Telecommunications Essentials Pt 4

The introduction of the modem did this. It took a higher speed set point converted into an analog format that would be compatible with the lower quality analog line. So, this was a modulator, demodulator type of processes was taking place called modem. It takes complex modulations to include at the digital bits onto the analog signal. The demodulation to receiver recovers the digital bits. So, we start out digital, end up digital but transport over an analog method. And the common interface here was the Rx-232, or now the EIA-232 interface.

So, here’s some of the strides/seen in modem development, the ITU standard starting here with me with the V.21         on up through V.92. We can see that the data rates increase substantially both uplink and downlink and the importance here is that what happened, we were able to live on more complex including schemes to allow us to get the higher performance.

We get access procedure here for modems. We see this place with V.34 for higher type of modems and this is the protocol used and provides, remember we talked about bit-oriented transmission earlier in our modules, links management, flow control using windowing and we talked about the benefit of that when we discussed TCPIP and error detection using Frames Check Sequence and ARQ techniques. So, a lot of the things that we applied to datacom technologies apply equally well here. And another thing that’s important dealing with low bandwidth channels is data compression.

So, what do we have in a way of modem connections, originally modem to modem path was all analog? But, we could have a modem to modem path that is predominantly digital, certainly, if we’re using the existing telephone cloud as our communication’s path. Remember, modern telephones networks have been upgraded to digital formats on trunk lines and all leased lined are digital. So, the only part of most modern circuit switched networks that are analog are the edge connections from the local exchange out to the actual customers’ equipment, most of everything is digital. So, here’s what we’ve seen is that what we’re goals is to ultimate to become all digital.

Let’s talk a little bit about the top of the line in the modems here. V.90 and V.92, it was higher but symmetric bandwidth and that’s good because most of our applications will be built around needing a higher downstream bandwidth namely downstream to me from a service provider rather than meeting the same high speed of luxury basis. So, what we’re showing here are the highest theoretically achievable decorates and we’ve noticed these are – they’re certainly not acceptable by most of today’s applications. However, if you’re in an area where this is all that’s available, then, it’ll still work.

ISDN, Integrated Services Digital Network, there are parts of the world of this technology suffered some delays in being deployed but allows us now to extend the digital format from the local exchange out to the actual subscriber lines. The datacom path is all-digital and offers a higher, symmetric data rates to subscribers. Well, this is higher, but how much higher? The common configurations in the private rate exclusively the basic rate interface are 64kb/s or 128 kb/s. At the time that this was envision in technology, then this work high data rates but by today’s standards, they are not. Data compression can be used to improve the throughput and here’s what we would have in ISDN type of connection out to say a small office or home office environment.

So, remember, ISDN services fall into the category of being circuit switched type of services and support two rates. The basic rate interface is 2b, two bert channels which are the information channels and consist of two 64kb/s channels and a control channel or their primary rate interfacing consisting of 23 information channels, data channels, bert channels plus a control channel. Notice that there are two different channels. One, for the North American technology of 23b + d or 30b + d based on the European developed technology.

Do you have any questions? Well, if you do, use the question/comment box. Thanks for joining us and I’ll see you in an upcoming module.

Telecommunications Essentials Pt 4

Telecommunications Essentials Pt 3

So, signaling method summarized the local exchanges handle signaling in the local loop, the analog or ISDN. So, I can do that pulse, DTMF or I can use digital signal or ISDN Q.931. Internally, we use SS7 and in private network signal we use QSIG.

Now, we can create calls. So, how do we hold them up? How many lines do I need to keep everybody happy? We can increase the number of trunk lines which will reduce the probability of blocking. Remember in circuit switching unlike packet switching, all typically all the resources can be used at one particular time. And then the next portion trying to gain access won’t be able to gain access, his call will be blocked. So, the more lines and trunks that we have, then the less likely that we have of call blocking, however, the network size becomes more efficient as the size increases, but also increases dramatically in cost. So, there’s a tradeoff. How much can we invest in having multiple lines to deal with that little surge of traffic that may occur only occasionally?

