SWITCHES & BRIDGES

INTRODUCTION

By now you can see the limitations of a simple hub and when you also read about Ethernet, you start to understand that there are even more limitations. The companies who manufacter hubs saw the big picture quickly and came out with something more efficient, bridges, and then the switches came along! Bridges are analysed later on in this section.

SWITCHING TECHNOLOGY

As we mentioned earlier, hubs work at the first layer of the OSI model and simply receive and transmit information without examining any of it.

Switches (Layer-2 Switching) are a lot smarter than hubs and operate on the second layer of the OSI model. What this means is that a switch won't simply receive data and transmit it throughout every port, but it will read the data and find out the packet's destination by checking the MAC address. The destination MAC address is located always at the beginning of the packet so once the switch reads it, it is forwarded to the appropriate port so no other node or computer connected to the switch will see the packet.

Switches use Application Specific Integrated Circuits (ASIC's) to build and maintain filter tables.
Layer-2 switches are alot faster than routers cause they dont look at the Network Layer (thats Layer-3) header or if you like, information. Instead all they look at is the frame's hardware address (MAC address) to determine where the frame needs to be forwarded or if it needs to be dropped. If we had to point a few features of switches we would say:

• They provide hardware based bridging (MAC addresses)
• They work at wire speed, therefor have low latency
• They come in 3 different types: Store & Forward, Cut-Through and Fragment Free (Analysed later)

Below is a picture of two typical switches. Notice how they looks similair to a hubs, but they aren't. It's just that the difference is on the inside!

  

THE THREE STAGES

All switches regardless of the brand and various enhancements they carry, have something in common, it's the three stages (sometimes 2 stages) they go through when powered up and during operation. These are as follows:

• Address Learning
• Forward/Filter decisions
• Loop Avoidance (Optional)

Let's have a look at them to get a better understanding!

ADDRESS LEARNING

When a switch is powered on, the MAC filtering table is empty. When a device transmits and an interface receives a frame, the switch places the source address in the MAC filtering table remembering the interface the device on which it is located. The switch has no choice but to flood the network with this frame because it has no idea where the destination device is located.

If a device answers and sends a frame back, then the switch will take the source address from that frame and place the MAC address in the database, associating this address with the interface that received the frame.

Since the switch has two MAC addresses in the filtering table, the devices can make a point-to-point connection and the frames will only be forwarded between the two devices. This makes layer-2 switches better than hubs. As we explained early on this page, in a hub network all frames are forwarded out to all ports every time. Most desktop switches these days can hold upto 8000 MAC addresses in their table, and once the table is filled, then starting with the very first MAC entry, the switch will start overwritting the entries. Even tho the number of entries might sound big .. it only takes a minute or two to fill it up, and if a workstation dosen't talk on the network for that amount of time, then chances are that its MAC address has been removed from the table and the switch will forward to all ports the packet which has as a destination this particular workstation.

And after the first frame has been successfully received by Node 2, Node 2 sends a reply to Node 1, check out what happens:

Notice how the frame is not transmitted to every node on the switch. The switch by now has already learned that Node 1 is on the first port, so it send it straight there without delay. From now on, any communication between the two will be a point-to-point connection :

FORWARD/FILTER DECISION

When a frame arrives at the switch, the first step is to check the destination hardware address, which is compaired to the forward/filter MAC database. If the destination hardware address is known, then it will transmit it out the correct port, but if the destination hardware address is not known, then it will broadcast the frame out of all ports, except the one which it received it from. If a device (computer) answers to the broadcast, then the MAC address of that device is added to the MAC database of the switch.

LOOP AVOIDANCE (OPTIONAL)

It's always a good idea to have a redundant link between your switches, in case one decides to go for a holiday. When you setup redundant switches in your network to stop failures, you can create problems. Have a look at the picture below and I'll explain:

The above picture shows an example of two switches which have been placed in the network to provide redundancy in case one fails. Both switches have their first port connected to the upper section of the network, while their port 2 is connected to the lower section of the same network. This way, if Switch A fails, then Switch B takes over, or vice versa.

