SS7 Node types:


SSP = Service Switching Point are Voice Switches with SS7 functionality. It processes voice media and performs SS7 signalling.
SSP can terminate and originate SS7 message but can't transfer it. All messages received with a different point-code than the Point code of SSP itself are discarded.

SCP = Service Control Point is an interface between Telcom Databases and the SS7 network. Usually, SSP sends a query to the database. SCP provides the response back to SSP. e.g. where number to forward the traffic for toll-free number. SCPs can be deployed in pairs for redundancy. Paired boxes share load.

STP = Signal Transfer Point is responsible for transferring SS7 messages between other SS7 nodes. Almost like a router does packet transfers in data network. This routing is done based on routing info in MTP3 routing label of SS7 messages. Hence STP is not the source or the destiation of any SS7 message. Standalone STPs can also filter messages, like a router with an ACL.
Though STP usually are standalone boxes, sometimes they come inegrated with SSP [service switching point]. Standalone STPs are deployed in mated pairs for redundancy. Normally, both boxes share the load.

SS7 Link types:


Access or A Links: provide access to the network. They Connect outer SPs (SSP ro SCP) to STP backbone. Usually SCP or SSP would connect to multiple STPs.

Bridge or B Links: connect STPs, that do not form a mated-pair, together.

Cross Links or C Links: conenct two STPs that form a mated pair. If one fails, other STP takes the load of both.

Diagonal Links or D Links: are same as B links but they connect STPs of different hierarchical levels or to different networks altogether. e.g. between IXC STP to LEC STP.

Extended Link or E Links: conect SSP and SCP to an STP pair as with A Links. E links are for non-home STPs.

Fully-associated or F Links: are used to connect SSPs and SCps directly to each other without using STPs.