The Difference Between Straight Through, Crossover, And Rollover Cables

There are generally three main types of networking cables: straight-through, crossover, and rollover cables. Each cable type has a distinct use, and should not be used in place of another. So how do you know which cable to use for what you need?

The Purpose of Straight-Through Cables

Straight-through cables get their name from how they are made. Out of the 8 pins that exist on both ends of an Ethernet cable, each pin connects to the same pin on the opposite side. Review the diagram below for a visual example:

Notice how each wire corresponds to the same pin. This kind of wiring diagram is part of the 568A standard. The 568B standard achieves the same thing, but through different wiring. It is generally accepted to use the 568A standard as pictured, since it allows compatibility with certain telephone hardware- while 568B doesn’t.
Straight-through cables are primarily used for connecting unlike devices. A straight-through cable is typically used in the following situations:
Use a straight-through cable when:

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• 1. Connecting a router to a hub
• 2. Connecting a computer to a swtich
• 3. Connecting a LAN port to a switch, hub, or computer

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Note that some devices such as routers will have advanced circuitry, which enables them to use both crossover and straight-through cables. In general, however, straight-through cables will not connect a computer and router because they are not “unlike devices.”

The Purpose of Crossover Cables

Crossover cables are very similar to straight-through cables, except that they have pairs of wires that crisscross. This allows for two devices to communicate at the same time. Unlike straight-through cables, we use crossover cables to connect like devices. A visual example can be seen below:

Notice how all we did was switch the orange-white and green-white wires, and then the orange and green wires. This will enable like devices to communicate. Crossover cables are typically used in the following situations:




 Use a crossover cable when:

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• 1. Connecting a computer to a router
• 2. Connecting a computer to a computer
• 3. Connecting a router to a router
• 4. Connecting a switch to a switch
• 5. Connecting a hub to a hub

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While the rule of thumb is to use crossover cables with like devices, some devices do not follow standards. Others provide support for both types of cables. However, there is still something that both crossover and straight-through cables can’t do.

The Purpose of Rollover Cables

Rollover cables, like other cabling types, got their name from how they are wired. Rollover cables essentially have one end of the cable wired exactly opposite from the other. This essentially “rolls over” the wires- but why would we need to do such a thing? Rollover cables, also called Yost cables, usually connect a device to a router or switch’s console port. This allows a programmer to make a connection to the router or switch, and program it as needed. A visual example can be seen below:

Notice that each wire is simply “rolled over.” These types of cables are generally not used very much, so are usually colored differently from other types of cables.

Cable Testing Devices

Cable Testing Devices

It’s generally considered vital to test a cable after it is made, repaired, or otherwise interfered with. We can do this via several different types of devices.

One of the easiest solutions to testing a cable is to look at a wire map. The device will output the wire map on a screen, so that you may review it and check for the correct wiring. A wire map can also tell us is there are any short-circuits, opens, or reversed-pair faults within the wiring. If one of these faults are indeed found, you’ll need to cut off the connector and reapply a new one- this time paying more attention to the wiring process. Below you can see some of the common wiring mistakes for a straight-through cable, as viewed from a wire map.

Wire Map

Other more advanced devices may test for thing such as propagation delay. Propagation delay is a measurement of how long a signal takes to get from one point to another on a cable. Obviously if there is an abnormally long wait time, we will need to adjust the length of the cable accordingly. However, most wiring jobs do not necessarily need tests such as these, and a wire map will suffice.

One last thing to keep in mind about cable testers is that they can indeed test for crosstalk. There are several types of crosstalk, each particularly harmful to your network. It is generally a good idea to test for crosstalk, although the skilled cable maker will know how to properly install a connector, and thus, this test isn’t as vital.

The Difference between Throughput and Bandwidth

The Difference between Throughput and Bandwidth

Although bandwidth can tell us about how much information a network can move at a period of time, you’ll find that actual network speeds are much lower. We use the term throughput to refer to the actual bandwidth that is available to a network, as opposed to theoretical bandwidth.

Several different things may affect the actual bandwidth a device gets. The number of users accessing the network, the physical media, the network topology, hardware capability, and many other aspects can affect bandwidth.
To calculate data transfer speeds, we use the equation Time = Size / Theoretical Bandwidth.

Bandwidth Throughput

Keep in mind that the above equation is actually what we use to find the “best download.” It assumes optimal network speeds and conditions since we use theoretical bandwidth. So to get a better idea on the typical download speed, we use a different equation: Time = Size / Actual Throughput.

What is Bandwidth?

  What is Bandwidth?