Things will work fine until a broadcast come along and causes alot of trouble. For the simplicity of this example, I am not going to show any workstations, but only the server which is going to send a broadcast over the network, and keep in mind that this is what happens in real life if your switch does not support Spanning-Tree Protocol (STP), this is why I stuck the "Optional" near the "Loop Avoidance" at the start of this section:

It might look a bit messy and crazy at a first glance but let me explain what is going on here.

The Server for one reason or another decides to do a broadcast. This First Round (yellow arrow) broadcast is sent down to the network cable and firstly reaches Port 1 on Switch A. As a result, since Switch A has Port 2 connected to the other side of the lan, it sends the broadcast out to the lower section of the network, this then is sent down the wire and reaches Port 2 on Switch B which will send it out Port 1 and back onto the upper part of the network. At this point, as the arrows indicate (orange colour) the Second Round of this broadcast starts. So again... the broadcast reaches Port 1 of Switch A and goes out Port 2 back down to the lower section of the network and back up via Port 2 of Switch B. After it comes out of Port 1 of Switch B, we get the Third Round (Cyan arrow), and then the Fourth Round (pink arrows) , Fifth Round (blue arrows) and keeps on going without stopping.....! This is what we call a Broadcast Storm.

A Broadcast Storm will repeat constantly, chewing up the valueble bandwidth on the network. This is a major problem, so they had to solve it one way or another, and they did... with the Spanning-Tree Protocol or STP in short. What STP does, is to find the redundant links, which this case would be Port 2 of Switch B and shut it down, thus eliminating the posibility of looping to occur.

LAN SWITCH TYPES

At the begining of this page we said that the switches are fast, therefor have low latency. This latency does vary and depends on what type of switching mode the switch is operating at. You might recall seeing these three switching modes at the beginning: Store & Forward, Cut-ThroughandFragment Free.

The picture below shows how far the different switching modes check the frame:

So what does this all mean ? Switching modes ? I Don't understand !

Let's Explain!

The fact is that switches can operate in one of the three modes. Some advance switches will allow you to actually pick the mode you would like it to operate in, while others don't give you any choice. Let's have a quick look at each mode:

STORE & FORWARD MODE

This is one of the most popular swtiching methods. In this mode, when the switch receives a frame from one of it's ports, it will store it in memory, check it for errors and corruption, and if it passes the test, it will forward the frame out the designated port, otherwise, if it discovers that the frame has errors or is corrupt, it will discard it. This method is the safest, but also has the highest latency.

CUT-THROUGH (REAL TIME)

Cut-Through switching is the second most popular method. In this mode,the switch reads the frame until it learns the destination MAC address of the frame it's receiving. Once it learns it, it will forward the frame straight out the designated port without delay. This is why we say it's -Real Time-, there is no delay or error checking done to the frame.

FRAGMENT FREE

The Fragment free switching method is mainly used to check for frames which have been subject to a collision. The frame's first 64 bytes are only checked before forwarding the frame out the designated port. Reason for this is because almost all collisions will happen within the first 64 bytes of a frame. If there is a corruption in the first 64 bytes, it's most likely that that frame was a victim of a collision.

Just keep one important detail in mind: When you go out to buy a switch, make sure you check the amount of memory it has. Alot of the cheap switches which support the Store & Forward mode have very small amounts of memory buffer (256KB- 512KB) per port. The result of this is that you get a major decrease in performance when you have more than 2 computers communicating via that switch cause there isn't enough memory to store all incoming packets (this also depends on th swtiching type your switch supports), and you eventually get packets being discarded.

The table below is a guide on what amounts of memory you should be looking at for switches of different configuration :

NETWORK BRIDGES

Bridges are really just like switches, but there are a few differences which we will mention, but not expand upon. These are the following:

• Bridges are software based, while switches are hardware based because they use an ASICs chip to help them make filtering decisions.
• Bridges can only have one spanning-tree instance per bridge, while switches can have many.
• Bridges can only have upto 16 ports, while a switch can have hundreds !

