Some of the typical requirements for a large office LAN are:

• More balanced data transfers (downloads compared to uploads) often including data transfers to other companies or clients, or off-site backups;
• High speed data connectivity;
• A large number of people requiring access, typically with desktops and/or laptops;
• Access by smartphones/tablets may be required, but often restricted;
• Telephone access between office staff members without additional call charges;
• Outgoing telephone connectivity from each staff to the general public;
• Incoming telephone access to dedicated staff (receptionist/s) or to IVRs;

In this scenario, your network would typically comprise of a routers/firewalls connecting to your ISP/s, an internal switched/routed network connecting all your desktops/laptops, a dedicated medium/large VoIP PABX and possibly other servers. Usually wireless connectivity is not required or may be provided and limited in usage.

The firewall/routers will be a business-grade models (eg: An appropriate Cisco router) or servers configured as routers and firewalls and dedicated to that role (NOT used by any staff as a desktop PC). A DMZ would be established where internet accessible servers would be located (such as email server, VoIP server, etc if providing those services to staff outside the office).