SS7 Signalling layers

Now let us compare the routing labels in ANSI and ITU

Point code - ANSI 24 bits , ITU - 14 bits

SLS - ANSI 5 bits , ITU - 4 bits

An ANSI routing label uses 7 octets; an ITU-T routing label uses 4 octets.

ANSI point codes use 24-bits (three octets); ITU-T point codes typically use 14-bits. For this reason, signaling information exchanged between ANSI and ITU-T networks must be routed through a gateway STP, protocol converter, or other signaling point which has both an ANSI and an ITU-T point code. (Note: China uses 24-bit ITU-T point codes which are incompatible with both ANSI and other ITU-T networks). Interaction between ANSI and ITU-T networks is further complicated by different implementations of higher level protocols and procedures.
An ANSI point code consists of network, cluster, and member octets (e.g., 245-16-0). An octet is an 8-bit byte which can contain any value between zero and 255. Telcos with large networks have unique network identifiers while smaller operators are assigned a unique cluster number within networks 1 through 4 (e.g., 1-123-9). Network number 0 is not used; network number 255 is reserved for future use.
ITU-T point codes are pure binary numbers which may be stated in terms of zone, area/network, and signaling point identification numbers. For example, the point code 5557 (decimal) may be stated as 2-182-5 (binary 010 10110110 101).

Signaling Link Selection (SLS)

The selection of outgoing link is based on information in the DPC and Signaling Link Selection field. The SLS is used to: Ensure message sequencing.

Any two messages sent with the same SLS will always arrive at the destination in the same order in which they were originally sent. Allow equal load sharing of traffic among all available links. In theory, if a user part sends messages at regular intervals and assigns the SLS values in a round-robin fashion, the traffic level should be equal among all links (within the combined linkset) to that destination.

In ANSI networks, the size of the SLS field was originally 5 bits (32 values). In configurations with two links in each linkset of a combined linkset (totaling 4 links), 8 SLS values were assigned to each link to allow an equal balance of traffic.
A problem arose when growing networks provisioned linksets beyond 4 links. With a 5 bit SLS, a configuration with 5 links in each linkset of a combined linkset (totaling 10 links) results in an uneven assignment of 3 SLS values for 8 links and 4 SLS values for the remaining 2 links. To eliminate this problem, both ANSI and Bellcore moved to adopt an 8-bit SLS (256 values) to provide better loadsharing across signaling links.
In ITU-T implementations, the SLS is interpreted as the signaling link code in MTP messages. In ITU-T Telephone User Part message, a portion of the circuit identification code is stored in the SLS field.
MTP Level 3 re-routes traffic away from failed links and signaling points and controls traffic when congestion occurs. However, a detailed discussion of this topic is outside the scope of this tutorial.
MTP Levels 2 and 1 can be replaced by ATM (Asynchronous Transfer Mode), a simple broadband protocol which uses fixed-length 53 octet cells. MTP Level 3 interfaces to ATM using the Signaling ATM Adaptation Layer (SAAL).