You probably already have a fairly good idea on what bandwidth is. It is technically defined as the amount of information that can flow through a network at a given period of time. This is, however, theoretical- the actual bandwidth available to a certain device on the network is actually referred to as throughput (which we’ll discuss further on in this section).

Bandwidth can be compared to a highway in many respects. A highway can only allow for a certain amount of vehicles before traffic becomes congested. Likewise, we refer to bandwidth as finite- it has a limit to its capability. If we accommodate the highway with multiple lanes, more traffic could get through. This also applies to networks- we could perhaps upgrade a 56K modem to a DSL modem and get much higher transfer speeds.
Bandwidth is measured in bits per second (bps). This basic unit of measurement is fairly small, however, and you’ll more than likely see bandwidth expressed as kilobits, megabits, and gigabits.

Unit Of Bandwidth

Make sure you make the distinction between bits and bytes. A megabyte is certainly not the same as a megabit, although they are abbreviated quite similarly. Since we know there are 8 bits in a byte, you can simply divide the number of bits by 8 to find the byte equivalent (or to convert from bytes to bits, multiply by 8).

Megabit megabyte

Lastly, it’s important to also make the distinction between speed and bandwidth. Bandwidth is simply how many bits we can transmit a second, not the speed at which they travel. We can use the water pipe analogy to grasp this concept further. More water could be transported by buying a larger pipe- but the speed at which the water flows is less affected.

How to Subnet a Network

In today's article we are going to learn about the concept of subnetting and how we can use it to divide our classful network into smaller networks that can operate in separate working zones. We'll also take a look at how we can conserve address space and save resources on process management with the use of subnetting.I'll use a few examples to clearly present the steps of subnetting and help you master this topic. And although at first this may seem difficult, don't give up! All it takes is some time and practice!

What Is Subnetting?

Subnetting is the process of stealing bits from the HOST part of an IP address in order to divide the larger network into smaller sub-networks called subnets. After subnetting, we end up with NETWORK SUBNET HOST fields. We always reserve an IP address to identify the subnet and another one to identify the broadcast address within the subnet. In the following sections you will find out how all this is possible.

Why Use Subnetting?

Conservation of IP addresses: Imagine having a network of 20 hosts. Using a Class C network will waste a lot of IP addresses (254-20=234). Breaking up large networks into smaller parts would be more efficient and would conserve a great amount of addresses.
Reduced network traffic: The smaller networks created the smaller broadcast domains are formed hence less broadcast traffic on network boundaries.
Simplification: Breaking large networks into smaller ones could simplify fault troubleshooting by isolating network problems down to their specific existence.

The Subnetting Concept

You will be surprised how easy the concept of Subnetting really is. Imagine a network with a total of 256 addresses (a Class C network). One of these addresses is used to identify the network address and another one is used to identify the broadcast address on the network. Therefore, we are left with 254 addresses available for addressing hosts.
If we take all these addresses and divide them equally into 8 different subnets we still keep the total number of original addresses, but we have now split them into 8 subnets with 32 addresses in each. Each new subnet needs to dedicate 2 addresses for the subnet and broadcast address within the subnet.
The result is that we eventually come up with 8 subnets, each one possessing 30 addresses available for hosts. You can see that the total amount of addressable hosts is reduced (240 instead of 254) but better management of addressing space is gained. I'll now use a couple of examples to help explain the process of subnetting as clearly as possible.

Subnetting a Class C Address Using the Binary Method

We will use a Class C address which takes 5 bits from the Host field for subnetting and leaves 3 bits for defining hosts as shown in figure 1 below. Having 5 bits available for defining subnets means that we can have up to 32 (2^5) different subnets.
It should be noted that in the past using subnet zero (00000---) and all-ones subnet (11111---) was not allowed. This is not true nowadays. Since Cisco IOS Software Release 12.0 the entire address space including all possible subnets is explicitly allowed.

Let's use IP address 192.168.10.44 with subnet mask 255.255.255.248 or /29.

STEP 1: Convert to Binary

STEP 2: Calculate the Subnet Address

To calculate the Subnets IP Address you need to perform a bit-wise AND operation (1+1=1, 1+0 or 0+1 =0, 0+0=0) on the host IP address and subnet mask. The result is the subnet address in which the host is situated.

STEP 3: Find Host Range

We know already that for subnetting this Class C address we have borrowed 5 bits from the Host field. These 5 bits are used to identify the subnets. The remaining 3 bits are used for defining hosts within a particular subnet.
The Subnet address is identified by all 0 bits in the Host part of the address. The first host within the subnet is identified by all 0s and a 1. The last host is identified by all 1s and a 0. The broadcast address is the all 1s. Now, we move to the next subnet and the process is repeated the same way. The following diagram clearly illustrates this process:

STEP 4: Calculate the Total Number of Subnets and Hosts Per Subnet

Knowing the number of Subnet and Host bits we can now calculate the total number of possible subnets and the total number of hosts per subnet. We assume in our calculations that all-zeros and all-ones subnets can be used. The following diagram illustrated the calculation steps.