That's pretty much as far as we will go with the bridges since they are pretty much old technology and you probably won't see many around.

HUBS & REPEATERS

INTRODUCTION

Here we will talk about hubs and explain how they work. In the next section we will move to switches and how they differ from hubs, how they work and the types of switching methods that are available; we will also compare them.

Before we start there are a few definitions which I need to speak about so you can understand the terminology we will be using.

Domain: Defined as a geographical area or logical area (in our imagination) where anything in it becomes part of the domain. In computer land, this means that when something happens in this domain (area) every computer that's part of it will see or hear everything that happens in it.

Collision Domain: Putting it simple, whenever a collision between two computers occurs, every other computer within the domain will hear and know about the collision. These computers are said to be in the same collision domain. As you're going to see later on, when computers connect together using a hub they become part of the same collision domain. This dosen't happen with switches.

Broadcast Domain: A domain where every broadcast (a broadcast is a frame or data which is sent to every comeputer) is seen by all computers within the domain. Hubs and switches do not break up broadcast domains. You need a router to achieve this.

There are different devices which can break-up collision domains and broadcast domains and make the network a lot faster and efficient. Switches create separate collision domains but not broadcast domains. Routers create separate broadcast and collision domains. Hubs are too simple to do either, can't create separate collision or broadcast domain.

Hubs & Repeaters

Hubs and repeaters are basically the same, so we will be using the term "Hub" to keep things simple. Hubs are common today in every network. They are the cheapest way to connect two or more computers together. Hubs are also known as Repeaters and work on the first layer of the OSI model. They are said to work on the first layer because of the function they perform. They don't read the data frames at all (like switches and routers do), they only make sure the frame is repeated out on each port and that's about it.

The Nodes that share an Ethernet or Fast Ethernet LAN using the CSMA/CD rules are said to be in the same collision domain. In plain English, this means that all nodes connected to a hub are part of the same collision domain. In a Collision domain, when a collision occurs everyone in that domain/area will hear it and will be affected. The Ethernet section talks about CSMA/CD and collision domains since they are part of the rules under which Ethernet functions.

The picture below shows a few hubs : 8 port Netgear and a D-link hub.

                              

The computers (nodes) connect to the hub using Unshielded Twisted Pair cable (UTP). Only one node can be connected to each port of the hub. The pictured hub has a total of 8 ports, which means up to 8 computers can be networked.

When hubs were not that common and also expensive, most offices and home networks use to install coax cable.

The way hubs work is quite simple and straightforward: When a computer on any one of the eight ports transmits data, this is replicated and sent out to the other seven ports. Check out the below picture which shows it clearly.

EXPLANATION:

Node 1 is transmitting some data to Node 6 but all nodes are receiving the data as well. This data will be rejected by the rest of the nodes once they figure out it's not for them.

This is accomplished by the node's network card reading the destination MAC address of the frame (data) it receives, it examines it and sees that it doesn't match with it's own and therefor discards the frame. Please see the Datalink layer in the OSI section for more information on MAC addresses.

Most hubs these days also have a special port which can function as a normal port or as an "uplink" port. An uplink port allows you to connect another hub to the existing one, increasing the amount of ports which will be available to you. This is a cheap solution when you need to get a few more computers networked and it works quite well up to a point.

This is how 2 eight port hubs would look when connected via the uplink port and how the data is replicated to all 16 ports :

In the above picture you can see that Node 1 is again transmitting data to Node 6 and that every other node connected to the hub is receiving the information. As we said, this is a pretty good and cheap solution, but as the network gets busier, you can clearly understand that there is going to be a lot of unecessary data flowing all over the network. All Nodes here are in the same broastcast and collision domain since they will hear every broadcast and collision that occurs.

This is the same situation you get when you use coax cable, where every node or computer is connected onto the same cable and the data that's put onto it travels along the cable and is received by every computer.

