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Why SS7 was needed in GSM?

Why SS7 was needed in GSM: This is a very important topic for GSM/BSS engineers. In this topic today we will know that  Why SS7 was needed in GSM.In GSM networks, signaling is not as simple as in PSTN. Due to the different architectures of the network in GSM there are additional signaling requirements for which large quantities of non-call related signaling are required.

In the first case, the subscriber mobile is opposite to the PSTN telephone, which is always in one place. Therefore, continuous tracking of mobile stations is required, resulting in the location update process.This process is an example of non-call related signaling, where mobile phones and networks are communicating, but no calls are being made. This requires an additional set of standard messages to meet the signaling requirements of the GSM network.
SS7 why needed in GSM network
Why needed SS7

Introduction about SS7

Short Message Service (SMS) was a relatively new feature in wireless telecommunications that allows short, text messaging for cellular telephones. SMS was extremely popular, gained more popularity in Europe and America and around the world.

SMS messages are transmitted on the control network, not all the allocated bandwidth channels for Signaling System 7 (SS7), and voice communication SS7 is the basis for all the control networks used by all major wireless and wireline telephone carriers disruption of SS7 operations can be devastating for PSN and NS / EP.

SMS and related services, Enhanced Messaging Service (EMS) and Multimedia Messaging Service (MMS) were becoming as popular in the US as they are presented in Europe as capable digital pictures and handsets of the Internet. These additional features will increase the processing requirements on SMS and, when combined with increasing popularity, will affect the future load on SS7.

Earlier there was two major SMS market; the United States and wonder that in Great Britain alone, over 68 million short messages were sent on Valentine’s Day, 2003. Also in Europe, 10 billion messages are sent each month. Even still in the United States, SMS traffic has increased tremendously to today where hundreds of millions of short messages are sent each month.

What is SS7

SS7 is the transport element for SMS traffic. This is the Network Control Protocol for telephone service providers worldwide. For OSI models in SS7, there are four levels in the map. One of the most important and basic use of the SS7 system is to control the network. It is used for data transfer for both landline and wireless telephone systems and has become standard for signaling across the globe.

Messages in SS7 travel freely from sound traffic, from one network to another, riding on the packet The protocol is used in both wireless and wireline networks, such as for call setup and teardown, 800 numbers and 900 numbers, routing for database lookup and registration in wireless network, location, facility through IS-41 To query. , Authentication, caller ID and SMS message delivery.

Additional protocol layers for SS7

There are three type of additional protocol layers which are mentioned below-
1. Base Station Subsystem Application Part (BSSAP)
2. Mobile Application Part (MAP)
3. Transaction Capabilities Application Part (TCAP)

1. Base Station Subsystem Application Part (BSSAP): The first of these additional protocol layers, which is specific to the GSM network, is the Base Station Subsystem Application Part (BSSAP). This layer is used when an MSC communicates with the BSc and mobile station. Since mobile stations and MSC have to communicate through BSC, therefore there should be a virtual connection, so SCCP service is also required.

Assigning the authentication verification process and assigning a new TMSI is done with the standard set of all BSSAP messages. Communication between MSC and BSC uses the BSSAP protocol layer. Therefore, BSSAP provides two purposes:

MSC-BSC signalling
MSC-MS signalling

image explain the location of BSSAP in SS7
SS7 layer

2. Mobile Application Part (MAP): The example of the previously described location update process is not limited to MSC-BSC sections, it spreads to many PLMNs. In the event of updating the location for the first time by the International Roaming Subscriber (where it is not in its home network), VLR will have to obtain data from the Subscriber's HLR through Gateway MSC. Customer's home network.

While a mobile terminated call is being handled, the HLR to HLR is to be requested by MSRN without routing the call. Therefore, another protocol layer was added to these cases as SS7, which is called Mobile Application Part (MAP). MAP is used to signal communication between NSS elements.

