WEBVTT 0:00:07.540000 --> 0:00:11.440000 This video, which is part of the CCT routing and switching series, is 0:00:11.440000 --> 0:00:16.120000 called Telco termination points and CSU DSUs. 0:00:16.120000 --> 0:00:21.520000 My name is Keith Bogart and I will be your instructor for this session. 0:00:21.520000 --> 0:00:25.400000 In this session I'm going to cover two of the bullet points that are in 0:00:25.400000 --> 0:00:30.000000 the CCT routing and switching Blueprint under section 3.0, namely the 0:00:30.000000 --> 0:00:34.320000 bullets that read described Telco termination points such as DMARC and 0:00:34.320000 --> 0:00:41.500000 etc. And describe what a CSU does, such as loopback processes and etc. 0:00:41.500000 --> 0:00:46.300000 So let's go into Telco terminology briefly to talk about these various 0:00:46.300000 --> 0:00:48.100000 different terms. 0:00:48.100000 --> 0:00:53.220000 So the idea here at a high level is that you have gone to a telephone 0:00:53.220000 --> 0:00:59.660000 company and you are getting a digital circuit from that telephone company. 0:00:59.660000 --> 0:01:01.560000 So let's go back in time a little bit. 0:01:01.560000 --> 0:01:06.060000 I know that now as I record this in 2017, most of the time you don't have 0:01:06.060000 --> 0:01:07.220000 to go to a Telco anymore. 0:01:07.220000 --> 0:01:10.520000 You go directly to an internet service provider and you ask for like a 0:01:10.520000 --> 0:01:14.640000 Metro Ethernet connection or maybe a cable or DSL connection. 0:01:14.640000 --> 0:01:19.560000 But let me take you back about 25 or 30 years because you'll still see 0:01:19.560000 --> 0:01:25.280000 this in parts of the world. 0:01:25.280000 --> 0:01:33.300000 So in those situations we know that back in the before the 1950s, so way 0:01:33.300000 --> 0:01:38.880000 before the invention of TCPIP, when you had Telco technology, what was 0:01:38.880000 --> 0:01:42.180000 it? Well you would pick up your phone at your house. 0:01:42.180000 --> 0:01:47.360000 You had some wires connecting that phone that would go a few miles to 0:01:47.360000 --> 0:01:51.660000 the telephone company's office where an actual human being would sit there 0:01:51.660000 --> 0:01:54.280000 at a switchboard and they would switch you. 0:01:54.280000 --> 0:01:58.640000 Basically they would connect the wires coming from your house and they 0:01:58.640000 --> 0:02:02.440000 would connect it with a cable to another port or jack that connected to 0:02:02.440000 --> 0:02:05.100000 somebody else's house or somebody else's business. 0:02:05.100000 --> 0:02:08.160000 So it was actually running a complete circuit between you and whoever 0:02:08.160000 --> 0:02:09.820000 you were talking up to. 0:02:09.820000 --> 0:02:16.900000 So every phone conversation consisted of a dedicated circuit between point 0:02:16.900000 --> 0:02:21.540000 A and point B. Even if you were talking across the country, that was still 0:02:21.540000 --> 0:02:24.280000 the case. It had to have dedicated wires. 0:02:24.280000 --> 0:02:28.040000 Well, in the 50s is more and more and more people start getting telephones 0:02:28.040000 --> 0:02:30.420000 and telephones became really popular. 0:02:30.420000 --> 0:02:33.280000 The telephone companies realized, uh oh, we have a problem here. 0:02:33.280000 --> 0:02:39.000000 If I've got, if we've got 10,000 people on the east coast talking to 10 0:02:39.000000 --> 0:02:43.480000 ,000 people on the west coast, that's going to be a lot of wires to connect 0:02:43.480000 --> 0:02:47.360000 all the way across the country to run all those conversations. 0:02:47.360000 --> 0:02:50.360000 Whoa, if it gets into the hundreds of thousands or even the millions. 0:02:50.360000 --> 0:02:55.680000 So the telephone company can invent something called a T1 line. 0:02:55.680000 --> 0:02:59.380000 T is in Thomas or T1 is in Telco. 