You probably also noticed the two orange boxes labled "50 Ohm". These are called terminating resistors and are used on both ends of the coax cable so when the signal gets to them, it's absorbed by them and that way you don't get the signal reflecting back. Think of them as shock absorbent and the data signal is the shock wave which gets absorbed when it reaches the terminating resistors. The coax cable can be up to 185 meters and can contain no more than 30 nodes per segment. What you're looking at in the above picture is one segment 25 meters long with 4 nodes attached to it.

Now coming back to the hubs, there are a few standard features most of them have these include a link and activity LED for each port, a power LED and collision LED. Some hubs have separate link lights and activity lights, others combine them into one where the link light will flash when there is activity, otherwise it remains constantly on. The Netgear hub which is displayed at the beginning of this page has two separate LEDs for the activity and link but the Compex hub below has only one.

This little hub also contains a special BNC connection so you can connect a coax cable to it. When you do connect it, the BNC light comes on. Notice the label at the top where they have written "8 port Ethernet Repeater". As we already have said, hubs are just simple repeaters.

The collision light on the hubs will only light up when a collision is detected. Collision is when 2 computers or nodes try to talk on the network at the same time. When this happens, their frames will collide and become corrupted. The hubs are smart enough to detect this and will light up the collision LED for a small amount of time (1/10 of a second for each collision). If you find yourself wondering why couldn't they make things work so more than two computers can talk on the network, it would be advisable to visit the Ethernet section where all this is explained in detail. Collisions and the fact that only one computer can talk on the network at any given time along with the cabling rules are all part of the Ethernet rules. Remember that any node connected to a hub becomes part of the same collision domain.

INTRODUCTION

MULTICAST - UNDERSTAND HOW IP MULTICAST WORKS

To understand what we are going to talk about, you must be familiar with how MAC addresses are structured and how they work. The MAC Addresses page is available to help you learn more about them.

A multicast is similar to a broadcast in the sense that its target is a number of machines on a network, but not all. Where a broadcast is directed to all hosts on the network, a multicast is directed to a group of hosts. The hosts can choose whether they wish to participate in the multicast group (often done with the Internet Group Management Protocol), whereas in a broadcast, all hosts are part of the broadcast group whether they like it or not!

As you are aware, each host on an Ethernet network has a unique MAC address, so here's the million dollar question: How do you talk to a group of hosts (our multicast group), where each host has a different MAC address, and at the same time ensure that the other hosts, which are not part of the multicast group, don't process the information ? You will soon know exactly how all this works.

To keep things in perspective and make it easy to understand, we are going to concentrate only on an Ethernet network using the IP protocol, which is what 80-90 % of home networks and offices use.

BREAKING THINGS DOWN...

In order to explain Multicasting the best I can and to make it easier for you understand, I decided to break it down into 3 sections:

1) Hardware/Ethernet Multicasting

2) IP Multicasting

3) Mapping IP Multicast to Ethernet Multicast

A typical multicast on an Ethernet network, using the TCP/IP protocol, consists of two parts: Hardware/Ethernet multicast and IP Multicast. Later on I will talk about Mapping IP Multicast to Ethernet Multicast which is really what happens with multicasting on our Ethernet network using the TCP/IP protocol.

The brief diagram below shows you the relationship between the 3 and how they complete the multicasting model:

HARDWARE/ETHERNET MULTICASTING

When a computer joins a multicast group, it needs to be able to distinguish between normal unicasts (which are packets directed to one computer or one MAC address) and multicasts. With hardware multicasting, the network card is configured, via its drivers, to watch out for particular MAC addresses (in this case, multicast MAC addresses) apart from its own. When the network card picks up a packet which has a destination MAC that matches any of the multicast MAC addresses, it will pass it to the upper layers for further processing.

