Design issues at Layer 1 include the type of cabling to be used, typically copper or fiber-optic, and the overall structure of the cabling. Layer 1 cabling media includes types such as 10/100BASE-TX Category 5, 5e, or 6 unshielded twisted-pair (UTP), or shielded twisted-pair (STP), 100BaseFX fiber-optic cable, and the TIA/EIA-568-A standard for layout and connection of wiring schemes.

Careful evaluation of the strengths and weaknesses of the topologies should be performed. A network is only as effective as its underlying cable. Layer 1 issues cause most network problems. A complete cable audit should be conducted, when planning any significant changes for a network, to identify areas that require upgrades and rewiring.

Fiber-optic cable should be used in the backbone and risers in all cable design settings. Category 5e UTP cable should be used in the horizontal runs. The cable upgrade should take priority over any other necessary changes. Enterprises should also make certain that these systems conform to well-defined industry standards, such as the TIA/EIA-568-A specifications.

The TIA/EIA-568-A standard specifies that every device connected to the network should be linked to a central location with horizontal cabling. This applies if all the hosts that need to access the network are within the 100-meter distance limitation for Category 5e UTP Ethernet.

In a simple star topology with only one wiring closet, the MDF includes one or more horizontal cross-connect (HCC) patch panels. HCC patch cables are used to connect the Layer 1 horizontal cabling with the Layer 2 LAN switch ports. The uplink port of the LAN switch, depending on the model, is connected to the Ethernet port of the Layer 3 router using a patch cable. At this point, the end host has a complete physical connection to the router port.

When hosts in larger networks are outside the 100-meter limitation for Category 5e UTP, more than one wiring closet is required. By creating multiple wiring closets, multiple catchment areas are created. The secondary wiring closets are referred to as intermediate distribution facilities (IDFs). TIA/EIA-568-A standards specify that IDFs should be connected to the MDF by using vertical cabling, also called backbone cabling. A vertical cross-connect (VCC) is used to interconnect the various IDFs to the central MDF. Fiber-optic cabling is normally used because the vertical cable lengths are typically longer than the 100-meter limit for Category 5e UTP cable.

The logical diagram is the network topology model without all the detail of the exact installation paths of the cabling. The logical diagram is the basic road map of the LAN including the following elements:

• Specify the locations and identification of the MDF and IDF wiring closets.
• Document the type and quantity of cabling used to interconnect the IDFs with the MDF.
• Document how many spare cables are available for increasing the bandwidth between the wiring closets. For example, if the vertical cabling between IDF 1 and the MDF is running at 80% utilization, two additional pairs could be used to double the capacity.
• Provide detailed documentation of all cable runs, the identification numbers, and the port the run is terminated on at the HCC or VCC.

The logical diagram is essential when troubleshooting network connectivity problems. If Room 203 loses connectivity to the network, by examining the cut sheet it can be seen that this room is running off cable run 203-1, which is terminated on HCC 1 port 13. Using a cable tester it can be determined whether the problem is a Layer 1 failure. If it is, one of the other two runs can be used to reestablish connectivity and provide time to troubleshoot run 203-1.