0:02:59.380000 --> 0:03:04.260000 And the idea behind a T1 line was, why don't we have one physical line, 0:03:04.260000 --> 0:03:09.020000 or I should say one cable that's fairly thick and shielded really well, 0:03:09.020000 --> 0:03:12.920000 but we're going to take all of those telephone conversations and multiplex 0:03:12.920000 --> 0:03:15.580000 them onto this one line. 0:03:15.580000 --> 0:03:17.320000 Now, what do I mean by multiplex them? 0:03:17.320000 --> 0:03:19.960000 The idea of multiplexing is very simple. 0:03:19.960000 --> 0:03:21.000000 Here's the idea. 0:03:21.000000 --> 0:03:26.660000 Imagine you have 10 people in a room and they all want to talk. 0:03:26.660000 --> 0:03:30.220000 Well, if they all talk at the same time, it's just going to be garbage. 0:03:30.220000 --> 0:03:32.900000 It's gibberish. No one's going to be able to understand anybody. 0:03:32.900000 --> 0:03:34.640000 So the idea of multiplexing is this. 0:03:34.640000 --> 0:03:36.300000 We say, okay, here's what we're going to do. 0:03:36.300000 --> 0:03:40.840000 We're going to take the next hour, a time slot, a period of time, and 0:03:40.840000 --> 0:03:42.300000 we're going to divide it up. 0:03:42.300000 --> 0:03:45.460000 And each person's going to get a unique slice of that time. 0:03:45.460000 --> 0:03:48.660000 So Tom, you can talk for the first five minutes. 0:03:48.660000 --> 0:03:51.100000 Sally, you get the next five minutes. 0:03:51.100000 --> 0:03:52.620000 John, you get the third five minutes. 0:03:52.620000 --> 0:03:54.440000 And then I'll roll back to John again. 0:03:54.440000 --> 0:03:58.080000 So every five minutes, you guys get your own dedicated time slot. 0:03:58.080000 --> 0:04:02.680000 You can only talk when it's your turn or your slot of time to talk. 0:04:02.680000 --> 0:04:06.340000 And if your turn comes around and you have nothing to say, well, we'll 0:04:06.340000 --> 0:04:09.900000 just sit here and twiddle our thumbs for five minutes because maybe you'll 0:04:09.900000 --> 0:04:11.220000 think of something. 0:04:11.220000 --> 0:04:13.700000 And then when your five minute time period is over, it goes on to the 0:04:13.700000 --> 0:04:18.900000 next time slot. Same idea was true with telco terminology and T1s. 0:04:18.900000 --> 0:04:22.080000 They say, we're going to take all these telephone calls and multiplex 0:04:22.080000 --> 0:04:23.760000 them onto this T1 line. 0:04:23.760000 --> 0:04:27.240000 This one phone call, he'll get this tiny little slice of time that's repeating, 0:04:27.240000 --> 0:04:28.740000 repeating, repeating, repeating. 0:04:28.740000 --> 0:04:30.000000 There'll be another telephone call. 0:04:30.000000 --> 0:04:32.600000 He'll get the slice of time right next to that. 0:04:32.600000 --> 0:04:36.320000 And so now we had instead of multiple wires carrying phone calls from 0:04:36.320000 --> 0:04:41.220000 everybody, we just had one cable that was digitally carrying all these 0:04:41.220000 --> 0:04:42.800000 phone calls on one circuit. 0:04:42.800000 --> 0:04:45.280000 So that was called a T1 line. 0:04:45.280000 --> 0:04:49.760000 Well, very soon people start realizing, hey, these T1 lines can carry 0:04:49.760000 --> 0:04:52.640000 a pretty good amount of bandwidth, go at a pretty good speed. 0:04:52.640000 --> 0:04:56.280000 I'd like to actually have a T1 line run all the way to my business. 0:04:56.280000 --> 0:05:01.140000 If I have a T1 line going all the way to my company, then I can get a 0:05:01.140000 --> 0:05:05.140000 much faster connection to the internet than if I'm just using a dial up 0:05:05.140000 --> 0:05:06.940000 modem or something like that. 0:05:06.940000 --> 0:05:10.020000 So that's where we are in this slide right here. 0:05:10.020000 --> 0:05:13.760000 So we're talking about the telephone company running a wire to your house, 0:05:13.760000 --> 0:05:18.300000 which is carrying a T1, the same type of T1 that they're using between 0:05:18.300000 --> 0:05:19.440000 their buildings. 