1) OSI model inside and out. Know what each layer does with the information passed from both above and below it. Know which applications use which layer, i.e, MPEG, JPEG, RPC, SQL, EBCDIC, ASCII, Encryption.
2) Frame Relay as applicable to WAN protocols. I had so many questions on Frame Relay it was unbelievable (10-15).
3) Had several questions on ISDN. Know the difference between TE1, TE2, NT-1, and U. Know the difference between BRI and PRI. There weren't that many questions, they were evenly divided between "gimmes" and impossible, but if you can study for the impossibles, they are some free points.
4) Know access lists. Know the implicit deny all, know how a deny filter that doesn't specifically state inbound or outbound will filter. Know wildcard masks (not just the 0 and 255 ones either).
The Application layer provides services to the software through which the user requests network services. This layer is not nor does it contain any applications, and your computer application software is not on this layer. In other words, a program like Microsoft Word does not exist at this layer, but browsers, FTP clients and mail clients do.
As its name suggests, this layer is concerned with the linkages and mechanisms used to move data about the network, including the topology, such as Ethernet or Token Ring, and also deals with the ways in which data is reliably transmitted.