Network hardware

Overview

Recall questions on network interface cards, hubs, switches and routers, wireless access points, modems, twisted pair, coaxial and fibre optic cable, MAC and IP addresses, and choosing hardware for a small office.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What does a network interface card do?

   Model answer

   It is the hardware that connects a device to the network, whether by socket or by radio. It converts the data the computer produces into the signals the medium carries and back again, assembles and disassembles frames at the link layer, and performs the error checking on them. Every network interface has a unique hardware address, the MAC address, assigned by its manufacturer, which identifies it on the local network; the interface reads frames addressed to that address and normally ignores the rest.

2. 2

   How does a hub handle the traffic it receives?

   Model answer

   A hub is the simplest connection device and works at the physical layer. It does not look at addresses at all: any signal arriving on one port is regenerated and sent out of every other port. Every device attached therefore receives every frame and must decide for itself whether the frame is addressed to it. Because the whole hub is one shared collision domain, two devices transmitting at the same time cause a collision and both must back off and retransmit, so as more devices are added the usable throughput collapses. It is also insecure, since every machine physically receives traffic meant for others and can read it.

3. 3

   How does a switch handle the traffic it receives, and how does it know where to send it?

   Model answer

   A switch works at the data link layer and forwards frames using MAC addresses. It maintains a table mapping each MAC address to the port that address was last seen on, and it builds this table by learning: whenever a frame arrives, the switch records the sender's MAC address against the port it arrived on. To forward a frame, it looks up the destination MAC address and sends the frame out only on the single port where that device lives; if the address is not yet in the table, it floods the frame to all other ports and learns the answer from the reply. Because traffic goes only where it is needed, several pairs of devices can communicate simultaneously without colliding, throughput is far better than a hub's, and other machines cannot casually read traffic that is not theirs.

4. 4

   What does a router do?

   Model answer

   A router works at the network layer and connects different networks together, which is what makes an internetwork possible. It examines the destination IP address in each incoming packet, consults its routing table to decide which of its interfaces leads towards that destination, discards the old link-layer frame and builds a new one addressed to the next device along that path, and forwards it. Routers exchange routing information with one another so their tables adapt when links fail or congestion changes, and they decrement the hop counter so that looping packets are eventually discarded. A home router usually also performs address translation, firewalling and DHCP address assignment, but those are extra functions rather than the definition of routing.

5. 5

   Be precise: what is the difference between a hub, a switch and a router?

   Model answer

   They differ in which layer they work at and therefore which address they use. A hub works at the physical layer and uses no address at all: it copies incoming signals to every other port. A switch works at the data link layer and uses MAC addresses: it forwards each frame only to the one port where the destination device is, and it operates within a single network. A router works at the network layer and uses IP addresses: it forwards packets between different networks by consulting a routing table, rebuilding the link-layer frame for each hop. Put simply: a hub broadcasts blindly, a switch delivers to one device on the same network, a router passes traffic between networks.

6. 6

   A school replaces the hubs in a lab with switches and the network gets noticeably faster although nothing else changed. Explain why.

   Model answer

   With hubs, every frame is copied to every port, so all the machines share one collision domain and one lot of bandwidth. Only one device can transmit successfully at a time; if two transmit together the frames collide and both must wait and retransmit, and the rate of collisions rises sharply as more machines become busy, wasting an increasing share of the capacity. A switch forwards each frame only to the port of its destination, so each link is a separate collision domain and several pairs of machines can exchange data simultaneously at full speed. The wiring, the cables and the computers are unchanged - what has changed is that traffic is no longer duplicated to machines that do not want it, and simultaneous conversations no longer destroy one another.

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