For CTC company that wants all its offices and departments networked, I would first recommend star WAN topology to connect all its offices. The headquarters in Washington DC will be the central point of connection with the other offices which is located across the U.S. This type of topology will best suit the company because first, it is very reliable because even if one site fails, data can still be rerouted so it would not affect the entire network system. Second is that this topology provides shorter data paths when all of its circuits are functioning. This means that data gets from one point to the other faster. Third is that expansion costs less. If for example, CTC wants to open another office in Florida, all it needs to do is to lease another dedicated circuit from Washington to Florida and none of the other offices would be affected by the change. .
The sites would be connected through the Internet and the way of achieving this is by T1 carriers. T1 would best suit this company because of the reasonable cost of installation and monthly service fees. Coaxial cable will used to accommodate multiple T1 connections. Also with the T1 connection, the terminal equipment will consist of bridges, routers, or a combination of two. The bridge or router would typically integrate two types of networks. The Internet and an Ethernet or token ring LAN at the customer's site. A router, which can convert TCP/IP to other protocols, is necessary if the internal LAN does not run TCP/IP. I would also recommend installing redundant bridges and routers in the headquarters in order for the system to be fault tolerant.
For the security aspect of the Wan, I would recommend installing packet-filtering firewalls at every WAN location. This type of firewall can be configured to examine the header of every packet of data that it receives to determine whether that type of packet is authorized to continue to its destination.