4-Understanding Ethernet Standards

8/12/2019 4-Understanding Ethernet Standards

1/11


2/11


3/11


4/11

Devices Transmits on Pins 3 and 6

Hubs

Switches

Determining which type of cable to use: Straight through or Crossover Cable

if 1 and 2 type device (PC)connect to 3 and 6 type device (Switch), requires Straight-Through Cable.

If 1 and 2 type device (PC)connect to 1 and 2 type device (PC)requires Crossover cable.

Now reverse it:

if 3 and 6 type device (Switch)connect to 1 and 2 type device (PC)requires Straight-Through Cable.

if 3 and 6 type device (Switch)connected to 3 and 6 type device (Switch)requires Crossover Cable.

UTP Cabling for 1000BASE-T

Interestingly, the same standard above for 10BASE-T and 100BASE-T pinouts works correctly for 1000BASE-T

cabling, except that 1000BASE-T uses the 4 pairs instead. Meaning, when you crimp the RJ-45 cable based on thecolor codes above either for Straight-Through or Crossover Cable, this pinouts configuration works correctly on 10,

100, and 1000BASE-T.

100BASE-T Straight-Through uses pin 1 to pin 1, pin 2 to pin 2, up through 8, just like in the earlier wiring of 10 and

100BASE-T, it uses all the wires inside the UTP cable. 1000BASE-T Crossover cable crosses the same two-wire pa

as the crossover similar to earlier 10 and 100BASE-T, the pair at pins 1 and 2 meet with pair 3 and 6. In addition,

another new pair added in case of 1000BASE-T that represents 4 and 5 to meet with 7 and 8 and crosses the pairs.

Its just another UTP cable using the complete 4 pairs instead of 2 pairs.

Forwarding Data inside an Ethernet Networks

Ethernet Protocol Standards rules and control Ethernet regardless of the type of physical Ethernet link used such

Copper or Fiber. Ethernet Protocols Controlhow Ethernet Hosts, Switches, and Routers forward Ethernet frames

through an Ethernet Network based on the MAC AddressSystem.

Ethernet Data Link Protocol

The Ethernet Data Link protocol defines the Ethernet frame:

An Ethernet Header at the front

The Encapsulated Data in the middle

And an Ethernet Trailer at the end

The following figure shows an Ethernet Frame.
http://www.ccnahub.com/ip-fundamentals/understanding-mac-address/


5/11

Breaking down the Ethernet Header to 5 sections

* The IEEE 802.3 specification limits the data portion of the 802.3 frame to a minimum of 46 and a maximum of 150

bytes. The Term Maximum Transmission Unit (MTU) defines the maximum Layer 3 packet that can be sent over a

medium. Because the Layer 3 packet rests inside the Data Portion of an Ethernet frame, therefore, 1500 bytes is thlargest IP MTU allowed over an Ethernet.


6/11

Hubs and Switches Overview

The use of modern switches allows the use of Full-Duplex logic, which is much faster and simpler than half-duplex

logic used by Hubs.

Sending Ethernet with modern Switches using Full-Duplex vs. Hubs using Half-Duplex

In order to understand the difference between Hubs and switches, we need to go back little bit in time, to the

beginning of the 1990s! IEEE introduced 10BASE-T standard in1990; 10BASE-T used a centralized cabling model

similar to todays ethernet LAN, meaning each device connecting to the LAN Hub using a UTP cable, using Categor

3 back then (CAT3 UTP Cable). However, instead of a LAN switch, the early 10BASE-T networks used Hubs,

because LAN switches had not yet been created.


7/11


8/11

1. When hubs receive an electrical signal on one port, the hub repeats the signal out all other ports.

2. When two or more devices send at the same time, an electrical collision occurs, making both signals corrupt. As a

result, devices must take turns by using CSMA/CD.

*CSMA/CD tell all devices on the a Collision Domain to back off for few seconds in order to overcome the collision

situation and send a signal again, this logic makes shared signal which is connected to Hub as Half-Duplex Network

Briefly, devices connected to Hub cant send and receive at the same time. Only one device at a time since it is Ha

Duplex. Consider a Classroom with one door, used to enter or exit only one student at a time.

And broadcasts sent by one device are heard by, and processed by, all other devices on the LAN. Broadcast basica

a drawback of both Hubs and Layer 2 Switches. Only Routers, Layer 3 switches, and VLANs can separate Broadca

Domains.

Besides, as One Collision Domain, Unicast Frames are heard by all other devices on a Hub LAN. * Switches

overcome this draw back since the switchs ports separate Collision Domains, making Unicast frames heard only

between 2 devices, which means, each switchs port is a Collision Domain by itself. Therefore, switch port allows

Full-Duplex between sending and receiving host using switch ports.

Over time and through 1990s, the performance of many Ethernet networks based on hubs started to degrade. Peop

developed too many Applications to take advantage of the LAN bandwidth provided by 10BASE-T. More devices weadded to each Ethernet that might have one Hub or multiple Hubs. However, the devices on the same Ethernet coul

not send (collectively) more than 10 Mbps of traffic because they all shared the 10 Mbps of Bandwidth. In addition, t

increase in traffic volumes resulted in an increased number of collisions, requiring more retransmissions and wastin

more LAN capacity.

