Another area of concern would be the frames and protocols loaded on workstations and servers. Today's networks are mostly the de-facto standard of TCP/IP. However, workstations and servers have the ability to load and communicate via NetBIOS, NetBEUI, IPX, SNA, APPLETALK, etc. Some workstations may have all of the protocols loaded. A workstation will communicate via each one of these protocols when it sends network requests. This can cause a packet to be repeated in each protocol and inflate traffic on an already congested link. Some of these protocols communicate advertisement services to other machines with the same protocols loaded. While they may never be used in actuality, this communication will continue. It is important to turn off all unnecessary protocols at all points on the network. This will also lessen security related issues. These errors will typically manifest themselves as unknown protocols (if the devices do not know what upper layer protocol is receiving the information) or just in elevated statistics on a port.
If a network already exists, planning becomes somewhat simpler, as there is real data and utilization statistics that can be analyzed. Many of the reporting tools available with a switch or another SNMP/RMON management tool will allow an administrator to see how many packets or octets (depending on the program) are traversing the network at any time. When new applications are added, or other changes are made to the network, these statistics should be reviewed. These planning tools will also report what protocols are loaded and communicating on the network links. This is a great place to find some of the mystery bandwidth thieves from ancillary protocols being loaded without having to physically go from machine to machine.
Everyone has heard the analogy that bandwidth is similar to the lanes of an interstate. The more lanes available, the more cars can travel that stretch. It is a common misconception that your transmissions are limited to your slowest link (WAN link). While traffic may slow for part of the journey, once on a faster segment, the speed will increase. This slow link is one source of a bottleneck. Think of it as a toll booth on the interstate. Each packet has some sort of timeout mechanism built into it that allows the sending station to know if the intended recipient received the packet.
For years, the most limiting factors have been bus speeds, network speeds and operating systems. Today's 64 bit operating systems have been developed in order to speed up transmissions, but you can also flood older electronics. The speeds should be matched for capabilities.