0:05:19.440000 --> 0:05:21.460000 So what's the terminology of all this? 0:05:21.460000 --> 0:05:25.440000 Let's start with a term of CPE. 0:05:25.440000 --> 0:05:30.500000 So you're going to have to have some equipment in your house or in your 0:05:30.500000 --> 0:05:34.900000 business that connects to this line that is your responsibility. 0:05:34.900000 --> 0:05:38.580000 If this equipment fails, you're responsible for replacing it. 0:05:38.580000 --> 0:05:41.980000 If you want to get something bigger, better, faster, it's your responsibility 0:05:41.980000 --> 0:05:43.120000 for replacing it. 0:05:43.120000 --> 0:05:44.960000 It's your responsibility. 0:05:44.960000 --> 0:05:48.240000 That is called the customer premises equipment. 0:05:48.240000 --> 0:05:52.220000 In this particular terminology or in this diagram here, that would consist 0:05:52.220000 --> 0:05:56.360000 of the router and then that router is connected to some kind of a box. 0:05:56.360000 --> 0:05:59.080000 And we'll talk about what that box is here in just a minute. 0:05:59.080000 --> 0:06:01.280000 And those two things are your job, your responsibility. 0:06:01.280000 --> 0:06:03.540000 So that is the customer premises equipment. 0:06:03.540000 --> 0:06:06.860000 Now, there's going to be a line that runs into your house or into your 0:06:06.860000 --> 0:06:10.260000 building and that line is going to stop at some point. 0:06:10.260000 --> 0:06:14.440000 And the telephone company is going to say, okay, if anything goes wrong, 0:06:14.440000 --> 0:06:18.840000 we're responsible for fixing it from this point on. 0:06:18.840000 --> 0:06:23.740000 So from this point, this jack or this wall plate in your wall and the 0:06:23.740000 --> 0:06:28.080000 line that goes outside of your building to us will take care of any problems. 0:06:28.080000 --> 0:06:32.000000 But beyond that point, you have to take care of the problem if the problem 0:06:32.000000 --> 0:06:32.920000 is on the other side. 0:06:32.920000 --> 0:06:35.940000 That point is called the demarcation point. 0:06:35.940000 --> 0:06:38.720000 Otherwise, frequently known as the demarc. 0:06:38.720000 --> 0:06:42.380000 So the demarc would typically look like some sort of wall jack or plate 0:06:42.380000 --> 0:06:46.480000 or maybe some sort of embedded box in your wall where the wires from the 0:06:46.480000 --> 0:06:49.500000 outside world is coming in and terminating right there. 0:06:49.500000 --> 0:06:51.740000 That is the demarcation point. 0:06:51.740000 --> 0:06:56.300000 Now the local loop is the actual physical wire that's going from the demarcation 0:06:56.300000 --> 0:07:00.280000 point out to the telephone company's buildings. 0:07:00.280000 --> 0:07:05.960000 So that local loop is probably a few miles long, but that is the local 0:07:05.960000 --> 0:07:09.620000 loop. Is that actual wire that's strung up on the telephone poles or maybe 0:07:09.620000 --> 0:07:12.400000 buried underneath the ground that goes between your house and your building 0:07:12.400000 --> 0:07:15.160000 to the telephone company's building? 0:07:15.160000 --> 0:07:18.100000 And now technically where that wire terminates to the other end, what 0:07:18.100000 --> 0:07:21.520000 I've been calling the telephone company's building, the technical term 0:07:21.520000 --> 0:07:23.820000 for that is the central office. 0:07:23.820000 --> 0:07:29.780000 So we have here, if we take a look at all this, we have the central office, 0:07:29.780000 --> 0:07:33.400000 they have a wire which is the local loop. 0:07:33.400000 --> 0:07:37.080000 That wire terminates at some point where they say beyond this point, if 0:07:37.080000 --> 0:07:39.320000 there's a problem it's your job to fix it. 0:07:39.320000 --> 0:07:40.840000 That's the demarcation. 0:07:40.840000 --> 0:07:45.740000 And then everything over here is the customer premises equipment. 