And this is how they do it:

Ethernet uses the low-order bit of the high-order octet to distinguish conventional unicast addresses from multicast addresses. A unicast would have this bit set to ZERO (0), whereas a multicast would be set to ONE (1)

To understand this, we need to analyse the destination MAC address of a unicast and multicast packet, so you can see what we are talking about:

When a normal (unicast) packet is put on the network by a computer, it contains the Source and Destination MAC address, found in the 2nd Layer of the OSI model. The following picture is an example of my workstation (192.168.0.6) sending a packet to my network's gateway (192.168.0.5):


Now let's analyse the destination MAC address:

When my gateway receives the packet, it knows it's a unicast packet as explained in the above picture.

Let's now have a look at the MAC address of a multicast packet. Keep in mind, a multicast packet is not directed to one host but a number of hosts, so the destination MAC address will not match the unique MAC address of any computer, but the computers which are part of the multicast group will recognise the destination MAC address and accept it for processing.

The following multicast packet was sent from my NetWare server. Notice the destination MAC address (it's a multicast):

Analysis of a multicast destination MAC address:

So now you should be able to understand how computers can differentiate between a normal or unicast packet and a multicast packet. Again, the destination MAC address 01-00-5E-00-00-05 is not the MAC address of a particular host-computer but the MAC address that can be recognised by computers that are part of the multicast group. I should also note that you will never find a source address that is a multicast MAC address, the source address will always be a real one, to identify which computer the packet came from.

The IEEE group used a special Rule to determine the various MAC addresses that will be considered for multicasting. This Rule is covered in the last section of this page, but you don't need to know it now in order to understand Hardware multicasting. Using this special rule it was determined that MAC address 01:00:5E:00:00:05 will be used for the OSPF protocol, which happens to be a routing protocol, and then this MAC address also maps to an IP address which is analysed in IP Multicast.

IP MULTICAST

The IP Multicast is the second part of multicasting which, combined with the hardware multicasting, gives us a multicasting model that works for our Ethernet network. If hardware multicasting fails to work, then the packet will never arrive at the network layer upon which IP multicasting is based, so the whole model fails.

With IP multicasting the hardware multicasting MAC address is mapped to an IP Address. Once Layer 2 (Datalink) picks the multicast packet from the network (because it recognises it, as the destination MAC address is a multicast) it will strip the MAC addresses off and send the rest to the above layer, which is the Network Layer. At that point, the Network Layer needs to be able to understand it's dealing with a multicast, so the IP address is set in a way that allows the computer to see it as a multicast datagram. A host may send multicast datagrams to a multicast group without being a member.

Multicasts are used a lot between routers so they can discover each other on an IP network. For example, an Open Shortest Path First (OSPF) router sends a "hello" packet to other OSPF routers on the network. The OSPF router must send this "hello" packet to an assigned multicast address, which is 224.0.0.5, and the other routers will respond.

IP Multicast uses Class D IP Adresses:

Let's have a look at an example so we can understand that a bit better:

The picture below is a screenshot from my packet sniffer, it shows a multicast packet which was sent from my NetWare server, notice the destination IP address:

The screenshot above shows the packet which was captured, it's simply displaying a quick summary of what was caught. But, when we look on the left, we see the above packet in much more detail.

You can clearly see the markings I have put at the bottom which show you that the destination IP for this packet is IP Address 224.0.0.5. This corresponds to a multicast IP and therefore is a multicast packet.

The MAC header also shows a destination MAC address of 01-00-5E-00-00-05 which we analysed in the previous section to show you how this is identified as a multicast packet at Layer 2 (Datalink Layer).

Some examples of IP multicast addresses:

224.0.0.0 Base Address (Reserved) [RFC1112,JBP]
224.0.0.1 All Systems on this Subnet [RFC1112,JBP]
224.0.0.2 All Routers on this Subnet [JBP]
224.0.0.3 Unassigned [JBP]
224.0.0.4 DVMRP Routers [RFC1075,JBP]
224.0.0.5 OSPFIGP OSPFIGP All Routers [RFC2328,JXM1]

Remember that these IP Addresses have been assigned by the IEEE !

Now all that's left is to explain how the IP multicast and MAC multicast map between each other...

