The MDF, or main distribution frame, is the central network room or closet where the carrier's circuit arrives and your core equipment lives: firewall, core switch and often servers. An IDF, or intermediate distribution frame, is a smaller closet that serves part of the space, such as another floor or a far wing, and connects back to the MDF over backbone cabling. Small offices often have only an MDF. Whatever the size, the closet needs dedicated power with a battery backup, enough cooling, physical security and labels someone else can understand.
Network closets tend to be designed last and noticed only when something goes wrong. A closet that doubles as a storage room, with no ventilation and a single power strip, is behind a lot of the "the internet keeps dropping" calls we hear. Getting it right is mostly about planning, not money. For the cable itself, see our office network cabling guide; this article covers the rooms.
MDF and IDF, side by side
| MDF | IDF | |
|---|---|---|
| Role | The hub of the network | A local distribution point |
| Usually contains | Carrier handoff or demarc extension, firewall, core switch, servers or storage, patch panels | Access switches, patch panels, sometimes a small battery backup |
| Connected to | Every IDF, and the outside world | The MDF, over backbone cabling |
| How many | One per site, normally | As many as distance and floor plan require |
| Most important needs | Power, cooling, security, space to grow | Ventilation, power, a clean path back to the MDF |
In a multi-tenant building there is another layer: the building's own main telecom room, where carriers bring fiber in, and telecom closets on each floor that serve the risers. Your MDF is inside your space and connects to those. If you manage the building, our guide to internet for multi-tenant buildings covers the building side.
Do you need an IDF?
The deciding factor is cable length. Copper Ethernet is designed for channels of up to 100 meters, including patch cords, from the switch port to the device. If every desk, access point and camera is within that distance of the MDF, measured along the actual cable path rather than in a straight line, you may not need an IDF at all. Typical reasons to add one:
- Another floor, or a separate suite you also lease.
- A long, narrow space where far ends exceed the distance limit.
- A warehouse, production floor or yard with drops far from the office.
- Large numbers of drops in one area, where running every cable back to the MDF would be bulky and costly.
IDFs usually connect to the MDF with fiber, which handles distance easily and is not affected by electrical interference, often with a spare pair or two for growth. Short links within one floor can be copper.
Where to put the closet
- Central, if you can. A central location keeps cable runs short.
- Close to the building's pathways. Near the riser or where the carrier's demarc extension arrives keeps that run short and simple.
- Away from water. Avoid spaces under bathrooms or kitchens, or with water pipes overhead.
- Not shared with cleaning supplies, files or a printer. Dust, chemicals and people looking for paper towels do not mix with equipment.
- Lockable, with a door that closes, and ideally a log of who has the key.
- Room to work. Leave clearance in front of and behind racks so someone can actually reach the back of a switch.
For small offices without a closet, a lockable, ventilated wall-mounted cabinet in a back room can do the job of an MDF.
Power
- A dedicated circuit for the closet, so a space heater or a microwave elsewhere cannot trip it. Your electrician can advise on capacity for the equipment you plan.
- A battery backup (UPS) sized to keep the firewall, switches and carrier equipment running through short power cuts, and long enough to shut servers down cleanly in a longer one.
- Count PoE load. Switches powering phones, access points and cameras draw much more than switches that do not. Size the UPS for the real load, not the switch's idle figure.
- Put the carrier's equipment on the UPS too. A firewall on battery is no use if the carrier's box beside it goes dark.
- Replace UPS batteries on schedule, and test the unit. A battery backup that has never been tested is a hope, not a plan.
Cooling
Switches, firewalls and servers turn electricity into heat, and a small closed room heats up fast. Overheated equipment slows down, reboots and fails early, often in ways that look like random network problems.
- Check the equipment's operating temperature range in its specifications, and keep the room comfortably inside it.
- Ventilation may be enough for a closet with a few switches: a louvered door, or an exhaust fan pulling warm air out.
- Rooms with servers or a lot of PoE often need dedicated cooling that runs when the building's air conditioning is off at night and on weekends.
- Watch the airflow direction. Most rack equipment pulls air in at the front and pushes it out the back. Keep the hot side clear.
- Monitor the temperature. Many switches and UPS units report it, and a cheap sensor that alerts someone is worth it. Our guide to network monitoring covers alerting.
Racks, cable management and labeling
A closet that only one person understands is a single point of failure, just like a single circuit.
- Use a rack or cabinet sized with room to grow. Two-post open racks suit patch panels and switches; four-post or enclosed cabinets suit deeper equipment and servers.
- Patch panels, then switches, with short patch cords between them, and horizontal cable managers so the front stays readable.
- Label both ends of every cable and every port on the patch panel, using a consistent scheme. The industry standard for labeling is ANSI/TIA-606, and a scheme based on it is easier for any contractor to follow.
- Label the carrier equipment with the carrier name, circuit ID and support number, and your firewall's WAN ports.
- Color-code by purpose if it helps: one color for the internet, one for phones, one for cameras.
- Keep a one-page diagram in the closet and a copy elsewhere, showing what connects to what.
Grounding and bonding of racks and cable pathways has its own standard, and your low-voltage contractor and electrician should handle it to code.
The demarc extension
The carrier's circuit ends at a demarc, often in the building's basement telecom room, which may be several floors away from your MDF. The demarc extension is the cabling that carries the service from that point to your closet. Who installs and pays for it varies by carrier and building, so ask before you order. It should be fiber for long runs or higher speeds, follow an approved pathway, be labeled at both ends and be tested before install day. Our guide to the demarc and inside wiring covers the details.
A worked example
Hypothetical: the company, floor plan and equipment are invented to show the planning.
A 60-person company leases two floors in an older building. The carrier's equipment is in the basement. On the lower floor, the plan is an MDF in an interior room near the riser, with a dedicated power circuit, a UPS sized for the firewall, core switch, carrier equipment and the PoE switch serving that floor's access points, and an exhaust fan with a temperature alert. On the upper floor, an IDF directly above it holds one PoE access switch, a small UPS and a patch panel, connected to the MDF by two fiber pairs, one in use and one spare.
The demarc extension runs as fiber from the basement to the MDF, labeled at both ends with the circuit ID. Every jack, patch panel port and cable is labeled to one scheme, and the diagram is taped inside the closet door. The extra cost over "a switch on a shelf in the supply room" is modest, and the first time someone needs to trace a problem, it pays for itself. The access points on both floors are placed from a survey, as our guide to access points vs mesh describes.
A closet checklist
- Lockable, dry, not shared with storage.
- Dedicated power circuit and a tested battery backup for every device, carrier equipment included.
- Ventilation or cooling that works nights and weekends, with a temperature alert.
- Rack or cabinet with space to grow and clearance to work.
- Every cable and port labeled to one scheme, and a current diagram.
- Fiber backbone to each IDF, with spare strands.
- A labeled, tested demarc extension from the carrier's equipment.
Planning the closet with the circuit
The best time to design a closet is before the circuit is ordered and the cabling is quoted. We price circuits from several Tier 1 carriers for your address and coordinate the handoff and demarc extension, and our managed network service covers switches, Wi-Fi and cabling coordination if you would rather hand off the inside too. The carrier pays FiberX, so nothing is marked up, and our lowest-price promise applies. The consultation is free, you usually hear back the same day, and Phil Morales is your one point of contact. Send us your address and floor plan, call 478-758-8091 or text (347) 870-0965.