0:07:45.740000 --> 0:07:52.040000 Okay, so let's now talk a little bit more about this thing right here. 0:07:52.040000 --> 0:07:56.960000 This box, what is that and why do we need it? 0:07:56.960000 --> 0:08:01.960000 So in this type of scenario where we have a T1 line or if you're in Europe 0:08:01.960000 --> 0:08:08.380000 in E1 line that's connecting into your company, that box would be what 0:08:08.380000 --> 0:08:14.560000 we call a CSU, DSU or a channel service unit, data service unit. 0:08:14.560000 --> 0:08:19.180000 And this is the fundamental device that connects their circuit, their 0:08:19.180000 --> 0:08:23.940000 line to your customer premises equipment, which is probably a router, 0:08:23.940000 --> 0:08:29.280000 your router. So connects DTE equipment, which is routers, to the digital 0:08:29.280000 --> 0:08:34.580000 lines. So even though it has two acronyms, channel service unit, data 0:08:34.580000 --> 0:08:39.280000 service units, one physical box, but we can sort of logically imagine 0:08:39.280000 --> 0:08:43.560000 the box being separated into two functional pieces. 0:08:43.560000 --> 0:08:48.360000 So some of the hardware in that box is designed to connect to the DTE 0:08:48.360000 --> 0:08:51.940000 equipment. It's designed to connect to your router. 0:08:51.940000 --> 0:08:55.320000 So that is the data service unit. 0:08:55.320000 --> 0:08:58.820000 So the DSU connects to your router to the DTE. 0:08:58.820000 --> 0:09:02.360000 Some other components within that same box, some other transistors and 0:09:02.360000 --> 0:09:06.700000 wires and buttons and jacks and everything connect to that cable that 0:09:06.700000 --> 0:09:09.040000 the telephone company has run into your building. 0:09:09.040000 --> 0:09:12.100000 That is called the channel service unit. 0:09:12.100000 --> 0:09:14.400000 Now, why do we need this thing? 0:09:14.400000 --> 0:09:15.220000 What's the purpose of it? 0:09:15.220000 --> 0:09:20.480000 Well, I'd create some whiteboards here that I want to help explain this. 0:09:20.480000 --> 0:09:26.540000 So back in the day, if you had a router and you need to connect it to 0:09:26.540000 --> 0:09:29.100000 a T1 line, you had to buy a CSU DSU. 0:09:29.100000 --> 0:09:30.400000 And why was that? 0:09:30.400000 --> 0:09:35.040000 Well, because on the router, your LAN interfaces were RJ45 interfaces. 0:09:35.040000 --> 0:09:36.520000 They were Ethernet. 0:09:36.520000 --> 0:09:38.260000 And Ethernet doesn't talk. 0:09:38.260000 --> 0:09:42.080000 Ethernet was not designed for wide area network protocols. 0:09:42.080000 --> 0:09:43.240000 It doesn't do that. 0:09:43.240000 --> 0:09:46.440000 So you had another type of interface on your router called a WAN interface, 0:09:46.440000 --> 0:09:52.080000 which might take the form of an RS-232 interface. 0:09:52.080000 --> 0:09:56.700000 And for example, if you have not seen an RS-232, there's a picture of 0:09:56.700000 --> 0:09:58.580000 what one looks like. 0:09:58.580000 --> 0:10:02.080000 You know, as the type of interface has got lots of different pins in it. 0:10:02.080000 --> 0:10:07.300000 And the point of an RS-232, as you can see here, lots of different wires, 0:10:07.300000 --> 0:10:12.820000 lots of different pins, each with its own distinct, separate functionality, 0:10:12.820000 --> 0:10:14.620000 its own purpose. 0:10:14.620000 --> 0:10:18.320000 Now for the purpose of this video and for the CCT, you don't need to know 0:10:18.320000 --> 0:10:20.080000 what all these pins do. 0:10:20.080000 --> 0:10:25.780000 For example, clear to send, data set ready to send. 0:10:25.780000 --> 0:10:29.020000 You don't need to know what those acronyms mean or why they're there. 0:10:29.020000 --> 0:10:32.900000 The main takeaway I want you to have right now is that your RS-232, this 0:10:32.900000 --> 0:10:38.180000 is your serial interface, on your router, has lots of different pins and 0:10:38.180000 --> 0:10:41.900000 each pin is actually connecting to a physical wire inside of a cable and 0:10:41.900000 --> 0:10:44.440000 each pin is doing its own unique thing. 