independent of voice channels. These signaling links form a network which is dedicated to the transmission of signaling, we call this network CCS7 Signaling Network.
It is a multifunctional supporting network, can be used in telephone network, circuit-switch data network, ISDN network, and intelligent network, etc..
OK, now, the question is: what is supporting network? Supporting network is named in contrast to service networks, such as PSTN, public switching telephone network, and PLMN, public landscape mobile network. As the name implies, supporting network provides support to the service networks. Generally speaking, there are three supporting networks, CCS7 signaling network, synchronization network and telecommunication management network (TMN). Now the next topic: the existing CCS7 signaling networks. The existing CCS7 signaling networks include international network, international standby network, national network, and national standby network. For most of the exchanges using CCS7, they are in the national network. For the international gateway exchange, which is the boundary for international tool call, it has two identities. First, it should be a member in the national network, so a local exchange inside the country could access it; and on the other hand, it should also be a member in the international network, so that it could transfer the call to the destination country's gateway exchange. And as for the national reserved network and international reserved network, generally they are used only for some special purpose. Therefore in CCS7, each signaling message carries a field called NI (network indicator) to indicate which of the four networks this message works in.
Now let's talk about the composition elements in CCS7 signaling network. CCS7 signaling network is composed of three elements: Signaling Point (SP), Signaling Transfer Point (STP) and Signaling Link. As shown in the figure on the right, in a CCS7 signaling network, usually SP is represented by a circle, STP is represented by a square, Link is represented by a dashed line, Voice path is represented by a solid line.
Now we come to the first element in CCS7 signaling network--SP. SP is the originating or destination point of a CCS7 message. In each signaling network, a SP has an exclusive signaling point code: SPC (14 bits). Since the four signaling networks assign the SPCs independently, only NI+SPC can uniquely locate a SP. Or, to put it another way, if you want to locate a SP, besides its SPC, you should also indicate its NI. For SP, please notice the following point: In the data setting for CC08, we describe SPC in Hex. On the slide there are two examples of the conversion between binary and hex for SPC.
From SPC, two new concepts come out: DPC, Destination Point Code, and OPC, Originating Point Code. A message going from one SP to another SP should bearing the SPC of the originating SP and the terminating SP, which are called OPC and DPC respectively. If we compare sending a message to send a letter, OPC and DPC are similar to the sender's address and receiver's address. This is the concept of DPC and OPC, later on, after we understand the function of STP, we will have a further discussion about DPC and OPC.
Now the second element, STP. STP is the network node which transfers CCS7 messages. Except for some management messages, normally STP does not send or receive messages. The last element is the link. Link is the data channel which connects the nodes (SPs and STPs) in CCS7 signaling network. Nowadays, usually we use digital signaling link, which is 64 kb/s, and is transmitted in one timeslot or one channel in a PCM system. By the way, in a PCM system, timeslot 0 is for carrying the synchronization signals, so except timeslot 0, all the other timeslots in a PCM system could be used as the signaling link. Some kinds of exchanges have restrictions on the timeslots to be used as link, but CC08 doesn't. Therefore when selecting the timeslots for carrying the link connecting to another exchange, CC08 can comply with the requirements of the exchange at the opposite side.
Working modes of CCS7 signaling network Working mode refers to the relationship between the signaling link through which a signaling message is transmitted and the voice channel to which the message refers. Presently two modes are in use. Associated mode and quasi-associated mode. For associated mode, the messages between two adjacent points are conveyed over a link-set directly interconnecting those signaling points, i.e., the link is parallel to the voice path.
Quasi-associated mode In the quasi-associated mode the message which is going to arrive at a SP goes through a path which is predetermined and via one or more STPs. Namely, as long as there is a STP on the way for the messages between two SPs, we can say that the quasi-associated mode is adopted. And if you draw a figure representing quasi-associated mode, the link and voice path are unparallel.
Here is a question to help you understand the function of STP better. The question is: When the links between any two offices are transferred by a STP, how will be the link path? For example, on the slide we give you a local network. We have SP A and SP B, the voice paths between them are transferred by a tandem office C, and there is a STP which provides link connection to SP A, SPB and tandem C respectively. Please remember, the sole purpose of using signaling is to serve the voice path. Suppose SP A make a call to SP B, how will be the link path? Here give you a hint: the only purpose of using signaling is to serve the voice path. The answer will be given to you on the next slide.
