For this control to be effective, there must be only a single entrance through which all traffic passes. The guard may function at different levels. Perhaps a name badge attached to a person's pocket is sufficient authentication. More rigorous authentication might require a combination of the name badge and a PIN, and include a computerized recording of the badge number and time. The most rigorous authentication would involve a gateway with two doors, and each person subjected to a retinal scan before being allowed to enter the inner door.
Armed guards stand by, just in case.
The firewall your organization uses provides the security at the entranceway to your network. If you permit individuals to use modems, your organization does not have a single entryway, but many ports of exposure. Most of all, your firewall is an implementation of your security policy. The policy itself should be based on examining your orgainzation's assets, and determing which level of access is appropriate when compared to the risk to those assets. Firewall technologies vary as to each one's strengths and weaknesses, and it is up to you to choose the right technology and vendor.