What is difference between MTP3 and M3UA?

Answer 1

Difference Between MTP and M3UA links?

M3UA [137] provides for the transport of MTP Level 3-user part signaling (such as ISUP and SCCP) over IP using SCTP. RFC 3332 defines and supplements it with an Implementers Guide [138]. M3UA provides for seamless operation between the user part peers by fully supporting the MTP Level 3 upper-layer primitives. M3UA can be used between an SG and an MGC or IP-resident database, or between two IPSP.

The most common use for M3UA is between a SG and a MGC or IP-resident databases (such as SCPs). The SG receives SS7 signaling over standard SS7 links. It terminates MTP Levels 1 to 3 and provides message distribution, or routing, of the user part messages that is destined for MGCs or IP-resident databases. The MGCs can send to other MGCs via the SG.

Figure 14-11 shows the protocol stacks at each network element for using M3UA between a SG and a MGC. The SEP, or SEP, is a node in the SS7 network. The NIF, or Nodal Interworking Function, provides for the interworking of SS7 and IP. RFC 3332 does not define the functionality of the NIF because it was considered out of scope.

The M3UA on the MGC or IP-resident database supports the MTP Level upper-layer primitives so the user parts are unaware that MTP is terminated on the SG. The MTP service primitives [49] consist of the following:

• MTP Transfer request and indication
• MTP Pause indication
• MTP Resume indication
• MTP Status indication

The MTP Transfer primitive is used to pass user data. MTP Pause indicates that an Affected Point Code is Unavailable, and MTP Resume indicates that an Affected Point Code is Available. MTP Status provides congestion and User Part Availability information on an Affected Point Code. Later, in the Messages and Formats description of M3UA messages, it will be clear how these primitives are supported.

The M3UA layer on the SGP must maintain the state of all the configured ASPs and ASes. M3UA at the ASP must maintain the state of all configured SGPs and SGs.

The M3UA layer on the SG supports message distribution of incoming messages from the SS7 and IP-based sources. The distribution is based on matching the incoming message against the Routing Keys. When a Routing Key is selected, the Application Server state is checked to see if it is active. An Active Application Server has at least one ASP that is ready to receive data messages. If the Application Server is active, the message is forwarded to the appropriate ASP(s) that support the AS.

To determine the appropriate ASP, the SG must take into account the AS's traffic mode. There are three possible traffic modes: Override, Load Share, and Broadcast. Override traffic mode is basically an Active-Standby arrangement in which one ASP is active for receiving data messages and one or more ASPs are Standby. In this case, the SGP sends to the active ASP. In Load Share mode, one or more ASPs can be active. The SGP load shares across the active ASPs using an implementation-specific algorithm. Finally, in Broadcast mode, one or more ASPs can be active, and the SGP sends the data message to each active ASP.

The M3UA layer on the ASP must also make decisions about the distribution of outgoing messages. To do so, the M3UA layer maintains the availability and congestion state of the routes to remote SS7 destinations. An M3UA route refers to a path through an SG to an SS7 destination. If an SS7 destination is available through more than one route (more than one SG), the M3UA layer must perform some additional functions. In addition to keeping the state of each route, M3UA must also derive the overall state from the individual route states. The derived state is provided to the upper layer. Also, if each individual route is available, the M3UA should load balance across the available routes. Further, if the SG consists of more than one SGP, M3UA should load share across the available SGPs.

The M3UA layer at the SGP and ASP must maintain the state of each SCTP association. M3UA uses a client-server model with the ASP defaulting to the client and SG as the server. However, both SG and ASP should be able to be provisioned as the client or server. The client side of the relationship is responsible for establishing the association.

During the establishment of the association, several inbound and outbound streams are negotiated between the SCTP peers. The M3UA layer at both the SGP and ASP can assign data traffic to individual streams based on some parameter that ensures proper sequencing of messages, such as SLS.

M3UA has an internet Assigned Numbers Authority (IANA) registered port number of 2905. It also has an IANA registered SCTP payload protocol identifier value of 3.