MAPPING IP MULTICAST TO ETHERNET MULTICAST

The last part of multicast which combines the Hardware Multicasting and IP Multicasting is the Mapping between them. There is a rule for the mapping, and this is it:

To map an IP Multicast address to the corresponding Hardward/Ethernet multicast address, place the low-order 23 bits of the IP multicast address into the low-order 23 bits of the special Ethernet multicast address. The rest of the high-order bits are defined by the IEEE (yellow colour in the example)

The above rule basically determines the Hardware MAC address. Let's have a look at a real example to understand this.

We are going to use Multicast IP Address 224.0.0.5 - a multicast for the OSPF routing protocol. The picture below shows us the analysis of the IP address in binary so we can clearly see all the bits:

It might seem a bit confusing at first, but let's break it down:

We have an IP Address of 224.0.0.5, this is then converted into binary so we can clearly see the mapping of the 23 bits to the MAC address of the computer. The MAC Address part which is in yellow has been defined by the IEEE group. So the yellow and pink line make the one MAC Address as shown in binary mode, then we convert it from binary to hex and that's about it !

You should keep in mind that multicast routers should not forward any multicast datagram with destination addresses in the following 224.0.0.0 and 224.0.0.255. The next page (multicasting list) gives a bit more information on this.

MEDIA ACCESS CONTROL - MAC ADDRESSES

INTRODUCTION

Media Access Control (MAC) addresses are talked about in various sections on the site, such as the OSI-Layer 2, Multicast, Broadcast and Unicast. We are going to analyse them in depth here so we can get a firm understanding of them since they are part of the fundamentals of networking.

MAC addresses are physical addresses, unlike IP addresses which are logical addresses. Logical addresses require you to load special drivers and protocols in order to be able to configure your network card/computer with an IP Address, whereas a MAC address doesn't require any drivers whatsoever. The reason for this is that the MAC address is actually "burnt-in" into your network card's memory chipset.

THE REASON FOR MAC

Each computer on a network needs to be identified in some way. If you're thinking of IP addresses, then you're correct to some extent, because an IP address does identify one unique machine on a network, but that is not enough.

The diagram below provides a visual representation how important MAC Addresses are on our network and at which layer of the OSI Model they exist:

You see, the IP address of a machine exists on the 3rd Layer of the OSI model and, when a packet reaches the computer, it will travel from Layer 1 upwards, so we need to be able to identify the computer before Layer 3.

This is where the MAC address - Layer 2 comes into the picture. All machines on a network will listen for packets that have their MAC address in the destination field of the packet (they also listen for broadcasts and other stuff, but that's analysed in other sections). The Physical Layer understands the electrical signals on the network and creates the frame which gets passed to the Datalink layer. If the packet is destined for the computer then the MAC address in the destination field of the packet will match, so it will accept it and pass it onto the Layer above (3) which, in turn, will check the network address of the packet (IP Address), to make sure it matches with the network address to which the computer has been configured.

EXAMINING A MAC ADDRESS

Let's now have a look at a MAC address and see what it looks like! We will use our workstation's MAC address as an example:

MAC Addresses are always presented in HEX format, just as our example above. It is very rare that a MAC address is represented in Binary format because it is simply too long and difficult to understand, as we will see futher on.

When a vendor, e.g Intel, produces network cards for computers, they don't just assign them any MAC address they like, this would create a big confusion in identifying who is the vendor of each network card and could possibly result in clashing with another MAC address from another vendor e.g D-link, who happened to choose the same MAC address for their network card!

To avoid these kind of problems, the IEEE group split the MAC address in half, and used the first half to identify the vendor, and the second half is for the vendor to allocate as serial numbers:

The Vendor code is specified by RFC - 1700. You might find a particular vendor having more than just one code; this is because of the wide range of products they might have. They just apply for more, as they need !

