 lands and wands, local area networks and wide area networks. More generally this topic is about different types of networks. So we've talked about two main concepts for networking, switching and routing. Now we're going to move into some more details about individual networks. For example, the network that connects all the PCs inside SIT or the network that connects our two campuses together. So what approaches can we use to build a network for serving, say, inside an organization or connecting multiple organizations together? So we'll first try and classify or categorize different types of networks and we'll define what we mean by lands and wands as we try and categorize networks. So there are many different technologies for building computer networks. I think you know of some. You use Wi-Fi, that's one technology. You use your wired LAN or sometimes called Ethernet, another technology for inside a small area. But there are many others that you use. You use ADSL, cable modems, satellite internet and then maybe even on the occasion Bluetooth for forming a small network, even just two devices nearby. And then there's some that you don't see. Some that connects, say, our campus to the rest of the world. Like we'll talk about PDH, SDH, different fiber, optical fiber-based technologies, ATM and others. So there are many different network technologies. We can sometimes categorize them based upon different characteristics and compare them. So some of the ways to categorize network technologies are by transmission medium, a wired network versus a wireless network. So some technologies are wired, some are wireless. And there are advantages and disadvantages of each. Based upon the link configuration, some networks use point-to-point links. Remember point-to-point, we have two devices and a link connects just those two devices. Point-to-multipoint, we can have multiple devices sharing that one link like in wireless communications. So some network technologies use different link configurations. Some networks support fixed users, whereas other support mobile users. So Wi-Fi, for example, allows you to move and still access the network. A wired LAN, you're basically a fixed user. If I plug the LAN cable into my laptop, I can move my laptop a little bit, but I cannot go far. So it's considered a fixed network technology. Some networks are for us human users to access the network, whereas other network technologies are to carry the data of other networks. So we talk about core networks and backbone networks. I'll show you a slide that explains that more. And some networks cover or intended to cover different geographical areas. Some are supposed to be used for communications across a small distance, centimetres, metres. Some are used for communications across a large distance. So we'll go through each of these. The first three or four quite quickly and then a bit more on coverage area. Wired versus wireless, which one's better? Well, there are advantages and disadvantages of each. With wired, when we transmit a signal across the wire, or even a fibre, if it's optical fibre, then usually that signal is contained inside that conductor or that fibre. So there's very little interference when you transmit a signal across that wire versus when someone's transmitting a signal across a neighbouring wire. The result is that we can generally achieve very high data rates because there's very little interference. And it's easy to upgrade capacity. If you have ADSL at home, anyone? ADSL? Yes, and what speed can you get download? What do you pay for? How many megabits per second? Anyone? 16 megabits per second. You pay whatever 800 baht per month and you get some download speed. And that link is from the telephone line. So you have one telephone line coming into your home and you get internet access via that one link. You can easily double your speed from 16 to 32 by buying another telephone line and having a second cable coming into your home. So in theory it's very easy to upgrade the capacity by just adding a new link because with two links coming into your home the transmissions across those two links will not interfere with each other because signals across cables usually are contained within those cables. So upgrading the capacity using wired networks is very easy. It's similar. I want to connect my laptop to this PC. I can have a wired LAN, so a LAN cable that connects them. And with a LAN cable I may be able to send it one gigabit per second. If I want two gigabits per second what I need is a second LAN port on my laptop and it's easy to buy a LAN card for the PC and plug in a second cable and now I've got two gigabits per second. Very easy to upgrade the capacity. Alright, not so common to do that for us end users but say the links between different cities, between different countries upgrading the capacity is quite simple. You just add new cables. With wireless that's hard to do because with wireless when you transmit you interfere with others. So you can't just upgrade the capacity easily because the more people using it the more interference and the lower the data rate. So it's not easy to increase the capacity of my link from my laptop to the access point. I can use different channels and so on but on the same channel I effectively cannot increase the capacity. So it's an advantage of wired systems. So generally compared to wireless better performance, higher data rates smaller delay, predictable delay. With wireless because of interference and varying conditions sometimes there's interference from different sources you generally get poorer performance compared to wired. Wired links are expensive to install in hard to access locations. You want to... SIT wants to upgrade the capacity from our campus to the other campus. So in theory we can just add another wire but if we need to dig another half hole over the 16 or so kilometers between the two campuses that's very expensive to do. If there's already an existing duct or hole under the ground that we can place our wire that's okay. But if we want to build a new network then digging holes under roads and under buildings and so on is very expensive. So adding wiring in many cases is quite expensive. Similarly inside your home maybe you want to have a network that covers all the rooms. Your home's already built. Putting in new cables is quite hard work. Either you need to run those cables along the floor and stick them on the walls which no one likes to look of or you need to put them inside the walls which