So this is the correct answer. The arrows indicate the paths that signaling messages go through. Remember, a signaling path always starts from one end of the voice path it is going to serve, and ends up at the other end of the voice path. Is your answer the same as this one?
Ok, now another question for you. This time we discuss STP in consideration of DPC and OPC. In the figure, in situation (a), SP A and SP C are connnected by a tandem exchange, SP B. If SP A wants to make a call to SP C, how will be the DPC and OPC of the messages transmitted on the link between A and B, and that between B and C? In situation (b), we replace SP B with STP B, and the voice path are also different, then how would be the DPCs and OPCs? Think about it and then go to the next slide for the correct answer.
So this is the correct answer. On the link between SP A and SP B, for the message sent from A to B, the OPC is A, and DPC is B; while on the link between SP B and SP C, for the message sent from B to C, the OPC is B, and DPC is C. But figure (b) presents a different situation. Remember, STP here only transfers the messages. So on the link between SP A and STP B, for the message sent from A to B, the OPC is A, and DPC is C. And this message got no change after it is transferred by STP B. So on the link between STP B and SP C, the messages still carry OPC equaling A, and DPC equaling C. So again emphasize: the only purpose of sending a message is to serve the voice path. Since in figure (a), there is a voice path between SP A and SP B, so the message in the same direction serves this voice connection from A to B. And similarly, the messages from B to C serve the voice path connection between B and C. That's why the answer for figure (a) and that for figure (b) are different.
1.1.3 Hierarchy of CCS7 Signaling System
Now we come to the third part of basic concepts, hierarchy of CCS7 signaling system.
The hierarchy of CCS7 system can be functionally divided into two parts, i.e. Message Transfer Part (MTP) and User Part (UP). First let's take a look of MTP. The Message Transfer Part, or MTP, provides the functions that enable User Part's significant information to be transferred across the signaling network to the required destination. In addition, functions are included in the MTP to overcome network and system failures that would affect the transfer of signaling information.
UP (User Part)
The User Part, or UP, is the "user" of MTP. It includes TUP (Telephone User Part), ISUP (ISDN User Part), etc. In the following, our explanation will focus on TUP, which transmits and receives interoffice control signals to/from the MTP for telephone calls during call process. The figure on the slide describes the relation between UP and MTP.
Three levels of MTP: MTP could be further divided into 3 levels, namely, level 1-- signaling data link, level 2 -- signaling link, level 3 -- signaling network. These three levels are similar to the first three levels of OSI model. Now let's take a look of the functions of these different levels. The 1st level, signaling data link. A signaling data link is a bi-directional transmission path for signaling, comprising two data channels operating together in opposite directions at the same data rate. It constitutes the lowest functional level (level 1) in the signaling System No. 7 functional hierarchy.
Level 2: Signaling Link. The second level provides such functions as signal unit delimitation, signal unit alignment, error detection, error correction, initial alignment, signaling link error monitoring and flow control. It, working together with level 1, ensures the reliable transmission of messages between two adjacent SPs. Level 3. Signaling Network. Functions on this level include signaling message handling and signaling network management. In the transmission of messages, the function of signaling message handling sends the messages to the proper link or user part; while malfunctions comes up, the network management function realizes the reorganization of the signaling network, and when congestion appears, the network management function carries out flow control at the network level, so as to ensure the reliable transmission of signaling under extreme conditions.
Now let's take a look at the working of the 4-level structure of CCS7. As shown in the figure, we have SP A and SP B, we will explain the procedure of sending a message from A to B. For easier understanding, we often compare the procedure of sending and receiving a message to that of sending and receiving a letter. First, the user part of SP A creates some information to be transmitted to SPB. It is like the letter inside the envelop. Then the information is sent to the third level. In the signaling network level, some fields are added to the information, which carries the networking information such as OPC and DPC. This is similar to write down the sender's address and receiver's address. Then the message is sent to level 2, so some more fields are added to the information, these fields realize the 2nd level functions as initial alignment, error detection and correction, and so on. You can compare this to putting the mailing letters into a safe bag or box. Then after all of this precaution, the message is send to level one, the physical link, so as to be sent to the opposite side. And for sending a letter, we have a similar procedure of transmission, maybe by train, or by airplane. Then on the other end of the link, the first level of SP B receives this message. So it reports the message to level 2. On level 2, the fields formerly added on level 2 are disbanded, and then send the message to level three. Then level 3 disbands the fields initially added on the 3rd level and performs "message discrimination": it compares the DPC of the message with the signaling point code of this exchange. If these two are the same, then it means the message is for SP B, then level 3 will perform "message distribution" to send the message to level 4. But if the DPC of the message is different from its own SPC, then level 3 will perform "message routing", adding back the fields just disbanded, and send it to level 2, so as to finally send the message onto the physical link again. And it is not difficult for you to notice that, the latter situation just describes what a STP does, transferring messages.
