The SCCP level network is a datagram network. To answer, the receiving equipment just inverts the called party and the calling party address. The subscriber profile includes all of the parameters shown in Table 1. Thus, in order that all roaming functions will work, the barring operator, operator A, must allow the E translation facility to be available and let the MAP messages addressed to the MGT range [that is the addresses of all its network equipment HLR, MSC ] go through.
The HLR sends the subscriber profile. This primitive is not an SMS primitive; however, in a nonstandard SMS procedure, obtaining the roaming number could be useful. It will be implemented later when it will be possible to initiate a WAP session from outside. Currently, a WAP session can only be established by a cell phone!
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This will be used to push data over a GPRS connection with the cell phone, but initiated from the network, not by the cell phone. A general implementation is still a ways off. An example follows of a Siemens billing record. The main fields are the A number, which is the cell phone number, and the B number, which is the service center GT.
It will be used by the visited network to charge the home network. Note that, unless additional software is used, the MSC SMC-MO CDR does not include the real destination cell phone number, which makes it impossible to charge different prices for different categories of destinations.
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Then it is possible to invoice only the successful SMS. How can the billing system make the difference? With a country code of 33 and a network destination code NDC of for Orange France, we can identify the sender. As we will see, it is directly proportional to the number of SS7 signaling links. With modern servers, the processing is not a limitation. We must also state what we measure.
Otherwise the buffer in the MSC, one for each number, would overflow. We also look in the logs to see if any warnings about congestion on the links are present. For very low sending rates less than 25 , the number of links does not matter: The three experimental curves follow the theoretical one, and the measured speed matches exactly the sending speed of the client PC.
When the number of SS7 links increases, the maximum reachable speed—without any congestion—also increases. From our results, one SS7 link gives a maximum speed of about 27 messages per second, two links give almost 50 messages per second without any congestion and 80 otherwise, and four links give around messages per second without any congestion. For an operator, even the smallest congestion has dramatic effects in that some messages may be lost! So sending at the appropriate rate, as determined by the number of SS7 links, is extremely important and is the reason for the great quality of service that is expected from an operator.
When sending messages at speeds higher than the congestion limit, we observe a saturation in the sending speed: The bandwidth of the SS7 links 64 Kbps in the outgoing direction is limiting the traffic. You should be wary if someone tells you that their equipment does SMS per second with only six signaling links. The correct value is 17 messages per second per SS7 link. Exercises 1. SS7 has been gradually developed to meet the increasing signaling requirements of new services developed in the s and s.
The Q series defines the procedures and protocols by which network elements in the public switched telephone network PSTN exchange services over a digital channel, such as call forwarding, calling party number display, conference calls. SCCP also supports the use of the SS7 network as an advanced packet switched network for direct communications between two network entities without physical ties to traffic circuits through connection-oriented or connectionless communications.
The PLMN benefits from the use of this protocol for operations such a location updating, which the HLR to be updated with information showing the location of mobile stations MSs. To execute these services, the service switching points SSPs must exchange data with the SCP in order to control the communication switch. As we can see from Figure 2. Then TCAP was implemented to provide a transactional layer [based on association control service element ACSE and remote operation service element ROSE ] to applications externalizing the service logic from the SS7 network and then to provide a reliable, efficient, and far more effective way of implementing new services in a telephony network.
It is far more flexible to update and install one SCP node implementing a telephony service rather than updating or changing all the SSPs of a telephony network. These point codes are carried in signaling messages exchanged between signaling points to identify the source and destination of each message. Each signaling point then uses a routing table to select the appropriate signaling link for each message. The SSP switches originate and terminate calls. An SSP sends signaling messages to other SSPs to set up, manage, and release voice circuits required to complete a call.
Signaling data link 2. An STP routes each incoming message to an outgoing signaling link based on routing information contained in the SS7 message. Because it behaves as a network hub, there is no need for direct links between signaling points. An STP may perform a GT translation, a transformation by which the routing is based on digits present in the SS7 message a dialed number or a mobile subscriber identification number.
To achieve fault tolerance, STPs or SCPs are usually provided in pairs and signaling data links and are bound into linksets a set of several links. In a linkset the traffic is shared across all of the links and can be rerouted to another link if one of them fails.