Keep in mind that even tho the MAC address is "burnt-in" to the network card's memory, some vendors will allow you to download special programs to change the second half of the MAC address on the card. This is because the vendors actually reuse the same MAC addresses for their network cards because they create so many that they run out of numbers ! But at the same time, the chances of you buying two network cards which have the same MAC address are so small that it's almost impossible !

LET'S START TALKING BITS AND BYTES!

Now that we know what a MAC address looks like, we need to start analysing it. A MAC address of any network card is always the same length, that is, 6 Bytes long or 48 Bits long. If you're scratching your head wondering where these figures came from, then just have a look at the picture below which makes it a bit easier to understand:

So that completes the discussion regarding MAC Addresses! We hope you have understood it all because it's very important so you can expand your knowledge and truly understand what happens in a network !

NETWORK DATA TRANSMISSION

INTRODUCTION

Routable protocols enable the transmission of data between computers in different segments of a network. However, high volumes of certain kinds of network traffic can affect network efficiency because they slow down transmission speed. The amount of network traffic generated varies with the 3 types of data transmissions:

• Broadcast
• Multicast
• Unicast

In the pages that follow, we are going to analyse each data transmission type because it is very important to understand the type of traffic they generate, what they are used for and why they exist on the network.

Before we proceed, please note that understanding the OSI Model (especially Layer 2 and 3), Ethernet and the way a packet is structured is fundamental to understanding broadcast, multicast or unicast type of traffic.

LAN NETWORK TOPOLOGIES

Network topologies can take a bit of time to understand when you're all new to this kind of cool stuff, but it's very important to fully understand them as they are key elements to understanding and troubleshooting networks and will help you decide what actions to take when you're faced with network problems.

This article explains the different network topologies found in today's networks. We examine Bus Topology, Ring Topology, Star Topology, Mesh Topology, Hybrid Topology and many more.

PHYSICAL AND LOGICAL TOPOLOGIES

There are two types of topologies: Physical and Logical. The physical topology of a network refers to the layout of cables, computers and other peripherals. Try to imagine yourself in a room with a small network, you can see network cables coming out of every computer that is part of the network, then those cables plug into a hub or switch. What you're looking at is the physical topology of that network !

Logical topology is the method used to pass the information between the computers. In other words, looking at that same room, if you were to try to see how the network works with all the computers talking (think of the computers generating traffic and packets of data going everywhere on the network) you would be looking at the logical part of the network. The way the computers will be talking to each other and the direction of the traffic is controlled by the various protocols (like Ethernet) or, if you like, rules.

If we used token ring, then the physical topology would have to change to meet the requirements of the way the token ring protocol works (logically).

If it's all still confusing, consider this: The physical topology describes the layout of the network, just like a map shows the layout of various roads, and the logical topology describes how the data is sent accross the network or how the cars are able to travel (the direction and speed) at every road on the map.

The most common types of physical topologies, which we are going to analyse, are: Bus, Hub/Star and Ring

THE PHYSICAL BUS TOPOLOGY

Bus topology is fairly old news and you probably won't be seeing much of these around in any modern office or home.

With the Bus topology, all workstations are connect directly to the main backbone that carries the data. Traffic generated by any computer will travel across the backbone and be received by all workstations. This works well in a small network of 2-5 computers, but as the number of computers increases so will the network traffic and this can greatly decrease the performance and available bandwidth of your network.

As you can see in the above example, all computers are attached to a continuous cable which connects them in a straight line. The arrows clearly indicate that the packet generated by Node 1 is transmitted to all computers on the network, regardless the destination of this packet.

Also, because of the way the electrical signals are transmitted over this cable, its ends must be terminated by special terminators that work as "shock absorbers", absorbing the signal so it won't reflect back to where it came from. The value of 50Ohms has been selected after carefully taking in consideration all the electrical characteristics of the cable used, the voltage that the signal which runs through the cables, the maximum and minimum length of the bus and a few more.

If the bus (the long yellow cable) is damaged anywhere in its path, then it will most certainly cause the network to stop working or, at the very least, cause big communication problems between the workstations.

