Hi, in most cases the access layer is populated by switches that provide network access to your end users. The size of a network has alot to do with where your routers are located, most times they are located at the core and distribution layers.
Some network this is combined with multilayer switches performing core and dist layer functions Your routers will reside between your network and the internet cloud, when several router are at this layer they are usually mesh together for redundency and transmit through a backbone connections. Your router can perform security function at this layer like NAT and use of ACL's to keep unwanted traffic out.
HTH
http://www.cisco.com/en/US/docs/switches/lan/catalyst4500/12.2/40sg/configuration/guide/l3_int.html