can be expensive to do. So wireless can be better in that case. Wireless is very convenient. We don't need to add those wires. Of course with wireless we also allow mobility. That's a key advantage of wireless communications. Whereas with wired our device locations are fixed. So some of the trade-offs between wired versus wireless. So depending upon your requirements you choose a network technology based upon the transmission medium. If you want mobility then you basically need wireless. If you want high performance but you don't need mobility then wired may be better. What else? Link configuration. Remember point to point is then we have one link and at the two endpoints we have two devices and they use that link just for themselves. No one else uses that link. So it's a dedicated link meaning the performance for those two devices is generally high. It doesn't have to be shared with anyone else the capacity and predictable. So if I connect my two computers by a one gigabit per second LAN interface then I'm pretty much guaranteed to be able to transfer close to one gigabit per second all the time. But if I connect my two computers via point to multi-point link like Wi-Fi then the performance depends upon how many other people are using that link. So point to multi-point link we have multiple users sharing one link. When I say link here it doesn't need to be a cable. So I can talk about I have a link to the Wi-Fi access point and everyone else uses that same link to the Wi-Fi access point. So dedicated performance is good for point to point links but it makes it more inconvenient if I want to now connect not just to this PC but to someone else's computer I need another link and to each destination I need a separate point to point link. And we'll look at some topologies in this topic that show some arrangements of nodes which are common. The problem with point to multi-point links it's good in that multiple users can share the one link very convenient but requires some way to allow them to share. We've talked about multiplexing remember multiplexing allow data from multiple users across one link in wireless links it's referred to as multiple access and usually we talk about multiple access or medium access control control who accesses the medium at what time. So a MAC protocol deals with allowing multiple users to share one medium. There are a few slides on that in this topic but I think we will not get time to cover it today. Point to point many wired links are point to point links some wireless links we can think of as point to point if we have highly directional antennas so if we have antennas pointing at each other then essentially they don't interfere with anyone else and it's a point to point link. Point to multi-point most other wireless systems which use omnidirectional antennas consider point to multi-point your laptop when you talk to the access point is using an omnidirectional antenna and that's considered a point to multi-point link because not just as my laptop talk to the access point so does everyone else's mobile phone and tablet in this room. Some wired links are point to multi-point we'll see some examples in a moment. Categorizing networks by transmission medium by link configuration by how who accesses and uses the network so we said we can distinguish between access and core networks let's try and explain this with this diagram we have end users the computers so the human users are using these devices for example this access network this cloud represents a network that I connect to and similar there's other people connecting to other access networks so this is access networks other networks which the end users directly connect to but the way that large networks work is that we connect multiple access networks together via other networks and those other networks refer to as core networks so these two access networks at the bottom are connected together by a third network which will refer to a core network and the distinguishing feature is that the end users directly connect to the access network the core network doesn't have end users connected to it the core network only connects to other networks so we often have different technologies for access networks and for core networks give me an example access network technology what's an example of an access network technology that you may use what's an example of a network technology that you use which is considered an access network a network technology so think about some of the protocols that you use to connect to other computers not HTTP not TCPIP think of the lower layers physical and data link layer how do you connect to from your computer to another device LAN so LAN is the general name that maybe the more specific name is Ethernet an Ethernet LAN or a wired LAN so that's one what I say network technology that allows you to connect to other devices and it allows you to access a larger network so we call that an access network technology the computer is attached via LAN cable to some other device a core network connects multiple access networks or other core networks together and we may even talk about backbone networks maybe across a country we have a large network that connects many core networks together and the way that users communicate is that they send their data across their access network and then their data traverses the core networks and maybe the backbone network and then to another destination access network we will see some examples of technologies used for core networks later in this topic how many networks in this picture how many networks where a cloud represents how many networks 13 networks you count the clouds there are 13 there just in this example well another way to think about there are 14 networks there are 13 component networks small ones and then one large network by joining them all together so we can think a network is this entire thing here we can talk about networks of networks or we connect different networks together to form one larger network and that's essentially what the internet is we have a topic on the internet but in brief the internet is many smaller networks connected together to create one large interconnected network one large internet so we will see when we go into detail of the internet it looks like this of course much more complex we connect many small networks together to form one large network the last categorization is by size or coverage area when we build a network about how many meters does it cover how far apart can