Satellite, ISDN, Cable, DSL, and dialup networks – CompTIA Network+ N10-006 – 1.4

| March 29, 2015


If you need a link to the Internet, you’re probably going to use one of the more common Internet options. In this video, you’ll learn about connectivity from satellite, ISDN, cable, DSL, and dialup networks.

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If you need to communicate over a network from wherever you might be, one of the ways to do that is with a satellite based network. This is where your in-station is literally communicating into outer space to a satellite. That’s satellite is feeding that back down to a ground station somewhere on Earth, and then the conversation completes by reversing that process. This is clearly something where you’re going to have high costs and limited speeds. Although the speeds have increased over the last number of years, it certainly is a much more expensive way of networking because you must have a satellite in orbit somewhere around the earth for this to happen. But if you’re at a remote site, you’re somewhere that does not have DSL, there’s very few options for dial connectivity, and you need some way to connect, satellite network maybe the right solution for you.

One of the challenges you have with satellite networking is the delays involved in sending something into outer space, and having that come back down to you. The latency numbers are relatively large when you compare those to a network that might be terrestrial-based. And because of that, certain applications probably would not work well over a satellite network, but if you have something that is very low bandwidth, or it’s not sensitive to large latencies in the communication, this would be the right solution for you.

The satellite communication operates, generally, on two gigahertz frequencies, back and forth to the satellite, so these are relatively high frequencies, that do suffer from things like rain fade. It’s very difficult for that high frequency to make it through any objects, and if a cloud happens to be between you and the satellite, and that cloud has a lot of water in it, you’ll find that the signal will not only decrease, sometimes it will disappear completely. So this is not a 100% connectivity type situation, but if this is your only choice, you can certainly wait for the sun to come out to be able to do what you need across the network.

One type of networking that we don’t see generally deployed these days, but it is still used in some very niche situations is ISDN. ISDN stands for Integrated Services Digital Network, and there are really two major types of ISDN that you need to know about. The first one is BRI, or basic rate interface. You’ll sometimes hear BRI referred to as 2B+D. That means there are two bearer channels and one data signaling channel. The two bearer channels are 64 kilobits, which means you could get a total of 128 kilobits per second of throughput through an ISDN connection. The D channel, the 16 kilobit per second signal on the channel, is used primarily for signaling and call set-up, and call break-down.

So you generally just have your data running over those very important B channels. That becomes very important when you look at the other kind of ISDN in connection which is the PRI, or primary rate interface. These are usually brought into a facility over T1 or an E1, and here in the United States, a T1 can support 23 B channels, and one additional D channel. We also have on the E1 side, 30 B channels, a D channel, and a separate alarm channel.

You’ll commonly see the ISDN connections being brought in for a traditional telephone system, connecting a PSTN, the public switched telephone network, to a local PBX, and if you have some of those older legacy non voice-over IP implementations, or you need to connect your local voice-over IP to the normal phone network. It’s very commonly done over these ISDN connections. DSL has become a very popular way of connecting everyone’s home to the internet. The most common type we see is ADSL or asymmetric digital subscriber line. And the reason it’s so popular is probably because it uses the existing phone lines that are going into your house. You don’t have to put anything extra into the ground. You don’t need to rewire your home, and you can have some relatively high speed internet connectivity.

What’s interesting about the Asymmetric DSL is that the download speed is going to generally be faster than the upload speed that’s the asymmetric part of it. Another challenge you’ll find with DSL is you have to be relatively close to a central office somewhere in the ’10 1,000 foot range, because that signal decreases quite a bit going through normal phone lines to get to your home. You’ll generally see about 24 megabit per second downstream and about 3.3 megabit per second upstream. But there are newer types of DSL that are constantly being worked on to get a higher speeds. One of those is VDSL, where you get a very high data rate DSL, and those are ranging somewhere between 3 megabits all the way up to 100 megabits per second. So as we see the technologies improve, we’re also going to see these DSL speeds get faster and faster.

Since we’re talking about DSL, lets also talk about PPPoE that stands for PPP, point-to-point protocol over ethernet. Point-to-Point protocol, of course, has been around for a very long time, and combining it with the ethernet has allowed us to take these DSL networks that we’re using, and be able to connect back to the central office, or the central ISP over that DSL line. It’s something that you can generally find supported in most operating systems. You don’t have to configure any routing or any extra configurations on the network side. It’s very similar to using an old dial up modem, except it’s now been created to use over an ethernet connection, and that’s perfect when you plug-in to one of those DSL ethernet bridges. Once you’re connected to the ethernet network, you use this PPPoE to now connect to the ISP that you’d like to communicate with. This is another way that you could have one provider give you the DSL connection, but have a completely different provider give you the internet connectivity.

Another popular networking type for internet connectivity in both the home and in businesses is a cable modem. This is running an internet connection over the coax and sometimes fiber that we have installed in the ground already for our television cable. This is something that is standardized as DOCSIS, or Data Over Cable Service Interface Specification, and the DOCSIS standards are improving all the time as the years go on. You’ll see the cable modem implementations use anywhere from 4 megabit download, all the way up to 250 megabit download, and the speeds continue to improve. Not only are you able to use data over this connection, along with the television signal that you’re already receiving, but there’s also a voice communication, as well. It’s very common for cable companies now to provide you with television, data, and voice, all with this cable modem set up.

If you don’t have a cable television connection, you’re too far away from a central office for DSL, and you don’t have anywhere to put up a satellite dish to communicate into space, maybe you just use a dial up modem. This connects to the traditional telephone lines that you can really find anywhere. We’ve had these phone lines for so many years that it is a very tried and true way of connecting up locations. You do have a limited frequency response with these modems. So your throughput is not going to be as high as a DSL or cable modem connection. Generally, you can see 56 kilobit modems, and some of the data that you could put through that connection can be compressed even up to 320 kilobits per second. Well, as you can tell, this is going to be a relatively slow communication, but if you have a system that just need to transfer certain amount of data over certain time every day, a modem may be easy way to do that.

And of course, if you need more connectivity, you could bring in more phone lines, but you’re going to need more modems. So this does not scale up to very high speeds. But it is something that you can connect up and have a modem on both sides and be able to transfer information. If you walk into a data center, you’re probably going to find a legacy system that’s got a modem connected to it because that’s the way the system was developed to transfer data from one side to the other. You may also see modems connected to networking equipment like routers, and switches, and firewalls, so that people can connect to those systems, even if the rest of the network is down. And if you look hard enough you can find a manufacturer out there who’s still making brand new motor. So if you do have a need for this kind of networking connectivity, you can still plug-in and connect to the traditional phone line, and still be able to communicate.

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Category: CompTIA Network+ N10-006

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