Point to be remember that MSC-MSC communication using MAP is used only in case of non-call related signaling. To route calls from an MSC to another MSC, TUP or ISUP is used.

3. Transaction Capabilities Application Part (TCAP): In MAP signaling, one MSC sends a message to a HLR, and that message requests (or invoices) a certain result. HLR sends the result back, which can be the end result or some other message can also follow (or it can not be the end result).

These invoices and results, which are sent back and forth between many elements using MAP, require any kind of secretary to manage the transaction. This secretary is called the Transaction Capabilities Application Part (TCAP). It caters to the SS7 protocol stack in the GSM network and their operations.

SS7 Layers in GSM Elements

SS7 requirements for individual GSM elements will explained in this section. I have already explained above that why SS7 was needed in GSM and which type of protocol layers that are used in SS7. It is useful to note that not all GSM elements have all protocols for the SS7 stack. For example, BSC will never need TUP because the call control is not the work of the BSC.

There are below mentioned SS7 layers in GSM elements-

Protocol Stack in MSC
Protocol Stack in HLR
Protocol Stack in BSC

Protocol Stack in MSC: Since MTP is the foundation on which SS7 is built, it will be necessary for every element that is capable of processing SS7. MSC is the element in GSM network that is responsible for call control, therefore, TUP / ISUP sits on top of MTP for that purpose.

MSC / VLR is also responsible for location updates and communication with BSC and HLR. For this reason it is necessary for BSSAP and MAP to sit at the top of the SCCP. There is also TCAP for providing services for MAP in MSc. Hence it can be seen that MSC / VLR has all SS 7 protocol stacks which have been implemented in it.

Protocol Stack in HLR: HLR is not responsible for call control, therefore, TUP / ISUP is not necessary. In addition, HLR does not communicate directly with BSC; Therefore, BSSAP is also not required, which leaves MTP, SCCP, TCAP and MAP as the signaling protocol in HLR.

Protocol Stack in BSC: BSC only requires BSSAP, but since BSSAP requires services of SCCP, for which MTP is required, the BSC consists of MTP, SCCP and BSSAP.
SS7 Protocols in vario us network elements
SS7 protocol
I have been trying my best to give you complete information about SS7.I have tried to cover more details about SS7 and Why SS7 was needed in GSM. I have also explained in details about the layers and role of elements in the GSM network.Subscribe to my page to receive more telecom network documents.

SS7 protocol  provides the following functionality

1. Load sharing across redundant links: An SS7 network node will deliver traffic equally across all links. Through this load-sharing scheme, no one will be harmed by the outage on a link, delay due to failure for an additional link, or any sudden cancellation of the transaction based on the messages sent through the link.

2. Link loading and load handling mechanisms: A typical SS7 network connects two-way redundancy (links and STP pairs are provided). Keeping this in mind, link loading is kept under 40% usage by a wireless carrier. This design allows an outage of one half pair, during the outage the traffic should be kept under 80% usage in the second half and 20% of its capacity is available for traffic peaks.If traffic is more than 100% constraint, handling procedures at each end will handle loads: buffering, processing messages only with high priority, or discard messages.

3. Link monitoring and status handling: Regular link status checks are performed on an SS7 network. If any link is unavailable, the messages are sent to ensure that both the attached nodes are aware of the outage, and the automatic link test and the link restart process has started. Unless the link resumption process is successful, a link will be unavailable to traffic until the problem is resolved.

4. Route redundancy: While there are physical paths between the link nodes, there are several links in the path which simultaneously carry the message from one node to another and does not require direct link connections. In a network of multiple pairs of STPs, there are several routes available for the destination, so that the failure of the link in the path in the path does not disrupt message delivery.

5. Route monitoring and status handling: When the routes become unavailable due to outage or maintenance, traffic management messages are sent to affected nodes, or network elements, which use a special route between two nodes. These nodes are conveyed that they have to send traffic on a pointless route. When a missing route returns to the service, traffic management messages are sent again to notify all related nodes about availability.

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