Transparent Bridge

To solve Hubs drawbacks, especially the ONE Collision Domain situation, vendors came up with a Transparent Brid

to divide a One Collision Domain to 2 Collisions Domains.

Bridges separated devices into groups called Collision Domains.

Bridges reduced the number of collisions that occurred in a Hubs network, therefore frames inside one collision

domain will not collide with frames in another collision domain.

Bridges increased bandwidth by giving each collision domain its own separate bandwidth, with one sender at a time

per collision domain.

Why hubs considered One Big Collision Domain (CD)? If we want to analyze an 8 ports Hub, we will find out that ev

though there are 8 ports it does not mean 8 separated collision domains (like Switches), Hubs are Layer 1 devices,

they are dump devices (Just Repeaters) and have no intelligence what so ever about where the electrical signal goi

through them, or where it might be going in the LAN, or where should be forwarded.


9/11

So all they do as dump devices with the incoming signal from a specific port is: to Flood it out through the rest of the

ports (except the incoming port where the signal arrived) with no intelligence such Layer 2 Switches where it

investigate each frame. Meaning, if PC1 in the figure above wants to talk to the Web-Server, PC2 has to listen as we

to the conversation which is Unicast communication and not broadcast!!!, but because hubs flood the signal to all

ports (even Unicast), PC2 will for sure hear the conversation; creating unnecessary traffic, that is sharing and wastin

bandwidth.

The Bridge, a predecessor to todays Ethernet LAN switch, uses logic so that the frames in one Collision Domain do

not collide with frames in the other Collision Domain. The bridge forwards frames between its two interfaces, andunlike a Hub, a bridge will buffer or queue the frame until the outgoing interface can send the frame. As shown in the

figure above, the 10BASE-T network on the left (CD1) has its own 10 Mbps to share, as does the network on the rig

(CD2). In this case, the total network bandwidth is doubled to 20 Mbps as compared with 10BASE-T One Big

Collision Domain previous figure above. Because of the Bridge, the devices on each side of the network (CD1 and

CD2) can send to each other at 10 Mbps at the same time, created a Full-Duplex connection between CD1 and CD2

at the Bridge side.

Todays Modern Bridge (LAN Switch)

If you replace a Hub with a LAN switch, a switch prevents collisions since it operates at the Data Link Layer, therefo

considered as Layer 2 device, meaning that it looks at the data link layer Header and Trailer. And most important aswell, a switch would look at the MAC addresses to determine frames destination, giving the switch a way to manage

the frames through some kind of forwarding to the right port, and even if the switch needed to forward multi-frames t

the same network device, the switch would send one frame and queue the other frames until the first frame was

processed and finished.

Each switchs port is a Collision Domain by itself. If only two devices connected to each other using Layer 2

intelligence (which means the signal will be addressed as Frame), they can use Full-Duplex. Switches use Full-Dup

on each link (One Pair to send and another Pair to receive), meaning one way to Enter and another way to Exit. But

the concept might be difficult to see. Full-Duplex means that a NIC or Switch port has no half-duplex restrictions. A

separate article Understanding Hardware Switching Concept will further explain how a switch function as Full-

Duplex device.

LAN switches perform the same basic core as bridges, but at much faster speeds and with many enhanced hardwa

features such Application Specific Integrated Circuit (ASIC) for faster frame forwarding. Thats why is very hard to

emulate a switch using GNS3, but you still can use the EtherSwitch model.

The following figure shows how a switchs ports acted like an independent bridge by itself, created 4 Collision

Domains and 4 Ethernet Segments.


10/11

Comparing Layer 1 Limitations vs. Layer 2 Functions

Layer 1 Limitation:

Cannot communicate or collaborate with upper Layers.

No Addressing Scheme to identify Network devices by Mac Addresses.

Only recognize streams of bits NO frames.

Cannot determine the source of a transmission when multiple devices are transmitting.

Layer 2 Functions:

The Ability to communicate and collaborate with upper Layers via the Sub-Layer of Data-Link Layer called Logical

Link Control.Uses Addresses Scheme to identify Network devices by MAC addresses.

Uses Frames to organize bits into groups.

Uses the sub-layer of Data-Link Layer Called Media Access Control (MAC) to identify transmission sources.

Next: Understanding MAC Address

Go to top

About Imad Daou

He is the founder of CCNA HUB, a CCNA Training HUB to help CCNA students get certified. Imad has more than 10

years of IT experience as Field Service and Consulting Engineer. A+, Network+, Server+, Security+, Storage+, HP,

Dell, and IBM Hardware Certified. He's a Professional SMB IT Consultant.

Facebook Twitter Linkedin Google

Copyright secured by Digiprove 2013 Imad DaouSome Rights Reserved
http://dprv_displaylicense%28%271170%27%29/http://www.digiprove.com/prove_copyright.aspx?id=P465207https://plus.google.com/+Ccnahubhttps://www.linkedin.com/in/emaddaouhttps://twitter.com/ccnahubhttps://www.facebook.com/ccnahubhttp://www.ccnahub.com/http://www.ccnahub.com/author/imad-daou/http://www.ccnahub.com/ip-fundamentals/understanding-ethernet-standards/#http://www.ccnahub.com/ip-fundamentals/understanding-mac-address/


11/11

4-Understanding Ethernet Standards

Documents

Transcript of 4-Understanding Ethernet Standards