the users be or the devices be when they communicate there is no one definition of these but I've roughly classified based upon centimetres that cover in the order of centimetres maybe hundreds of centimetres metres kilometres and mega metres mega metres really is thousands of kilometres so centimetres networks that cover or allow communication around a person your headset to your mobile phone so that's a simple network to communicate from the headset to the mobile device or to your watch so sometimes called a personal network or a body area network maybe some medical devices which are monitoring a patient in a hospital that can form a small network or between objects my wireless keyboard wireless mouse my PC and maybe other objects form a network between each other usually in the order of up to a few metres in distance some of the example network technologies are infrared Bluetooth and some newer ones Zigbee and IEEE 802.15.4 and there are others these last two are mainly used for connecting different devices to allow them to automate things for example we want to connect all the air conditioning control units and connect them to some other device so we can control them remotely so we could use maybe Zigbee or 802.15.4 to wirelessly talk between the different air conditioning units to control the air conditioning so small networks and the acronyms can personal area network is commonly used we move up in size covering metres, tens of metres maybe hundreds of metres covering homes, offices, buildings even a campus in some cases we have local area networks that's the general name that we use a network that covers a home or a building some specific instances of a local area network are a home area network may refer to a specific for a home which may have different requirements to some other locations or maybe you have in a data centre or in a company you have many file servers you need to connect together to share storage so you can hear people talk about storage area networks essentially instead of having a set of hard disks in one computer hard disks are spread across many devices and they communicate across a network and the main technologies that you come across are wired LAN and the general name is Ethernet the formal name is IEEE 802.3 and wireless LAN the general name is Wi-Fi or the marketing name and the formal name is IEEE 802.11 but there are others others for more specific purposes and some older ones and there are many others but those are the main two we come across moving up connecting across cities across countries between countries across oceans across a continent so in the order of kilometres, hundreds of kilometres we generally talk about a wide area network so there will be different technologies that will be used to cover across such a large area a WAN if we're talking especially about cities sometimes we refer to a metropolitan area network but the more common one is a wide area network so the technologies that we connect cities together and countries together will be much different than the technologies we use to connect inside our home inside our home we may use Ethernet or Wi-Fi but between Bangkok and Singapore we may use other technologies and some of them will mention a bit more detail but PDH, SDH, ATM frame relay satellite access and there are many others in fact so for long distance links usually or large networks there are a few details about the first three our next topic talks about Ethernet in more depth covering the globe covering the entire earth sometimes we refer to as a global area network or maybe just the internet so the internet is an example of a network that covers the earth it really is made up of connecting many different wide area networks and local area networks together so like in this picture the internet can think of many smaller networks connected together if we go beyond the earth we can connect between different planets and different objects out in space satellites spaceships and so on and people talk about interplanetary networks the main point is that the technologies that we choose usually differ depending upon how big the network will be what we use for personal area networks is usually different from what we use inside the home which is different again from what we use to connect between cities in this topic we don't go through all of these technologies but just be aware that we usually classify by say personal area local area and wide area networks and then the internet as the global network questions on how to categorize networks or anything among network technologies so far WiMAX is a wireless technology for larger distance than Wi-Fi so Wi-Fi generally covers what meters tens of meters maybe in extreme circumstances with very large antennas maybe kilometers WiMAX is an alternative really to your 3G wireless access so 3G there's a tower a cell phone tower or a base station and you talk to your mobile phone wirelessly WiMAX is similar but there's some towers and they talk to say to your laptop for long distance wireless access or talk to a device in some home so WiMAX is wireless coverage over distances of usually kilometers maybe 10, 15 kilometers PDH, SDH we'll talk about technologies for connecting across cities using PDH using electrical cables SDH mainly uses optical fibers ATM and frame relay networks which are usually formed across cities and across countries for example in the past and still in many networks I think you know that with your mobile phone you connect to a cell phone tower or a base station you see them around but then those towers connect via wired links so AIS for example has towers all through the country and those towers are all connected together via a wired network and one technology to connect them together was ATM it doesn't give you money it gives you connectivity between different devices and frame relay is really an older technology than ATM for that any other questions on network technologies why is WiMAX not so popular so WiMAX is maybe considered as an alternative to using say 3G for wireless access across a large distance why is it not popular maybe for many reasons it was new compared to mobile phone networks that have gone from much longer periods so people are used to the technologies and using them it can generally provide maybe 5 or so years ago it could provide much higher data rates than your mobile phone but mobile phone technologies have improved a lot recently now you can get up to 40 Mbps maybe 100 Mbps to your mobile phone which is matching