Thinnet - 10 Base2, also known as coax cable (Black in colour) and Thicknet - 10 Base 5 (Yellow in colour) is used in these type of topologies.

THE PHYSICAL HUB OR STAR TOPOLOGY

The Star or Hub topology is one of the most common network topologies found in most offices and home networks. It has become very popular in contrast to the bus type (which we just spoke about), because of the cost and the ease of troubleshooting.

The advantage of the star topology is that if one computer on the star topology fails, then only the failed computer is unable to send or receive data. The remainder of the network functions normally.

The disadvantage of using this topology is that because each computer is connected to a central hub or switch, if this device fails, the entire network fails!

A classic example of this type of topology is the UTP (10 base T), which normaly has a blue colour. Personally I find it boring, so I decided to go out and get myself green, red and yellow colours :)

THE PHYSICAL RING TOPOLOGY

In the ring topology, computers are connected on a single circle of cable. Unlike the bus topology, there are no terminated ends. The signals travel around the loop in one direction and pass through each computer, which acts as a repeater to boost the signal and send it to the next computer. On a larger scale, multiple LANs can be connected to each other in a ring topology by using Thicknet coaxial or fiber-optic cable.

The method by which the data is transmitted around the ring is called token passing. IBM's token ring uses this method. A token is a special series of bits that contains control information. Possession of the token allows a network device to transmit data to the network. Each network has only one token.

THE PHYSICAL MESH TOPOLOGY

In a mesh topology, each computer is connected to every other computer by a separate cable. This configuration provides redundant paths through the new work, so if one computer blows up, you don't lose the network :) On a large scale, you can connect multiple LANs using mesh topology with leased telephone lines, Thicknet coaxial cable or fiber optic cable.

Again, the big advantage of this topology is its backup capabilities by providing multiple paths through the network.

THE PHYSICAL HYBRID TOPOLOGY

With the hybrid topology, two or more topologies are combined to form a complete network. For example, a hybrid topology could be the combination of a star and bus topology. These are also the most common in use.

STAR-BUS

In a star-bus topology, several star topology networks are linked to a bus connection. In this topology, if a computer fails, it will not affect the rest of the network. However, if the central component, or hub, that attaches all computers in a star, fails, then you have big problems since no computer will be able to communicate.

STAR-RING

In the Star-Ring topology, the computers are connected to a central component as in a star network. These components, however, are wired to form a ring network.

Like the star-bus topology, if a single computer fails, it will not affect the rest of the network. By using token passing, each computer in a star-ring topology has an equal chance of communicating. This allows for greater network traffic between segments than in a star-bus topology

INTRODUCTION TO NETWORKING

INTRODUCTION

A network is simply a group of two or more Personal Computers linked together. Many types of networks exist, but the most common types of networks are Local-Area Networks (LANs), and Wide-Area Networks (WANs).

In a LAN, computers are connected together within a "local" area (for example, an office or home). In a WAN, computers are further apart and are connected via telephone/communication lines, radio waves or other means of connection.

HOW ARE NETWORKS CATEGORIZED?

Networks are usually classified using three properties: Topology, Protocol and Architecture.

Topology specifies the geometric arrangement of the network. Common typologies are a bus, ring and star.You can check out a figure showing the three common types of network typologies here.

Protocol specifies a common set of rules and signals the computers on the network use to communicate. Most networks use Ethernet, but some networks may use IBM's Token Ring protocol. We recommend Ethernet for both home and office networking. For more information, please select the Ethernet link on the left.

Architecture refers to one of the two major types of network architecture: Peer-to-peer or client/server. In a Peer-to-Peer networking configuration, there is no server, and computers simply connect with each other in a workgroup to share files, printers and Internet access.

This is most commonly found in home configurations and is only practical for work groups of a dozen or less computers. In a client/server network there is usually an NT Domain Controller, to which all of the computers log on. This server can provide various services, including centrally routed Internet Access, mail (including e-mail), file sharing and printer access, as well as ensuring security across the network. This is most commonly found in corporate configurations, where network security is essential.