Now let's take a look of the realization of the 4 levels of No.7 signaling system in C&C08. As you can see from the figure on the slide, the 1st level function of CCS7 is realized by DT board, and the level 2 function is realized by LPN7 board. As for Level 3 and level 4, their functions are rather complicated, and they are realized by MPU board.
1.2 Signaling Units
OK, that's all for section 1, the basic concepts in CCS7. Now we come to section two, signaling units.
First let's take a look of the definition of Signaling unit. Signaling unit (SU) is the minimum unit used to carry the various signaling messages in CCS7 signaling system. In fact, every message in CCS7 appears in the form of a SU.
Now we will discuss the classification of signaling units. According to the sources, SU could be classified as MSU, LSSU and FISU. 1. MSU, or Message Signaling Unit, is used to transfer the signaling message from the 4th level or the signaling network management message from the 3rd level. 2. LSSU, Link Status Signaling Unit, it originates and terminates at the 3rd level, carrying no detailed signaling message, used to indicate the link status when the link is put into use or malfunctions, so as to set up or recover the signaling links. 3. FISU, Fill-in Signaling Unit, it originates and terminates at the 2nd level, used to fill in the vacant locations when the link is free or congested, so as to keep the link in the status of communication, and sometimes it is also used to confirm the reception of message from the opposite side.
The figures on this slide illustrate the composition structure of MSU, LSSU and FISU. You can see that the fields making up FISU can also be found in LSSU and MSU, because FISU only includes the fields which could realize the level 2 functions. And for LSSU, it only has one field that is unique for it, that is SF. So SF is the field that carries the information about the link status. As for MSU, since it really contains the information, it has more fields and is more complicated. So we have the special fields SIO and SIF for MSU. SIO contains the information added on the third level. In SIF, besides some information added on the third level, the information created by user part is also included here. As for the functions of these fields, since these are not closely related to our operation and maintenance job, it is not explained here, if you are interested, you can refer to any training documents for CCS7.
Now let's take a look of some important fields in TUP and MSU. Here explain CIC and SLC. CIC refers to Circuit Identification Code, and SLC means Signaling Link Code. The figure on the slide gives an example of two exchanges being connected with CCS& links and speech channels. So there are multiple circuits between the two exchanges, here "circuits" has the similar meaning as timeslot or channel. If SPA want to have a talk with SPB, it should first send a message to SPB, in which it should tell the opposite side the circuit it plans to use for sending the voice. Therefore the two exchanges should have a same method to identify these circuits. So we assign each of the trunk circuits a code, CIC, which should be the same on both end of the trunk. So CIC should be unique in each direction. "Direction" is a term used in C&C08, later on in the course "Trunk Data" we will explain it in detail, here, to put it simply, one direction means connecting to one exchange. If the local exchange is connected to two exchanges using CCS7, then we say the local exchange has two directions. And CIC is coded in 12 bits, which implies that in one direction we can have as many as 4096 circuits, and the value range of CIC is [0,4095].
Now let's look deeper into CIC. CIC has 12 bits, so how do we assign these CIC? The general principle is that the lower 5 bits represent the PCM timeslot number, and the higher 7 bits represent the PCM system number. So why we do this? for E1, one PCM system contains 32 timeslots, if we want to represent these timeslots in binary, how many bits do we need? Yes, 5 bits. So for easier recognition, we just use the lower 5 bits of CIC to represent PCM timeslot number. You may argue that in some types of exchanges, the timeslots taken for carrying clock signal or link do not need a CIC. No problem. In those exchanges, the CICs for these timeslots are reserved. For C&C08, the assignment of CIC is very flexible. It can assign CIC to the timeslots or not just according to the demands of the exchange at the other side. On the right of the slide, there is an example. Say we use two E1s to connect with another exchange. The E1s' PCM system number is 0 and 1. So we have altogether 64 circuits, and we can assign the CIC numbers as shown in the figure. Now suppose the opposite exchange does not assign CICs to timeslot 0, and the timeslot 1 of the number 0 E1 is used for link, and requires no CIC, then we can make the following change to suit its requirement: in the figure, CIC 0, 1 and 32 are changed to 65535, which means invalid. Why this is the solution? Please think about it.