Subnetting a Class C Address Using the Fast Way

Now let's see how we can subnet the same Class C address using a faster method. Let's again use the IP address 192.168.10.44 with subnet mask 255.255.255.248 (/29). The steps to perform this task are the following:
1. Total number of subnets: Using the subnet mask 255.255.255.248, number value 248 (11111000) indicates that 5 bits are used to identify the subnet. To find the total number of subnets available simply raise 2 to the power of 5 (2^5) and you will find that the result is 32 subnets.
Note that if subnet all-zeros is not used then we are left with 31 subnets and if also all-ones subnet is not used then we finally have 30 subnets.
2. Hosts per subnet: 3 bits are left to identify the host therefore the total number of hosts per subnet is 2 to the power of 3 minus 2 (1 address for subnet address and another one for the broadcast address)(2^3-2) which equals to 6 hosts per subnet.
3. Subnets, hosts and broadcast addresses per subnet: To find the valid subnets for this specific subnet mask you have to subtract 248 from the value 256 (256-248=8) which is the first available subnet address.
Actually the first available one is the subnet-zero which we explicitly note. Next subnet address is 8+8=16, next one is 16+8=24 and this goes on until we reach value 248. The following table provides all the calculated information.
Note that our IP address (192.168.10.44) lies in subnet 192.168.10.40.

Test Your Subnetting Knowledge and Practice, Practice, Practice!

Don't get discouraged if you didn't understand every little detail I went over in this article. Subnetting is not really that difficult, but it does require a bit of practice.
Start with testing your knowledge of subnets and make sure you feel confident about this before you move on to designing your own subnets. But remember, if you're on the Cisco Networking track you will have to deal with subnetting sooner or later, so grab this opportunity and start testing yourself.
Go ahead and subnet the network address 192.168.10.0 address using the subnet mask 255.255.255.192 (/26). Find the valid subnets, host ranges and broadcast addresses per subnet. If you want to double-check your answer, feel free to leave me a comment and I will provide you with the correct solution.

An Introduction to Ethernet Switching

When you stop to think how well the internet is put together, you start to wonder how the internet coexists with all the chaos that results from broadcasts, data collision, and data loops. Yet, despite the odds, the internet is still (currently) alive and thriving today. But what can we attribute this achievement to? As you’ll find in the coming section, we owe a lot of gratitude towards a little something Cisco likes to call Ethernet switching.

But before we jump into the fun theories such as Spanning Tree Protocol (STP), let’s take a look at the devices we are dealing with.

Layer 2 Bridges

As you’ll recall from the OSI model, layer 2 corresponds to the Data Link layer- the layer that deals directly with MAC addresses. In this case, we are dealing with bridges that, coincidentally, handle MAC addresses.

The purpose of a bridge is simple: divide a network into two separate pieces so we can save bandwidth. That way if a computer on one segment of the bridge needs to communicate with another computer on that side of the bridge, the connection remains local. The other segment will not be bothered with the request.

This also effectively gives us two separate collision domains. This will help cut down on data collision, which is a major cause of network latency. Note that in the above example, computer A sends information to the bridge first, which makes the decision to route the request to computer B, while filtering the data out of the segment on the right.

Things to Remember About Bridges

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• 1. Bridges provide switching via comparing destination MAC addresses found in the data being sent to MAC addresses stored in its tables.
• 2. If the source MAC address is not already known, the bridge creates a new entry in the MAC address table with the source port. This will be used for future switching operation.
• 3. If the destination MAC address is not known by the bridge, a broadcast will be sent to all segments in a process called flooding. Note that a broadcast is not sent out in the port the data was received on.
• 4. If the bridge determines the destination MAC address is not from the same network segment as the sending device, it will forward the data to the appropriate segment.

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Layer 2 Switches

Layer 2 switches are essentially the same as bridges, only they have multiple ports and can use microsegmentation to decrease collisions and increase throughput. They also have support for full-duplex operation and spanning tree protocol (STP).

Full-duplex operation enables devices to have support for both receiving and sending information at the same time. This eliminates the problem of data collision altogether. Keep in mind that if a device such as a hub were used, full duplex operation could not be possible because hubs lack microsegmentation.