0:10:44.440000 --> 0:10:48.160000 However, the wire that's coming into your office that's carrying that 0:10:48.160000 --> 0:10:52.660000 T1 line is coming in on an RJ48 interface. 0:10:52.660000 --> 0:10:55.520000 That only has six wires inside of it. 0:10:55.520000 --> 0:10:58.300000 Well, only six of them are actually used. 0:10:58.300000 --> 0:11:02.060000 So right here, you can see there's, we need something to translate because 0:11:02.060000 --> 0:11:08.140000 on the one hand, our router has a connector that's got like 15, 20 different 0:11:08.140000 --> 0:11:13.100000 connectors in it, different pins and that certainly cannot connect to 0:11:13.100000 --> 0:11:16.200000 another wire that's only got six pins in it. 0:11:16.200000 --> 0:11:17.340000 That's not going to work. 0:11:17.340000 --> 0:11:19.900000 The two wires just won't connect. 0:11:19.900000 --> 0:11:23.700000 So we had a need to put something in the middle here that can connect 0:11:23.700000 --> 0:11:25.940000 to these two different connectors. 0:11:25.940000 --> 0:11:28.740000 Not only that. Okay. 0:11:28.740000 --> 0:11:30.360000 Look at the signal levels here. 0:11:30.360000 --> 0:11:34.100000 Now you don't need to know what's a one and what's a zero as far as voltage 0:11:34.100000 --> 0:11:35.280000 levels are concerned. 0:11:35.280000 --> 0:11:40.720000 But I've drawn this in a very specific way to indicate that your serial 0:11:40.720000 --> 0:11:47.280000 interface, your voltage levels are about that high and but the voltage 0:11:47.280000 --> 0:11:52.280000 levels coming in off of the T1 are about that high. 0:11:52.280000 --> 0:11:57.700000 Because remember, keep in mind, the WAN circuit, the T1 line has to be 0:11:57.700000 --> 0:12:02.880000 able to put a one or a zero on a wire and have that one or zero go for 0:12:02.880000 --> 0:12:07.660000 possibly miles down that wire and still be recognized. 0:12:07.660000 --> 0:12:12.840000 That electrical pulse still has to be recognizable as a one or a zero, 0:12:12.840000 --> 0:12:17.640000 five, 10, 15 miles down the wire when it terminates at the demarcation 0:12:17.640000 --> 0:12:21.840000 point. So we need a lot more electrical energy to push that down the wire 0:12:21.840000 --> 0:12:23.760000 so it doesn't get degraded. 0:12:23.760000 --> 0:12:27.880000 But a serial interface on a router was never designed to connect to a 0:12:27.880000 --> 0:12:32.000000 wire that goes three, four, five, 10, 15 miles. 0:12:32.000000 --> 0:12:33.580000 It wasn't designed for that. 0:12:33.580000 --> 0:12:36.440000 So a serial interface, when it's pumping out its voltage levels for ones 0:12:36.440000 --> 0:12:41.020000 and zeros, that voltage level is much less than the voltage level used 0:12:41.020000 --> 0:12:44.640000 in your T1. So that's another reason why we need something in the middle 0:12:44.640000 --> 0:12:48.140000 here to be able to handle these different voltage levels and translate 0:12:48.140000 --> 0:12:56.480000 between them. And then lastly, notice that on the left, on the RS-232 0:12:56.480000 --> 0:13:00.220000 interface, I mentioned that we have these dedicated wires that are used 0:13:00.220000 --> 0:13:02.160000 just for signaling. 0:13:02.160000 --> 0:13:05.240000 So for example, here I have my wire that's just for my transmit. 0:13:05.240000 --> 0:13:06.940000 That's for my transmitting and my data. 0:13:06.940000 --> 0:13:13.220000 But this other stuff, RTS, CTS, clocking, DCD, that's not concerned with 0:13:13.220000 --> 0:13:14.680000 actually carrying the data. 0:13:14.680000 --> 0:13:16.580000 That's not carrying your ethernet frames. 0:13:16.580000 --> 0:13:19.340000 That's not carrying your web browsing or your telnet. 0:13:19.340000 --> 0:13:22.440000 That's carrying signaling or control information. 