Fundamentals of Ethernet

Networks can be separated in to 2 general types, WAN and LAN. LANs typically connect near by devices: devices in the same room. WANs connect devices that are far apart. Together WAN and LANs create a Enterprise network, working together to do the Job of a computer network, delivering data from one device to another.

While many types of LANs have been commonly used of the years, todays networks use mainly wireless LAN and Ethernet LAN. LANS typically use cabling for the link between nodes. Because LANs use typically use copper cabling to transmit and receive data they are known as wired LANs, instead of using radio waves between nodes.

An overview of LANs

The Term Ethernet refers to a family of LAN standards that together define the physical and Datalink layers of the most popular wired LAN Technology. First the LAN needs an Ethernet device such as an Ethernet switch. An Ethernet switch which provides many allows for cable connections which conform to several Ethernet formats. The LAN uses Ethernet cables to connect Ethernet devices or nodes which connect to the ports of the Ethernet switch.

SOHO networks Small Office Home office.

A SOHO network is a small network that is used at home or in the office. The SOHO network typically consists of standard devices ie. PC, printers and other various devices. These Ethernet devices typically connect to a switch / router and can communicate with each other. Before today’s modern routers a standard network would have been setup using a small LAN switch, today in most SOHO networks the switch and a router / modem and Wi-Fi are integrated into one device. As said today’s SOHO LANs support wireless LAN connections, however you can build a LAN with switch a Router and add a wireless AP. The wireless AP acts as a wireless access point for wireless devices such as PDAs, mobile phones, wireless laptops. The AP remains as an access point to for devices and they are contained with the LAN. The advantage of this is clear, the AP is connected physically to the LAN somewhere, and this allows users to move around without the need for a physical cable attached to their device. But as said, today manufactures have all features of an AP (WLAN) Switch and Router.

Typical Enterprise LANs

Enterprise LANs have similar needs compared to a SOHO network on a bigger scale. For example, enterprise Ethernet LAN begins with a with a LAN Switch, the LAN switches are normally locked away in a room preventing unauthorized people from putting network cables into the switch or taking them out, also preventing intruders from accessing the network. Normally and electrician will install the Ethernet cabling from the closet to office or conference rooms for devices that might connect to the LAN. Most Enterprises might support wireless LANs with in the same space to support wireless devices for people that are moving with devices throughout the LAN area. So once again the AP will be connected to a port on the Switch which will allow uses to move above or below floors as long as the wireless signal is within range.

The term Ethernet refers to a family of standards. Some standards define the specifics of how the data is sent over a particular type of cabling, and at a particular speed. Other standards define rules or protocols that other Ethernet devices must follow to be part of the network. The Ethernet standards are defined by IEEE and include 802.3 as the beginning of the standard name.

Ethernet supports a variety of options for physical Ethernet links over the last 40 years. Ethernet standards today includes many different standards for optical a copper cabling, speed of data transfer vary greatly from 10mbps up to 100Gbps. The standard also differs depending on the type of cabling and the allowed length of cabling.

The most fundamental cabling choice has to do with the materials used inside the cabling for the physical transmission of bits: either copper wires or glass fibers. The unshielded twisted pair (UTP) cabling saves money compared to fiber optic.

To be ready to choose products for an Ethernet LAN, a Network Engineer must know the names and features of different Ethernet Standards supported for Ethernet products. The IEEE defines physical layer Standards using a couple of naming conventions. The formal name begins with 802.3 followed by some suffix letters. The IEEE also uses more meaning full shortcut names that identify the speed as well as a clue about whether the cabling is UTP (with that suffix that included a “T”) or fiber (with a suffix that includes an “X”.

Here is a list of the 4 most common UTP and an example of a Fiber optic.

10 Mbps Ethernet 10BASE-T 803.3 100 meters, copper
100 Mbps Fast Ethernet 100BASE-T 802.3u 100 meters, copper
1000 Mbps Gigabit Ethernet 1000BASE-LX 802.3z 5000 meters, copper
1000 Mbps Gigabit Ethernet 1000BASE-T 802.3ab 100 meters, copper
10 Gbps 10 Gig Ethernet 10GBASE-T 803.an 100 meters, copper

*note fiber cable cabling contains thin strands of fiber glass and the attached nodes send light over the fiber cabling, encoding the bits as changes in the light. Consistent behavior over all links using the Ethernet Data link layer.

Although Ethernet includes many physical Standards, Ethernet acts like a single LAN technology because it uses the same data link standard over all types of Ethernet physical links. The standard defines a common Ethernet header and trailer. No matter whether data over a UTP cable, or any kind of fiber cable, no matter what speed, the header and trailer use the same format.