Lastly, switches use what is called spanning tree protocol. Spanning tree protocol is used to help prevent loops from forming. Imagine that switch A forwards data to switch B, since it is unsure where the data should go. Switch B isn’t sure either, and forwards the data back. This creates a never ending loop in theory, but thankfully we can make use of STP. In the example below, you can see that there are four physical links maintained, while two of the links are purely logical- they do not technically exist to the switches.

To counteract the threat of loops, switches send messages called bridge protocol data units, or BPDUs, out every port to let other switches know if its existences. Redundant paths are then shut down through port blocking, and we result with a path free of loops. (These paths can be opened again, however, in case a line goes down and the redundancy is needed.) In the example below, you can see that there are four physical links maintained, while two of the links are purely logical- they do not technically exist to the switches.

Layer 2 Switch Modes of Operation

But what sets them apart from bridges even further is that they can operate in three different modes- Store-and-Forward, Cut-Through, and Fragment-Free.

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• 1. Store-and-Forward is the method with the slowest operation speed. This is due to the fact that it checks incoming frames of data for integrity. If the frame has errors, it is discarded. Otherwise, it is sent to its destination. This error checking can be quite costly to network performance, however.
• 2. Cut-Through switching is considered to be the bare minimum- and thus much faster. It only requires that the beginnings of the frame up to the destination MAC address be read before the frame of data can pass through the switch.
• 3. Fragment-Free switching is a modified form of the Cut-Through method. Fragment-free switching filters out collision fragments, which is where the majority of packets errors originate. To do this checking, the switch must wait for the entire packet of information to be received before the filtering takes place. Obviously, it’ll not be as quick as the cut-through method.

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A Last Note on Broadcasts and Collisions

Keep in mind that since we have been dealing with layer 2 devices, we do not divide the broadcast domain with these devices. Only a router can divide a broadcast domain. As for collision domains, each of these devices creates more of them (this is a good thing). As for hubs and repeaters, they only extend the collision domain.

Also keep in mind that if a frame is bearing the format of FFFF.FFFF.FFFF, it will be automatically received by all NICs on the network, as this is, in fact, a broadcast address in hexadecimal.

Notes on Fiber Optic Media

Notes on Fiber Optic Media

If you’ve worked with fiber optics, you know that optical fiber is resistant to noise and outside interference- unlike copper. Thus, we only have one concern when dealing with fiber optic cable: keeping the signal strength strong. Since we don’t have to 

worry about interference, we can cable fiber optic media much farther than twisted-pair cable.

We noted earlier that computers communicate with high values and low values. However, it would be more accurate to claim that they instead communicate via “on” and “off” indications. Fiber optic media uses light to signal this on or off state.
You’ll generally want to use fiber optic cable when copper media proves to be too limited for long distances, or noisy environments. It may cost a little more, but the speed and efficiency fiber optic cable provides is well worth it. And in longer cable runs, fiber optic cable will actually cost less than copper media.

How to Avoid Signal Attenuation

How to Avoid Signal Attenuation

Attenuation is the decrease in signal amplitude. If we have a small signal, it becomes increasingly harder to decipher the signal. Much like a yell is easier to understand that a faint whisper, computers appreciate healthy signal amplitudes.

Signal Attenuation

You’ll notice in the above diagram that as distance increases, amplitude decreases. This becomes a problem since computers communicate in this instance via two values; high values and low values. Since the high value becomes increasingly similar to a low value, the signal will eventually become worthless for communication.
Several factors create attenuation- mostly resistance in the copper wire and leaked signal energy. So to fix the problem, we try to stick with the recommended cable lengths. If you indeed need a longer cable run, you may clean the signal by adding a repeater, switch, router, or other devices to your network so that the signal can be regenerated.
It’s also important to note that as frequency (the number of cycles a second) increases, so does the noise and interference.

How to Avoid Crosstalk

How to Avoid Crosstalk

You’ll come to find that the most common mistakes are usually the simplest to avoid. For instance, cross talk is very commonly created when connectors are not installed properly at both ends of the cable. Cross talk is the effect we get when electromagnetic energy from one cable leaves an imprint on adjacent cables. (You’ll often see this referred to as “noise.”)This usually isn’t a problem, however, since we twist wires inside Ethernet cable to cancel out this effect. So how, then, does cross talk become such a problem?

Cross Talk

You’ll notice that the Ethernet cable on the right has too much wire left over- we actually need to crimp the connector to the point where no internal wiring is visible. It’s important to note that while the wires do need to be separated to properly install a connector, they should only be separated as little as possible. Otherwise the lack of cancellation will create cross talk- and possibly cause hard-to-track failures in a network.