0:13:22.440000 --> 0:13:24.740000 Can I go? Do I need to stop? 0:13:24.740000 --> 0:13:26.060000 Hold on a second. 0:13:26.060000 --> 0:13:30.000000 Pause. That's all the types of signaling that these wires are carrying 0:13:30.000000 --> 0:13:32.740000 on completely separate wires. 0:13:32.740000 --> 0:13:37.900000 But in a T1, signaling is actually embedded in the exact same wires, the 0:13:37.900000 --> 0:13:42.300000 exact same cables that your data is being transmitted on. 0:13:42.300000 --> 0:13:46.480000 So as 1s and 0s are coming in at these really high voltage levels, some 0:13:46.480000 --> 0:13:48.420000 of them carry your data. 0:13:48.420000 --> 0:13:52.540000 But some of those 1s and 0s right next to it are interpreted completely 0:13:52.540000 --> 0:13:56.060000 differently as signaling bits or control bits. 0:13:56.060000 --> 0:14:01.060000 So that's another reason why we need our CSUDSU to be able to interpret 0:14:01.060000 --> 0:14:05.400000 between signaling that's on separate pins and separate wires as opposed 0:14:05.400000 --> 0:14:09.340000 to signaling it's being embedded within the signal itself. 0:14:09.340000 --> 0:14:15.220000 So the long and short of it is to say that the CSUDSU is a box, and you 0:14:15.220000 --> 0:14:18.740000 can see here an example of what one of them looks like, that translates 0:14:18.740000 --> 0:14:24.460000 or converts the signal coming in from a digital line like a T1 or E1 into 0:14:24.460000 --> 0:14:29.060000 something that your RS-232 serial interface on your router can actually 0:14:29.060000 --> 0:14:34.060000 understand. Now one other thing about the CSUDSU I want to point out that 0:14:34.060000 --> 0:14:39.940000 was on the blueprint for X, they want you to know what a loopback test 0:14:39.940000 --> 0:14:46.680000 is. Okay so let's say that I'm back here and I have a problem. 0:14:46.680000 --> 0:14:48.900000 I'm not getting any data through here. 0:14:48.900000 --> 0:14:50.960000 Everything seems to be set up. 0:14:50.960000 --> 0:14:55.680000 The telco has told me that they've set up my circuit and yet I can't get 0:14:55.680000 --> 0:15:00.400000 any data through this router to my office at the other end of the T1 which 0:15:00.400000 --> 0:15:04.260000 is in Boston or in New York City or Lando or something. 0:15:04.260000 --> 0:15:06.400000 I'm just not able to reach them. 0:15:06.400000 --> 0:15:10.240000 Or maybe I am able to reach them but there's a lot of noise on the circuit. 0:15:10.240000 --> 0:15:11.820000 It's really slow. 0:15:11.820000 --> 0:15:13.300000 I'm getting a lot of delay. 0:15:13.300000 --> 0:15:15.700000 A lot of the bits are having to be reset. 0:15:15.700000 --> 0:15:18.700000 And I'm wondering okay where is the problem? 0:15:18.700000 --> 0:15:21.380000 Is the problem on my serial interface? 0:15:21.380000 --> 0:15:23.340000 Is the problem with this cable? 0:15:23.340000 --> 0:15:28.160000 Could the problem possibly be my CSUDSU is messed up and I need to get 0:15:28.160000 --> 0:15:34.140000 another one? Or could the problem be beyond my DMARC and into the realm 0:15:34.140000 --> 0:15:35.820000 of responsibility of the telco? 0:15:35.820000 --> 0:15:38.240000 Could it be on their line or something like that? 0:15:38.240000 --> 0:15:43.160000 That's where we would use loopback tests to try to identify and isolate 0:15:43.160000 --> 0:15:45.340000 where that problem is. 0:15:45.340000 --> 0:15:48.620000 So there's two different kinds of loopback tests. 0:15:48.620000 --> 0:15:50.400000 Software and hardware. 0:15:50.400000 --> 0:15:55.280000 So the main sort of idea behind a loopback test is we're going to do something 0:15:55.280000 --> 0:16:01.920000 somewhere so that when I send my bits out I know exactly where they stop 0:16:01.920000 --> 0:16:04.420000 and they're loopback to me. 0:16:04.420000 --> 0:16:08.120000 So in other words when I send a bit out I know that I've at this particular 0:16:08.120000 --> 0:16:11.820000 point this point knows it needs to take that bit and basically reflect 0:16:11.820000 --> 0:16:13.920000 it right back to me again. 