Straight-Through and Cross-Over Cables- What are they and When to use them? What does cooling fans do?

WHEN TO USE STRAIGHT-THROUGH CABLE and WHEN TO USE CROSSOVER CABLE:

To connect two similar devices, we use Crossover cable.

To connect two dissimilar devices, we use Straight- through Cables

If we want to connect a host to console port on a router/ switch, then we use Rollover Cables.

A straight- through cable is needed to connect Hosts and Hubs since they are different devices. If same devices are connected, then, we must use a Crossover Cable

The below diagram provides an easy way to know what type of cable to use where:

MDI and MDIX:

The standard wiring for end stations is known as “MDI” (Media Dependent Interface), and the standard wiring for hubs and switches is known as “MDIX” (Media Dependent Interface with Crossover).

AUTO-MDIX:

Some devices support something called as Auto-MDIX wherein it doesn't matter whether you use straight-through or crossover cables to connect your devices. This is called as Auto-MDIX and what it does is it detects the type of cable used and sends signals accordingly. On these devices, it is possible for hardware to automatically correct errors in cable selection, making the distinction between a “straight through” cable and a “crossover” cable unimportant. This capability is known as Auto-MDI/MDIX.

WHAT DO PHYSICAL TOPOLOGY DIAGRAMS MEAN:

A network topology is the pattern in which nodes (i.e., computers, printers, routers or other devices) are connected to a local area network (LAN) or other network via links (e.g., twisted pair copper wire cable or optical fiber cable).

There are four principal topologies used in LANs: bus, ring, star and mesh.

The most widely used of these is bus, because it is employed by Ethernet, which is the dominant LAN architecture.

The several basic network topologies can be combined in various ways to form hybrid topologies, such as a ring-star network or a tree network.

Why we need cooling fans and how they work?

Ideally, all processors should have at least two fans: one fan to intakes cool air, while the other fan expels heat by blowing the hot air out. The specification of the heat sinks and cooling fans depends on its compatibility with the processor, and on how much they are expected to perform. The fan is attached to this fin-like structure and improves the transfer of hot air by pulling the hot air from the electrical heat generated by the components and pushing in cooler air between the aluminum fins, thus keeping the processor cool.

There are two types of switches- F and R.

• R= Rear . here, the hot air comes out of the rear
• F= Front. The hot air comes out from front

This helps in datacenter design requirements

Network Repeaters and Hubs

REPEATERS AND TYPES OF REPEATERS:

There are 4 types of repeaters

1. Telephone Repeaters- It is usually present in the trunked lines for long distance calls and include amplifier circuit to increase the power and compensate for attenuation. Since the telephone is a duplex, telephone repeaters should amplify the signal in both directions without causing feedback

2. Optical Communication Repeater- This is used to increase the range of signals in a fiber optic cable. Digital information travels through a fiber optic cable in the form of short pulses of light. The light is made up of particles called photons, which can be absorbed or scattered in the fiber. An optical communications repeater usually consists of a phototransistor which converts the light pulses to an electrical signal, an amplifier to increase the power of the signal, an electronic filter which reshapes the pulses, and a laser which converts the electrical signal to light again and sends it out the other fiber.

3. Radio repeater- This is used to extend the range of coverage of a radio signal. A radio repeater usually consists of a radio receiver connected to a radio transmitter. The received signal is amplified and retransmitted, often on another frequency, to provide coverage beyond the obstruction.

4. Multiport Repeater (Hub)- A network hub is an unsophisticated device in comparison with a switch. As a multiport repeater it works by repeating bits (symbols) received from one of its ports to all other ports. It is aware of physical layer packets, that is it can detect their start (preamble), an idle line (interpacket gap) and sense a collision which it also propagates by sending a jam signal. A hub cannot further examine or manage any of the traffic that comes through it: any packet entering any port is flooded on all other ports.

FUN THOUGHT: WHAT DOES A HUB CIRCUIT LOOK LIKE:

It can be made using anti-parallel diodes for each port of the hub so that information sent on one port goes only to the other ports and does not come back in a loop- sort of way.

The topology is similar to Bus topology because all devices are connected to a central cable, called the bus or backbone.

ETHERNET HUBS:

• They are also known as Ethernet Hubs/ Networking Hubs 
• They operate at Physical Layer (Layer 1 ) of the OSI model since they cannot switch, they just send the electrical signal they receive on one port to all other ports. 
• It is aware of physical layer packets, that is it can detect their start (preamble), an idle line (interpacket gap) and sense a collision which it also propagates by sending a jam signal. A hub cannot further examine or manage any of the traffic that comes through it: any packet entering any port is rebroadcast on all other ports. 
• A hub/repeater has no memory to store any data in – a packet must be transmitted while it is received and it is lost when a collision occurs (the sender should detect this and retry the transmission). 
• Due to this, hubs can only run in half duplex mode. Consequently, due to a larger collision domain, packet collisions are more frequent in networks connected using hubs than in networks connected using more sophisticated devices like switches. 
• Most hubs detect typical problems, such as excessive collisions and jabbering on individual ports, and partition the port, disconnecting it from the shared medium. 
• Historically, the main reason for purchasing hubs rather than switches was their price. This motivator has largely been eliminated by reductions in the price of switches.