WiMAX the biggest difference in speed now maybe it was due to the implementation everyone has a mobile phone for WiMAX you need a new chip a new wireless chip in your device maybe in your laptop and there was a cost involved in creating them maybe the companies that built the mobile phone networks didn't support WiMAX so much so there wasn't much support behind it it's used in some cases though there are some large networks in the US that use WiMAX other questions on network technologies what about multiple input multiple output MIMO so maybe when you hear about different wireless technologies WiMAX maybe new mobile phone new Wi-Fi technologies especially Wi-Fi access points have multiple antennas this one has two but it's very old but some of the new ones have three antennas and they talk they're starting to support a technology called MIMO multiple input multiple output the idea is that multiple input or multiple output is that you transmit out multiple copies of your signal one through each antenna so with three antennas for example when you transmit your data you really send three different or three copies of that signal one via each antenna and multiple input there are three receive antennas so they receive three different signals and the idea is that with three different signals they are separated in space those antennas so they're a little bit apart a few centimetres apart and the signals that are received are of the same data the signals are slightly different they're received at different times because of different propagation delay and with different characteristics because of interference and it turns out that it's easier for the receiver to process the received data when it receives say three copies of the signal versus one copy and it can process the data better as a result you can send faster because it can handle processing at higher data rates there's much more about it than that but multiple input, multiple output is really transmitting multiple signals receive multiple signals and the processor at the receiver is much smarter to be able to work out what that signal represents and allows you to send faster let's in the last so that's not related to that's way out of scope for this topic but interesting topic about wireless access we're going to focus mainly on wide area networks and local area networks the smaller networks often have similarities to local area networks different technologies but some of the concepts are similar between LANs and PANs so we will not specifically talk about the smaller networks and really most large networks can be similar to classified as wide area networks so that's a simple classification wide area networks usually connect devices as we said across a large area large geographical area examples between campuses so our campuses for SIT we use a wide area network technology to connect them between office buildings cities and countries LANs within campuses whether it's a university campus or a company within buildings, within homes usually wide area networks are owned and operated by some organization that runs it on behalf of other people on behalf of the users that organization is usually a telecom company or an internet service provider an ISP so the government the government telecom companies for example in Entile NTOT CAT and the many different ISPs which run their own network so these companies build a wide area network they own it, they operate it but then they lease or rent access to that network to the users where the users may be different organizations universities, companies, other internet service providers for example we want to connect our two campuses together via a wide network SIT is not going to build a network between our two campuses what we do is we lease or rent access to a network from some telecom company so they already have the network connecting the two locations they just we pay per month for access to their network so the WANs usually they're owned and operated by the organization using the network so the wide LAN inside our campus here it's made up of cables different LAN switches that network is owned and operated by SIT, SIT built it and paid for all the equipment it's inside the walls and the different rooms and we are the user of that network that's one difference between WANs and LANs and it has an impact on what technology can be chosen if you own and operate and build the network then you have more choices of what technology to use if you want to rent off someone else well basically you have to choose from whatever they will provide and usually there's less choices of technologies what else can we compare them although it changes a little bit recently but typically LANs have a higher data rate than wide area networks one example is the SIT LAN the wide LAN supports data rates up to 1 gigabit per second so if I want to communicate say my office computer to another office computer inside our campus I can send data up to 1 gigabit per second if I want to communicate from my office computer to someone out on the internet I'm really limited by the link from our campus to the next location our wide area network link which may be in the order of tens of megabits per second I don't know the exact data rate that we support so I think the WAN link from Bunkeri to the rest or to the next location is usually much smaller than the internal LAN so LANs are usually higher data rates than wide area networks because the local area network supports not just communications from me out to the internet but also me to other locations internal so it needs to support more data but there are some exceptions to that it's not always true the last thing today we'll look at again generally with networks some topologies which are common a few and then next week we'll give a few examples of wide area network technologies I'll just mention two or three and then the next topic is on Ethernet which is how LANs, wide LANs work so we'll spend some time on that and the last topic for this course is on the internet which can be thought simply as connecting many different WANs and LANs together to form one large network so let's finish with look at some common topologies how we connect nodes together in different networks remember a topology is how we arrange nodes and links so in the topic on routing and switching we saw that example network topology of those six nodes and those links between them that's a topology so the nodes or devices well really we have two different types of devices in a