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1.3 Compare physical interface and cabling types

Compare physical interface and cabling types

In this section we will explore the physical media (interfaces and cabling) used in networking, the purpose behind them and the key specifics. There are two key methods of transmitting data across a link which are either light, or electricity (wireless transmission will be discussed in further articles). You will commonly see the term Ethernet when looking at Layer 1 equipment (cabling or interfaces). Ethernet in it's simplest definition is a family of technologies and protocols that standardise how frames are forwarded through a network. This is called the IEEE 802.3 protocol suite.

Physical Interfaces

The interface is the physical socket that is used to connect network appliances to one another. The following table outlines the interface types found on most Cisco appliances:

Interface Types

Fibre Optics

Fibre optic cables use an extremely small glass or plastic core (as thin or thinner than a human hair) to transmit a beam of light between two nodes. These nodes must have optical transmission capability, or a converter such as an SFP (Small Form-Factor Pluggable) to turn the light signals into electrical signals. There are multiple ways of transmitting data using light, we will explore these in the next section.

Single-mode Fibre

A single-mode fibre optic cable allows for a single beam of light to be sent into the wire. These cables have very small cores which reduces the degradation of the signal over longer distances, with 1Gbps transmit speeds seen up to 40 miles between nodes.

SM Fibre
Multi-mode Fibre

Multi-mode fibre cables are similar in design to single-mode with one key difference, the core is much larger. With this larger core it is possible to send light signals at different angles where they will refract off of the cladding surrounding the core. Due to this refraction multi-mode cables cannot transmit light the lengths seen with single-mode. Multi-mode fibre media has a larger amount of attenuation as compared to Single-mode, therefore the distances covered are greatly impacted. This is because energy is lost each time the light signal refracts off the outer core wall.

multimode fibre
Key Differences

Below are some of the key differences between single-mode and multi-mode cable types.

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Interfaces

Below are some of the key differences between single-mode and multi-mode cable types.

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SFPs

An SFP (Small Form-Factor Pluggable) is found on the vast majority of enterprise/business level networking appliances. They provide a hot-plug in which a number of different fibre-optic connectors can be used. SFPs provide for flexibility within the hardware layer of the network topology, removing constraints of being bound to one type of physical media. To the right is an example of a typical duplex LC SFP.

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Copper

Typically, most Ethernet networks use twisted pair wiring with RJ45 connector blocks at each end. Within the twisted pair wire, there are 8 wires of which makes 4 pairs of twisted wires. The wires are twisted together to minimise the effects of electromagnetic interference and noise as reliable transmission of bits across the wire. These wires are allotted into categories as improvements have been made and features added:

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STP (Shielded Twisted Pair)

These wires are best used over longer distances or where there may be increased electromagnetic interference from lights or electrical wiring. Each pair of wires is contained within an aluminium foil providing additional protection over UTP. STP cabling carries a higher cost due to the additional materials and manufacturing processes.

UTP (Unshielded Twisted Pair)

This is the most common twisted pair found in most networks due to their low cost. Like STP, there are 4 twisted pairs made up from 8 individual wires.

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RJ45

At the end of each side of the copper cable, an RJ45 (Registered Jack 45) is used to terminate to the network or end user appliance. The RJ45 plug has 8 pins and a locking tab to minimise network issues due to accidental removal. There are two configurations for which the order of the pins are housed (T568A and T568B which will be explored below). You may hear the term patch lead/cable, which is a popular term to describe a copper Ethernet cable with an RJ45 jack at each end of the wire.

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T568A / T568B Pin Layouts

Looking at the below image, you can see that pins 1,2,3 and 6 are different between the T568A and B pin layouts. These pins are used for the transmission of data between network appliances. Pins 4,5,7 and 8 are used solely for (PoE) Power over Ethernet which allows low power consuming devices such as desk phones to connect to the network and be powered from a single cable.

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Crossover Cable

The network interface on network appliances and EU devices can differ between the transmit and receive pins. Because of this, depending on the requirement either a straight through or crossover cable will be needed. A straight through cable will have the same T568 plug at each end, whereas a crossover cable will have T568A at one end, and T568B at the other. A crossover cable is only required where each interface on the network appliances have matching transmit (and receive) interface pins. Below are some examples of where a crossover cable is required:

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Straight-Through Cable

A straight through cable on the other hand does not need to hand the pins differ on each end of the copper wire. This is because the network devices themselves have differing pins within the interfaces for receive and transmit. For example a network switchport has a different internal pin layout as opposed to an end user device such as a laptop or wireless access point.

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1.3.b Connections (Ethernet shared media and point-to-point)

Ethernet Shared Media

Prior to point - to - point networks with layer 2 switches etc, most networks used a share media topology such as a bus network or a hub to transmit data. Due to the nature of the shared media, it was highly likely that collisions would occur on either a hub or bus network. For this reason, half-duplex is used alongside CSMA/CD (Carrier Sense Multiple Access with Collision Detection).

 

  • Hubs - hubs operate at half-duplex and traffic is flooded out of ALL ports, meaning that all devices connected to that hub are part of the same collision domain. The more devices connected to the same hub, the greater the degradation of network performance. Should two devices attempt to transmit data at the same time, a collision will occur and the data will be dropped. CSMA/CD will detect this and provide the necessary workflow to re-transmit the data.

  • Bus Topology - EU devices connected to a single bus cable will be sharing the transmission with other devices. Again like with a hub, this introduces the collision problem by there being only a single collision domain.

Point - to - Point Network

Modern networks have done away with Ethernet shared media, and most of the issues that were present with older networks. With the introduction of switches (L2 & L3) each port is it's own collision domain. This therefore allows devices to transmit and receive at full-duplex. This has improved network stability and performance massively. Collisions in a modern switched Ethernet network are virtually impossible if everything is configured correctly.

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