0:16:13.920000 --> 0:16:17.540000 So if I can send bits out and I get those same bits back I know that everything 0:16:17.540000 --> 0:16:20.160000 from here to here is okay. 0:16:20.160000 --> 0:16:22.440000 Then I can move my loopback maybe a little bit further out. 0:16:22.440000 --> 0:16:26.400000 And now if I send bits and I don't get anything back well I know okay 0:16:26.400000 --> 0:16:29.720000 it's between this point and this point right here. 0:16:29.720000 --> 0:16:31.720000 So that's what we're talking about. 0:16:31.720000 --> 0:16:33.760000 So there's two different kinds of loopback tests. 0:16:33.760000 --> 0:16:38.940000 Software loopbacks and hardware loopbacks and where can they be implemented? 0:16:38.940000 --> 0:16:44.300000 Well within the CSU DSU or within the telcos connection to your CSU DSU. 0:16:44.300000 --> 0:16:50.600000 As an example. Let's say I had a physical CSU DSU like that box that we 0:16:50.600000 --> 0:16:54.560000 just saw that's like so big on the slide just a second ago and I've got 0:16:54.560000 --> 0:16:58.160000 that sitting at my table and that's connected to my router. 0:16:58.160000 --> 0:17:02.320000 Well one thing I could do is on the CSU DSU there's probably a button 0:17:02.320000 --> 0:17:07.340000 I can press or maybe a digital keypad I can type something in which will 0:17:07.340000 --> 0:17:09.960000 cause the CSU DSU to perform a loopback. 0:17:09.960000 --> 0:17:16.820000 So if I press that button then that means that the received side of the 0:17:16.820000 --> 0:17:20.380000 DSU is loopback to the transmit side. 0:17:20.380000 --> 0:17:25.740000 So once I press that button or take whatever that action is on the CSU 0:17:25.740000 --> 0:17:30.220000 DSU. Now what I can do is I can say okay this right here is some interface 0:17:30.220000 --> 0:17:35.200000 like serial 0 slash 0 on my router. 0:17:35.200000 --> 0:17:39.080000 And that interface has some IP address on it. 0:17:39.080000 --> 0:17:41.560000 Let's just say 1.1.1.1. 0:17:41.560000 --> 0:17:45.760000 Okay now they performed a loopback so this would be a hardware loopback 0:17:45.760000 --> 0:17:50.340000 within the CSU DSU itself because I'm actually using its internal circuitry 0:17:50.340000 --> 0:17:54.760000 to do this. Then I would just ping myself with an extended ping. 0:17:54.760000 --> 0:18:00.340000 Do like several thousands of pings to 1111 which is yourself. 0:18:00.340000 --> 0:18:04.320000 Now if I don't get those pings back or if I'm only getting some of them 0:18:04.320000 --> 0:18:07.240000 back. Hey I know it's not the telcos problem now. 0:18:07.240000 --> 0:18:12.260000 Now the problem is either with the cable itself or with the CSU DSU. 0:18:12.260000 --> 0:18:18.560000 If I suspect now I could narrow it down even further a hardware loopback 0:18:18.560000 --> 0:18:23.640000 would be simply meaning okay let me just unplug this cable entirely. 0:18:23.640000 --> 0:18:27.520000 And I'll actually take the end of that cable cut it off. 0:18:27.520000 --> 0:18:32.080000 I'll pull out the wires and I will physically attach the receive wire 0:18:32.080000 --> 0:18:33.520000 to the transmit wire. 0:18:33.520000 --> 0:18:34.980000 I will twist them together. 0:18:34.980000 --> 0:18:38.620000 Now I've created another type of a hardware loopback that doesn't even 0:18:38.620000 --> 0:18:40.940000 involve my CSU DSU at all. 0:18:40.940000 --> 0:18:45.960000 It's like a similar analogy would be taking a switch, an ethernet switch 0:18:45.960000 --> 0:18:49.680000 and taking a crossover cable for one port and connecting it to another 0:18:49.680000 --> 0:18:51.780000 port on that exact same switch. 0:18:51.780000 --> 0:18:54.980000 Except here I've got one cable coming out and on the end I've twisted 0:18:54.980000 --> 0:18:57.800000 the wires together creating a loopback. 