network the devices which have data to send so the data is created by some devices and sent to some devices sometimes we call them stations so my laptop or my PC or your mobile phone can be called a station or a host or even an end node so the devices that we use will refer to as stations for example but inside some networks there are other devices devices that the human doesn't use but is used to support the data delivery we've already mentioned in switch networks we have switches and we'll see similar we'll refer to switches but there may be other devices like repeaters and hubs some common names the links are either point to point or point to multi-point we will go through these four topologies over the next few slides with some pictures and there's a hybrid topology which is combining them in different ways so we'll go through them with it more detail first when we want to build a network and we need to choose a topology some things that we would desire are listed here we should allow stations to communicate with any other station I have a network to be built across our campus we've got many PCs in offices, in lecture rooms, in labs we'd like to be able to have any PC to communicate with any other PC so that's a common requirement we don't want to limit the network such that this PC can only talk to some of the PCs in the network we generally want to allow anyone to communicate usually using point to point links is better than using point to multi-point remember we said point to point we can get higher performance than multi-point so if we can, point to point links is better for performance maybe a conflicting requirement is that we often want to use as few links as possible the more links, the more cables we need and the more maintenance and the more complex it is to install the network so minimise the number of links we would like a network that scales well which means that if I build a network today in some arrangement, some topology and then in next week I want to add a new network node, a new PC it should be easy to add a new node to the network, a new station take a little effort to build or upgrade the network often we'd like more tolerant networks if something goes wrong on say one link or in one node we don't want the whole network to fail that would not be good we want to tolerate some faults for example this PC has a link, a cable going to it goes up through the wall if the link fails here maybe someone pulls out the cable or accidentally cuts the link we don't want that to affect the other PCs in the network so we'd like to choose a topology that allows for fault tolerance or if some device fails in the network we'd like the other devices to still be able to communicate in some cases we'd like to be able to quickly detect faults if someone pulls a link out of this PC maybe we'd like to be able to detect that that's happened automatically so that someone who manages the network can go and fix it that's not so much an important requirement for LANs but in wide area networks that's often very important you have a link we have a network connecting one country to another if there's a a link that fails for some reason we'd like to be able to automatically detect that and switch over to a backup link so fault detection is important in some networks so keep in mind these requirements of what we'd like to achieve and we'll go through four or five different topologies which are common first is a mesh topology the example picture gives us six nodes A through to F and in a mesh topology we connect each of those nodes to every other node via a point to point link so there's a point to point link from A to B so when A wants to send data to B it uses that link point to point links are good for performance the link between A and B is not used by anyone else so they get the performance that the link offers it's not shared if A wants to send it C it sends via a different link and so on so we get this full mesh connecting all the nodes together this is with six nodes how many links count the links how many links in the network count the lines or the links sure did you count the 30 it's not 30 can anyone count if you look at those links I think you'll find there are 15 links there's a one from A to B there's C A to D A to E A to F that's five links then from B there's one from B to A but we just counted that then B to C D E F so another four so that's nine then another three for C that's 12 another two from D it's 14 another one from E that's 15 links this is a simple example now consider SIT computers, office computers lecture computers maybe one thousand computers how many links needed so imagine we have a thousand computers we want to connect in a mesh topology turns out the equation is the number of nodes times by the number of nodes minus one divided by two which is with a thousand computers about a thousand times 999 divided by two which is about 500,000 so if we want to connect all our PCs in a mesh topology inside SIT we'd need 500,000 cables going across the campus just wouldn't be possible and also each each node has five cables coming out of it so we've got five cables plugged into each node again we've got a thousand computers I'd need a thousand different cables plugged into this PC again not possible mesh networks full mesh networks are only used in small scenarios when we have a few nodes to connect usually not in LANs mostly in small wide area networks becomes too complex too many cables too many links as the number of nodes increase but good for performance because every pair of node gets a dedicated link guaranteed performance an extension or a sub-case is a partial mesh don't have links for all pairs of nodes less links that's good but the problem is A can't talk to F without a link from A to F it cannot communicate with F unless there's some way that A for example could send to E could forward or onto F which is really what our switching approach does in our general networking so such a topology is commonly used for large networks like wide area networks you cover the country, you have links between many different nodes but not a full mesh just a partial mesh not used in LANs for example a bus topology you probably studied buses in terms of computer hardware you can communicate across a bus similar in a network we have our six nodes attached to a bus this solid black line think of that as the communications bus and the way that a bus works is you transmit onto the bus and the data travels across the bus and you can think for