Now we come to discuss SLC. Similar to CIC, if there are multiple links connecting two adjacent SPs, to distinguish them, we should sign each of the links a code, which is called SLC. So SLC is unique in each direction, and it is coded in 4 bits, which implies that in each direction we can have as many as 16 links, and the value range of SLC is o to 15. On the slide there is a figure which illustrates the coding of CIC and SLC. In the middle it is our local SP, and it has two directions. For the connection to the left side exchange, we can have CICs coded as 0,1, 2, and so on, SLCs as 0,1, and so on. And for the connection to the right side exchange, when we assign CICs and SLCs, we do not need to pay attention to the codes we used for the left side. So again we can use CICs as 0, 1, 2, and so on, and SLC also starting from 0. So we can say CIC and SLC should be unique within each direction.
Now we come to another topic, Dual Seizure. We know that in CAS signaling, usually we use uni-directional trunk circuits. For example, we have 30 voice circuits connecting to an opposite exchange, then we divide the circuits into half, half for incoming and the other half for outgoing. But for this kind of practice, the efficiency of the utilization of the trunk circuits is not very high. For example, suppose at a certain moment, the outgoing traffic is much higher than the incoming traffic, and all the 15 outgoing are busy when a subscriber wants to make a call to the opposite exchange, then his requirement will be refused even when there are idle incoming circuits. So in CCS7 we usually use bi-directional trunk circuits, which mean the trunk circuits can either be used for outgoing call or incoming call. But this kind of practice also has some disadvantages, because it induces the possibility of selecting the same circuit from the two offices at the same time, this is what we call "dual seizure". To prevent this, for each circuit, we define different control priority to the SPs at two ends, one has "master" control right, and the other has "non-master" or "slave" control right. So how could we assign the master and non-master control rights over the trunk circuits to the two SPs on both ends? Usually we assign the master control rights according to the circuit's CIC number : if the CIC number is even then its master control right is given to the office which has larger SPC (Signaling point code), and if the CIC number is odd then its master control right is given to the office which has smaller SPC. On the next slide we will give you an example of the distribution of CIC as well as the master control rights.
So let's take a look of the example on the slide. Suppose there are two adjacent SPs, A and B. The signaling point code for SPA is 0002, and that for SPB is 0001. So the SPC for A is larger than that for B. And suppose the CIC for the trunk circuits between A and B are coded as 0, 1, 2, and so on. So the even CICs 0,2,4, and so on, their master control right is given to SPA, and SPB only has "non-master" control rights over these circuits, while for the odd CIC numbers, the situation is the reverse.
Now the question is: what kind of mechanism does CCS7 adopt to avoid dual seizure? Here is the explanation: When the local office wants to make an outgoing call, it first tries those circuits on which it has master control right; only when all these circuits are busy, will the local SP try the circuits it has slave control right. So when the slave circuits are to be selected, the possibility of "dual seizure" arises again. Hence different "circuit selection modes" are used: for the circuits on which the local office has master control right, "FIFO" mode is adopted, which means the circuit bearing the longest idle period will be selected; while for the circuits on which the local office has slave control right, "LIFO" mode is adopted, which means the circuit bearing the shortest idle period will be selected. In this way, most of the danger of "Dual seizure" can be avoided.
But still there is an extreme occasion when there is only one idle circuit left for selection from both sides. Then if both sides try to use this circuit at the same time, the possibility of dual seizure rises again. In this case, the principle is the "slave" side should give way to the "master" side.
1.3 Signaling Procedure
So up to know, we have finished the topics in Section two, now let's turn to Section three, CCS7 signaling procedure.
This slide explains the basic signaling procedure of a successful ordinary call. The caller picks up the phone and dials the number for an outgoing call, MPU analyzes the called prefix and finds out that this is an outgoing call, so it will choose a voice channel as well as a signaling link for this call. Then on the link chosen, The local office sends the first message, initial address message IAM to the opposite office, this message contains the called number as well as the CIC of the circuit chosen to carry the voice. If the opposite office judges the information as correct, it returns Address Complete Message (ACM) after receiving all digits. At this moment ring back tone is sent to the caller from the opposite office through the newly setup speech channel, and at the same time the telephone set of the called party rings. Then called party picks up the phone. So the opposite office sends Answer Charge Signal (ANC), indicating the beginning of conversation and charging. After the talk, suppose the caller hangs up first, then the local office Clear Forward Signal (CLF) to the opposite office, and the opposite side will return a RLG message to release the voice path taken for this call.