While the physical layer standard focus on sending bits over the cable, the Ethernet data link protocols focus on sending Ethernet frames from the source to destination node. From the Data link perspective, nodes build and forward frames. The “TCP/IP and OSI networking models” the term “frame” specifically refers to the header and trailer of the data link protocol, plus the data encapsulated inside that header and trailer. The various Ethernet nodes simply forward the frame, over all the required links, to deliver the frame to the correct destination.

Figure 2-4 in CCNET Book shows an example of the process.

In this case PC1 sends an Ethernet frame to PC3. The frame travels over the UTP link to Ethernet switch SW1, then over fiber links to Ethernet Switch SW2 and SW3, then finally over a UTP cable to PC3, note the bits actually travel a 4 different speeds in this example, 10mpbs 1Gps 100mbps and 10mbps.

So what is an Ethernet LAN, it is a combination of devices, switches, and different kinds of cabling, each link can use different kinds of cabling and switches. However they can all work together to deliver Ethernet frames from one device to another device.

Building a physical Ethernet Networks with UTP.
This section focuses on the 3 most commonly used Ethernet standards 10BASET 100BASET ANS 1000BASET (or GE). Specifically, this section looks at the details of sending data in both directions over a UTP cable. It then examines specific wiring of the UTP cable used for 10mbps and 100mbps and 1000mbps Ethernet.
Transmitting data using Twisted pairs.

While it is true that Ethernet sends data over UTP cables, the means to send the data uses electricity that flows over the wires inside the UTP cable. To better understand how Ethernet sends data using electricity, break the idea down into 2 parts: how to create an electrical circuit and then how to make an electrical circuit and then how to make the electrical signal communicate 1s and 0s.

First, to create one electrical circuit, Ethernet defines how to use the 2 wires inside a single twisted pair of wires as shown in figure 2.5, The figure does not show a UTP cable between 2 nodes, it simply shows 2 wires inside a UTP cable. An electrical circuit requires one pair to create a complete loop, so the 2 nodes, using circuitry on their Ethernet ports, connect the wires in one pair to create a loop, allowing electricity to flow.

Figure 2.5 Creating one electrical Circuit over one pair to send in one direction.
To send data over the 2 devices follow rules called encoding scheme. The idea works like 2 people talking, using the same language: The speaker says some words in a particular language, and the listener, because she speaks the same language, can understand the words. With an, the transmitting node changes the electrical signal over time, while the other node, the receiver, using the same rules, interpretates those changes as either 0 or 1s (for example 10BASE-T uses an encoding scheme that encodes a binary 0 as a transition from a higher voltage during a middle of a 1/10,00,000,000 of a signal a second interval.)
Note that in an actual UTP cable, the wires will be twisted together and not parallel, as shown in Figure 2-5, the Twisting helps over come some physical transmission issues. When electrical current passes over any wire, it creates electrical magnetic interference (EMI) that interferes with the electrical signals in nearby wires, including the same wires in the same cable. EMI is also known as cross talk. Twisting the wire pair’s together halps cancel out most of the EMI, so most networking physical links that use copper wires use twisted pairs.

Breaking down the UTP Ethernet link

The term Ethernet refers to any physical cable between 2 ethernet nodes. To lean about how UTP Ethernet link works, it helps break down the physical link into those basic pieces as shown in figure 2-6: the cable itself has connectors at the end of the cable which in turn have matching ports on the devices into with the connectors will be inserted.

First think about the UTP cable itself. The cables holds some copper wires, grouped as twisted pairs. The 10BASE-T and 100BASE-T standards require 2 pairs of wires, while the 1000BASE-T standard requires 4 twisted pairs. Each wire has a color-coded plastic coating, with the wires in a pair having a colour scheme. For example the blue wired pair, one wire is blue and the other is blue white striped.

Many Ethernet UTP cables use a RJ45 connector on both ends. The RJ-45 has 8 physical pin locations into which the 8 wires can be inserted, called pin positions, or simply pins. These pins create place where the ends of the copper can touch the electronics inside the nodes at the end of the physical link so the electricity can flow.

To complete the physical link both nodes need a RJ-45 Ethernet port that matches each connectors at both ends of the cable. PCs today come with an onboard NICs ( Network interface card ). PCs may also have a NIC that is fitted into the PC in the form of an expansion card.

Figure 2-7 shows RJ-45 ports on a switch, RJ-45 connector on a UTP cable. Be aware the RJ-11 cable looks similar to an RJ-45, however the RJ-45 cable is slightly wider. The RJ-11 is typically used for a telephone connection.
Finally, while RJ-45 coonetors with UTP cabling can be common. CISCO LAN switches often support other types of connectors aswell.