0:18:57.800000 --> 0:19:03.540000 Now when I ping myself if the ping is fine I say oh okay when I did the 0:19:03.540000 --> 0:19:06.960000 loopback test on the CSU DSU my pings were messed up. 0:19:06.960000 --> 0:19:10.600000 I wasn't getting them but when I removed the CSU DSU and I actually physically 0:19:10.600000 --> 0:19:14.720000 twisted the wires of the cable together my ping is working. 0:19:14.720000 --> 0:19:18.760000 That tells me the CSU DSU is at fault. 0:19:18.760000 --> 0:19:25.020000 Also you might say okay well the CSU DSU is a two part box right it's 0:19:25.020000 --> 0:19:26.840000 got a DSU and a CSU. 0:19:26.840000 --> 0:19:31.760000 When you press that button on here it was doing this. 0:19:31.760000 --> 0:19:34.680000 There's another way you could do is you could say all right let me take 0:19:34.680000 --> 0:19:40.340000 this cable out right here let me take that one out and put a wire in there 0:19:40.340000 --> 0:19:42.840000 and loop loop that back. 0:19:42.840000 --> 0:19:49.200000 Now it's testing not only the DSU portion but the CSU portion as well. 0:19:49.200000 --> 0:19:52.400000 Now let's say I do all that that's that's the extent to what I can do 0:19:52.400000 --> 0:19:56.180000 and my pings work all along the way my pings work. 0:19:56.180000 --> 0:20:01.100000 So my cable connecting to the CSU DSU is okay both the DSU and the CSU 0:20:01.100000 --> 0:20:04.280000 portions of that box are okay. 0:20:04.280000 --> 0:20:09.680000 At that point the next step in the line would be to get the telco involved. 0:20:09.680000 --> 0:20:12.920000 You'd have to call them on the line and say hey can you put your circuit 0:20:12.920000 --> 0:20:15.720000 into a loopback for me because I need to do some testing. 0:20:15.720000 --> 0:20:17.420000 And that's exactly what they would do. 0:20:17.420000 --> 0:20:21.100000 So if they do that they would go on to their WAN switch their telco switch 0:20:21.100000 --> 0:20:28.260000 and they would take the receive and loop it back to the transmit. 0:20:28.260000 --> 0:20:33.260000 And now if I do my ping test and it fails I know okay the problem is here 0:20:33.260000 --> 0:20:38.180000 it's on this wire or it's on the WAN switch it's but so these loopback 0:20:38.180000 --> 0:20:43.120000 tests are ways of identifying where the circuit is broken. 0:20:43.120000 --> 0:20:45.740000 And like I said there's software or hardware. 0:20:45.740000 --> 0:20:47.920000 Now in this particular case pretty much everything I've been describing 0:20:47.920000 --> 0:20:49.800000 has been hardware loopbacks. 0:20:49.800000 --> 0:20:55.060000 A lot of Cisco routers these days have CSU DSUs actually built inside 0:20:55.060000 --> 0:20:56.320000 the router itself. 0:20:56.320000 --> 0:21:00.240000 Like you'll actually see the router has an RJ48 interface that connects 0:21:00.240000 --> 0:21:05.740000 right to the D-mark and internal to the router it has a CSU DSU. 0:21:05.740000 --> 0:21:08.860000 You'll actually see on the RJ48 interface of the router you'll see CSU 0:21:08.860000 --> 0:21:10.960000 DSU across the top of it. 0:21:10.960000 --> 0:21:15.460000 So you could still do a hardware loopback by putting a special plug or 0:21:15.460000 --> 0:21:19.740000 a special cable in there that's loopback or you could actually go on to 0:21:19.740000 --> 0:21:24.860000 that T1 interface in your Cisco iOS software and type in a loopback command. 0:21:24.860000 --> 0:21:30.120000 For example the command might be loopback local or loopback T1 or something 0:21:30.120000 --> 0:21:32.700000 like that but it's an interface level command. 0:21:32.700000 --> 0:21:37.120000 And that's programming the internal CSU DSU to loopback to itself and 0:21:37.120000 --> 0:21:40.480000 that's what we call a software loopback. 0:21:40.480000 --> 0:21:44.480000 So that concludes this section of this video.