This level defines the requirements that need to be met by the physical circuit pulse code modulation [PCM] channel, for instance. SS7 messages are exchanged over or Kbps bidirectional channels called signaling links. Signaling occurs out of band on dedicated channels signaling networks rather than in band on voice channels. Out-of-band signaling provides faster call setup times, more efficient use of voice circuits, an improved control over network usage and support for IN services SCPs do not manage voice trunks.
Once the signaling terminals in the traffic switching or network intelligence nodes are interconnected by physical circuits, they represent a signaling link. MTP layer 1 is equivalent to the OSI physical layer and defines physical, electrical, and functional characteristics of the digital signaling link. For this purpose, the signaling link functions implement separation of messages, error detection, error correction, and supervision.
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FISUs carry basic level 2 information only e. Link status information is used to control link alignment and to indicate the status of a signaling point to the remote signaling point. MSUs carry call control, query and response, network management, and network maintenance data in the signaling information field SIF. MSUs have a signaling label contained into the SIF field that allows an originating signaling point to send information to a destination signaling point across the network.
Otherwise, the LI is set to 63, which indicates that the message length is equal to or greater than 63 up to a maximum of octets. The flag indicates the beginning and end of a message. The opening flag serves as a closing flag for the previous SU the bit pattern is When a signal unit is ready for transmission, the signaling point increments the FSN by 1. The CRC value is then calculated and appended to the forward message.
On receiving the message, the remote signaling point checks the CRC and copies the value of the FSN into the BSN of the next available message scheduled for transmission back to the originating signaling point.
If the CRC is correct, the backward message is transmitted Figure 2. If the CRC is incorrect the remote signaling point toggles the BIB, indicating a negative acknowledgment, and sends the backward message. When the originating signaling point receives a negative acknowledgment, it retransmits all forward messages beginning with the corrupted one, with the FIB toggled. The 7-bit FSN can store values between 0 and Then a signaling point can send up to signal units before requiring acknowledgment from the remote signaling point.
The BSN indicates the last in-sequence signal unit received correctly by the remote signaling point.
The BSN acknowledges all previously received signal units as well. Message priority is only considered during congestion conditions. Low-priority messages may be discarded during periods of congestion. Status field SF is carried only by the LSSU message and indicates the state assumed by a signaling link side after a change of state.
It consists of 8 or 16 bits. The CRC is used to detect correct data transmission errors. MTP reroutes traffic from failed links and signaling points and controls traffic when congestion occurs. It is equivalent to the OSI network layer. Signaling network func- 2. If an MSU is addressed to another signaling point in the network, then it is routed to a suitable signaling link according to instructions from the signaling network management functions.
SMS and MMS Interworking in Mobile Networks
An MSU that is to be sent from a signaling point is routed to the signaling link—in a signaling linkset—selected for the transport of this MSU. Incoming MSUs are separated. If an MSU addressed to another signaling point is received, it will be routed to the appropriate new linkset. Depending on the state of the network—such as performance affected by defective signaling links or signaling points—signaling traffic may have to be rerouted over alternative routes to reach its destination.
In situations where rerouting is not possible, the traffic must be stopped or restricted at the source. The main difference between these frames is the field circuit identification code CIC , which is 12 bits long and includes signaling link selection SLS , in MSUs for user parts. They include the following fields depending on the type of routing label Figure 2. Data contain up to octets of information intended for users, network management, or testing.
The routing label is composed of the destination and originating point codes and the SLS. Points codes are numeric addresses that identify each signaling point in the SS7 network. When the DPC in a message indicates the receiving signaling point, the message is forwarded to the appropriate user part e.
Messages for which the DPC is different from the receiving one are routed to the correct DPC, provided that the receiving entity has message transfer capabilities like an STP. ITU-T point codes are binary numbers that may be stated in terms of zone, area, network, and signaling point identification numbers.
For example, the point code is stated ANSI point codes consist of network, cluster, and members octets e.
This ensures that message sequencing will occur that is, any two messages sent with the same SLS will arrive at the DPC in the same order in which they were originally sent. Moreover, the SLS allows equal load sharing of traffic among all available links. If a user part sends messages at regular intervals and calculates the SLS values by means of a round-robin fashion, the traffic level should be equal among all links.
Signaling message handling can be made independent of the traffic part, and it is also possible to make connection-oriented transfers in case many messages or long messages must be sent to the same destination. It begins with an octet field indicating the SCCP message type followed by data, the coding of which depends on the message type.