On this slide, it is another example. This is for the call connection through tandem office. The local exchange and the destination office is connected by a tandem office. This time, as we can see from the slide, instead of sending one IAM to give the tandem office the complete called number, here IAM only carries the first two digits of the called number, then the local exchange uses several SAM and SAO messages to send the rest of the called number. SAM means subsequent address message, it often follows IAM, used to send the subsequent digits of the called number, SAO also means subsequent address message, but every time it can send only one digit. Whether sending just one IAM or sending a group of messages including one IAM and several SAM or SAO, this is decided by the data setting in the local exchange. Then, for the tandem office in the example, after receiving all the called digits, it uses one IAM to send the complete called number to the destination exchange. Then the destination exchange responses with an ACM, address complete message, and the tandem exchange will transfer this message to the local exchange. At this moment the caller will listen to ring back tone, and for the called party, the telephone set rings. Then the called party picks up the phone, then ANC message is sent and transferred to arrive at the local exchange, so charging and conversation begin. After the conversation, if the called party hangs up first, then CBK message, clear backward, will be sent, this message, again, will be transferred by the tandem office to the local exchange. At this moment the caller will listen to busy tone, so he hangs off too, then the local exchange sends CLF, clear forward message. And then the destination office will send release guard message, RLG to the tandem office to release the voice path between the tandem and the destination seized for this call, then the tandem office will also send RLG to the local exchange to release the voice channel seized for this call.
Now we will check a new situation. The opposite exchange requires caller number so the local exchange sends the caller number initially. In the example, the call from the local exchange to the destination exchange should go through two toll offices, the originating toll office and the destination toll office. And the originating toll office needs the caller number for the purpose of charging. So, instead of IAM, this time the first message is IAI. IAI means initial address message with additional information. If the caller number is to be sent initially, then IAI is used. As you can see from the figure, this IAI contains two digits of the called number, and the complete caller number, then the local exchange uses several SAM and SAO messages to send the rest of the called number. After all of these messages arrive at the originating toll office, the office takes the caller number, then it uses one IAM to send the complete called number to the destination toll office. And the following procedure is familiar with you, so it is not explained here. Ok, here we just use this slide to explain the usage of IAI, which is used to send caller number initially.
So this slide describes a different situation. The caller side does not send the caller number initially, but the opposite side asks for it You can find application for this method in MCT and CLID, etc. MCT refers to malicious call tracing, and CLID means caller line identification display. Now let's take a look of the example on the slide. This is a local call through a tandem office. To initiate the call, the local exchange sends an IAM to send the complete called number to the tandem office. Then the tandem office transfers the message to the destination office. So the destination office analyzes the message it receives and finds out that there is no caller number it requires. So it sends GRQ message. GRQ means general request message. It is used to ask for caller number and some other information. And GRQ is transferred to the local exchange. In response, the local exchange uses GSM message to send the caller number. GSM means general forward setup information message, it is used to send caller number and some other information. So after receiving GSM, the destination office decides that the address is complete, so it sends back ACM. OK, again, the following procedures are familiar with you, and it is not explained here. So this slide just explains the usage of messages GRQ and GSM.
This slide illustrates the cooperation between CCS7 and R2 signaling. The tandem office uses CCS7 to be connectted with the local exchange and R2 to connect with the destination exchange. Since we have explained the basic signaling procedures for CCS7 and R2 respectively, here we will not go to details. You can read the slide carefully and explain to yourself the cooperation procedures.
So that's the introduction of CCS7 signaling. In fact, CCS7, as a complete and multi-purpose system, is quite complicated and needs a long time to study. Here, in our course, we have only paid attention to the knowledge which is related to the routine operation and maintenance of CCS7 trunk, as well as trunk data setting. If you are interested in the CCS7 system, you can look for a tracing manual or just ITUT documents to study. Now before we end up, let's make a summary: This course describes the following important aspects of CCS7: Basic concepts of CCS7: which includes signaling, CCS7 signaling network and CCS7 hierarchy Signaling Units, in which CIC, SLC, etc. are the most important concepts. And signaling procedure for common situations.
Thank you
