The redback SMS is a combined router and Subscriber Management Server
It was sold in 3 models, 500, 1800, 10000. It is designed fro ISP / DSL service providers to allow them to manage subscriber sessions.
It supports all common authentciation protocols (PPP, RADIUS, etc) and a large number of edge router functions (RIP, OSPF etc). IT also supports common protocols like DHCP, VLAN (802.1xx) as well
If you read the DSLAM BASICs sticky in this forum, the SMS was designed to perform the fucntions of the BRAS - Broadband Remote Access Server.
2 unique features made this highly desirable to ISPS. The first was the concept of CONTEXTs. A Context is like a virtual router/sms within the redback SMS. This allows the SMS to have multiple running configurations for each port. very useful for wholesaling ISP access - 1 ISP = 1 context.
The second feature is it's architecture - pull a control card our and you should find the line cards still carry & process traffic. The configuration for each line card is held on the card, as is the route processor ensuring the card can run on it's own. Improves reliability a great deal.
The SMS products are all End Of Life, having being replaced by the SE - SmartEdge range of products which are amazing Edge routers. They do tend to make ciso 76xx series routers rather basic.