example if A transmits data to D it transmits the data onto the bus and the signal during that data travels across the entire bus in both directions so in fact every other node can receive that signal the way that it works usually in networks is that A transmits to D it creates a frame and sets the destination address to be D sends it onto the bus the bus delivers it to everyone B, C, E and F ignore it because they are not the destination because it is the destination this bus link is a point to multi-point link one node transmits multiple receive so it's in fact a shared link and that's a disadvantage in that the performance depends upon how we share that amongst the multiple users bus topologies were used in early wired lands so maybe 15 years ago now that many of the wired lands inside campuses and inside offices used a bus topology basically there was a cable that passed all the PCs and the PC had a special attachment on the bus on the back that connected to that one bus cable it's a so there's a single multi-point link connecting the stations it's a point to multi-point medium A transmits everyone receives what else should we stay here it's generally easy to install compared to the mesh we just need one long cable that bypasses all of the or goes past all the stations so if I want to connect all the PCs in the building then we need a cable that passes all of those PCs and they essentially connect onto the bus if we want to add a new one we just need to make sure that that bus passes that new node the problem with this in terms of performance is with point to multi-point we need some way to allow the users to share the medium the problem is if A transmits and at the same time B transmits the signals for those transmissions will interfere with each other and no one will receive the data so we need some way to share that reduces the performance if the bus fails no one can communicate that's a problem as well so if there's some error on the bus then nothing will work a similar topology is take the bus but join the two endpoints together take the dots here and join them together and you really get a ring topology it can be either a join bus or we can even just have point to point links between each pair of nodes to get this ring topology where A transmits the data you can think the data is placed on the ring and passes all the way around and back to A and then no more transmissions occur and again if A transmits to D the destination address is included in the frame and the frame is ignored by F E D takes a copy and ignored by C and B and really you can think A removes it from the bus so a ring topology we transmit our data all the way around the ring whoever is the destination takes a copy of that data it's very good if you want to be able to send data to a selection of the nodes you want to send data to both D and C then since the frame passes all nodes it's very simple and D and C take a copy it usually moves in one direction but you can have a bidirectional ring that is you could basically have two cables so if you have a single direction then you have some performance problem if A wants to send to B it needs to go all the way around the ring but a way to overcome that is to have a second cable that goes the other direction and choose the one that works best it's not so common in lands today it was used in old lands there was something called token ring but in wide area networks and across cities metropolitan area networks it's still common where there are multiple rings in both direction and it supports fault detection quite well in that if there's a fault if we send data if there's a fault between E and D this portion of the link has a fault the data gets to E and D doesn't get the data well we've identified the portion of the network that has the fault because if E got the data and D did not then we know that the fault is between E and D that's very useful in large networks across cities and between countries because once you detect a fault then you can switch over to a backup link and in large networks they have multiple rings so essentially multiple cables in both directions it's still a point to multi-point medium in that we need some way to share the medium if two users transmit at the same time they'll interfere with each other we'll come to that ways to share the medium next week the main one used in lands today is a star topology we connect all our stations to some special central node it's not a station it's often referred to as a hub and we usually use point to point links from the station to the central node and if A wants to communicate with D, A transmits to the central node and it's got a dedicated link to do that no one else can use this link and the central node then recognises from the destination address it needs to go to D so it transmits across this link to D so we now introduce the new device which supports the communications and this is really what's in use in most wired lands today so this PC has a cable going through the walls down to the third floor in there there's this central device a switch and that central device has many cables going into it one from each PC inside the land and you have that at home you may have your home ADSL or home router and on the back there may be four ports sometimes you see those yellow or blue ports on the back which means you can plug in four LAN cables and connect say four laptops or four PCs to that central device that's the switch or the central device if you want to add a new node then you just connect to the switch the central node so add a new link as long as this central node has enough ports to plug in a new cable if node D fails it doesn't affect the other nodes all the other nodes can still communicate with each other if the link from D to the central node fails then again all other nodes can still communicate so that's good in terms of fault tolerance but if the central node fails the whole network fails and that's bad with respect to fault tolerance so there's a high dependence upon this central node that needs to be reliable that's used in most lands today and in the next topic on Ethernet we'll see a few more examples of how that setup is used in a real land so a quick overview of some common network topologies some of those topologies require sharing the medium so the next part is upon how do we control who has access to the medium at what time that's next week and then we'll summarize with a few example network technologies and then look specifically about Ethernet