How to Cable LANs and WANs

By | 23rd November 2015

Cabling LANs and WANs.

When planning the installation of LAN cabling, choosing the cables necessary to make a successful LAN or WAN connection requires consideration of the different media types.
There are many different Physical layer implementations that support multiple media types:
• UTP (Category 5, 5e, 6, and 7)
• Fiber-optics
• Wireless
Each media type has its advantages and disadvantages. Some of the factors to consider are:
• Cable length – Does the cable need to span across a room or from building to building?
• Cost – Does the budget allow for using a more expensive media type?
Bandwidth – Does the technology used with the media provide adequate bandwidth?
• Ease of installation – Does the implementation team have the ability to install the cable or is a vendor required?
• Susceptible to EMI/RFI – Is the local environment going to interfere with the signal?

Making LAN Connections

UTP cabling connections are specified by the Electronics Industry Alliance/Telecommunications Industry Association (EIA/TIA).


The RJ-45 connector is the male component crimped on the end of the cable. When viewed from the front, the pins are numbered from 8 to 1.
When viewed from above with the opening gate facing you, the pins are numbered 1 through 8, from left to right.
This orientation is important to remember when identifying a cable.

rj45 connector

rj45 connector explained

Types of Interfaces

In an Ethernet LAN, devices use one of two types of UTP interfaces – MDI or MDIX.

The MDI (media-dependent interface) uses the normal Ethernet pinout. Pins 1 and 2 are used for transmitting and pins 3 and 6 are used for receiving.
Devices such as computers, servers, or routers will have MDI connections.

The devices that provide LAN connectivity – usually hubs or switches – typically use MDIX (media-dependent interface, crossover) connections.
The MDIX connection swaps the transmit pairs internally. This swapping allows the end devices to be connected to the hub or switch using a straight-through cable.

Typically, when connecting different types of devices, use a straight-through cable. And when connecting the same type of device, use a crossover cable.

Straight-through UTP Cables

A straight-through cable has connectors on each end that are terminated the same in accordance with either the T568A or T568B standards.

Identifying the cable standard used allows you to determine if you have the right cable for the job. More importantly, it is a common practice to use the same color codes throughout the LAN for consistency in documentation.

Use straight-through cables for the following connections:
Switch to a router Ethernet port
Computer to switch
Computer to hub

straight through cable

Crossover UTP Cables

For two devices to communicate through a cable that is directly connected between the two, the transmit terminal of one device needs to be connected to the receiving terminal of the other device.

The cable must be terminated so the transmit pin, Tx, taking the signal from device A at one end, is wired to the receive pin, Rx, on-device B. Similarly, device B’s Tx pin must be connected to device A’s Rx pin. If the Tx pin on a device is numbered 1, and the Rx pin is numbered 2, the cable connects to pin 1 at one end with pin 2 at the other end. These “crossed over” pin connections give this type of cable its name, crossover.

To achieve this type of connection with a UTP cable, one end must be terminated as EIA/TIA T568A pinout, and the other end terminated with T568B pinout.

Crossover cables directly connect the following devices to a LAN:
• Switch to switch
• Switch to hub
• Hub to hub
• Router to router Ethernet port connection
• Computer to computer
• Computer to a router Ethernet port

crossover cable


Use straight-through cables for connecting:
• Switch to router
• Computer to switch
• Computer to hub

Use crossover cables for connecting:
• Switch to switch
• Switch to hub
• Hub to hub
• Router to router
• Computer to computer
• Computer to router

MDI/MDIX Selection

Many devices allow the UTP Ethernet port to be set to MDI or MDIX. This can be done in one of three ways, depending on the features of the device:

1. On some devices, ports may have a mechanism that electrically swaps the transmit and receive pairs.
The port can be changed from MDI to MDIX by engaging the mechanism.

2. As part of the configuration, some devices allow for selecting whether a port functions as MDI or as MDIX.

3. Many newer devices have an automatic crossover feature.
This feature allows the device to detect the required cable type and configures the interfaces accordingly. On some devices, this auto-detection is performed by default.
Other devices require an interface configuration command for enabling MDIX auto-detection.


Making WAN Connections

By definition, WAN links can span extremely long distances. These distances can range across the globe as they provide the communication links that we use to manage e-mail accounts, view web pages, or conduct a teleconference session with a client.

Wide area connections between networks take a number of forms, including:
• Telephone line RJ11 connectors for dial-up or Digital Subscriber Line (DSL) connections
• 60 pin Serial connections, Cisco routers use one of two types of physical serial cables.
Both cables use a large Winchester 15 Pin connector on the network end. This end of the cable is used as a V.35 connection to a Physical layer device such as a CSU/DSU.

The first cable type has a male DB-60 connector on the Cisco end and a male Winchester connector on the network end.
The second type is a more compact version of this cable and has a Smart Serial connector on the Cisco device end.
It is necessary to be able to identify the two different types in order to connect successfully to the router.

Data Communications Equipment and Data Terminal Equipment – DCE/DTE

DCEs and DTEs are used in WAN connections. The communication via a WAN connection is maintained by providing a clock rate that is acceptable to
both the sending and the receiving device. In most cases, the telco or ISP provides the clocking service that synchronizes the transmitted signal.

The following terms describe the types of devices that maintain the link between a sending and a receiving device:
• Data Communications Equipment (DCE) – A device that supplies the clocking services to another device. Typically,
this device is at the WAN access provider end of the link.
• Data Terminal Equipment (DTE) – A device that receives clocking services from another device and adjusts accordingly.
Typically, this device is at the WAN customer or user end of the link.
If a serial connection is made directly to a service provider or to a device that provides signal clocking such as a channel service unit/data service unit (CSU/DSU), the router is considered to be data terminal equipment (DTE) and will use a DTE serial cable.

For example, if a device connected via a WAN link is sending its signal at 1.544 Mbps, each receiving device must use a clock,  sending out a sample signal every 1/1,544,000th of a second. The timing, in this case, is extremely short.
The devices must be able to synchronize to the signal that is sent and received very quickly.

By assigning a clock rate to the router, the timing is set. This allows a router to adjust the speed of its communication operations,  thereby synchronizing with the devices connected to it.


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