Telephone companies aimed for around one percent blocking probability and even then, it doesn’t work well all the time and we can each think of one day at least in the United States    where we’re pretty much assured to have some form of call blocking and we all know what day that is. If you don’t know, then send me a note and we’ll talk about it.

How do we load up our circuits here? The call activity is statistical measure of traffic over one hour period and there are two different measurement units. One, that’s used most around the world pretty much, Erlangs, based on the work of a Swedish telephone engineer. And then they have, I believe the US version is called Centruy Call Seconds. So, an Erlang is equal to 36 CCS per hour. So, that is an essence of Erlang is one line being tied up for one hour and that’s the measure for traffic density. So, traffic loading, then, is based on the average arrival rate of calls multiplied by their holding time. So, top phone companies come up with wide range techniques to try to predict what is the best size network to build to maintain in around one percent blocking probability.

Now, let’s look at some of the signal formats, frequency versus time multiplexing, the most practical form now. We used to have a lot of frequency multiplexing telephony but no longer do. It’s all based on time division multiplexing. This provides a mean for us to bring together a lot of our digital type of signals, allows many applications to time-share a high-bandwidth circuit. This has been the challenge in the past. How can we effectively tie together a lot of higher capacity lines between switches those support of very limited number of users? In IS, we want to get many, many, many users on the more expensive circuits and let them share them in other frequency in time and time is the wise way to do it. Well, here’s an example, four telephone 64kb/s telephone calls would have an overall capacity requirement 256 kb/s and we see here the 8-bits refrain in the eight thousand times per second that was dictated when we set up the tedium type of system. So, in the example, we have only four channels being supported. In reality, in the U.S., the basic building block for the digital lines is 24 channels of 64kb circuits and in Europe the E1 technology supports 30 voice channels.

So, let’s take a look at how this stuff is evolved worldwide. The T technology is what we have in North America and the E technology is what we would see supported pretty much the rest of the world. Notice that there’s nothing compatible here except all of these are built around 64kb multipliers. The interoperability between the North American and the European systems is non-existent, though. This is a fact and it was through the introduction of some recent technologies, SDH we found a way to work around these incompatibility issues.

Now, we’ll step away from time multiplex and take a look at dealing with communications through modem. Voice-grade modem means we’re going to be operating over a lower quality telephone circuit and in the past, in order to deal with potential interferences and so forth, the system had filters and place that would be a filter removed frequencies outside the voice passed band of around three hundred to thirty four hundred hertz. So, the band with the on line was intentionally limited just to allow the passage of voice. Well, if we want to get a higher speech, we’re going to need a higher frequency range and that means we’re going to get rid of the filters.

Telecommunications Essentials Pt 3
telecommunications expert witness referrals

Telecommunications Essentials Pt 2

If we’re using a digital type of communications method, standardized voice channel became a sixty four kilobit digital channel. By today’s data speed, that’s not very high but it certainly had an adequate speed for moderate quality voice transmission and this particular channel involved from some work done by night quest in studying how to digitize analog signals and to be able to recover the digitized signals and reproduce the analog signal at the destination.

So, this is where the 64kb/s channel came from that we actually use 8 bits to give the dynamic range we needed the signal and a sample grade of eight thousand times a second which is twice of the highest frequency supported which is around 4kHz.

So, eight thousand times eight gives a sixty four thousand bits per second. The 64kb/s channels are equally adept at carrying bursty data. The downside knowing no one has been circuit which environment is data’s bursting when there’s no data than the channel is setting idle to keep a circuit synchronized in digital operations here, the gaps between the packet bursts must be filled with idle bits. We don’t like to do that, however, if we have only available in a circuit switch connection and that connection is better none at all.

How do we set up the channels? One of the original methods which we’re dealing with in a very limited capacity, how do we do this kind of stuff? So, we said, “Let’s include within the actual communications channel a means for creating performing the signal that is identifying that I’m trying to make a call where need to make a call and so forth.” So, this is called Channel Associated Signaling. I can use DTMF, Dual-Tone Multi-Frequency, or pulse dialing to invoke signaling process and the signaling information now here is carried within the traffic channel and no one of the first digital intrusions here was called or something called Robbed-bit signaling.

And what we would do is borrow bits out of the octets, remember the eight bits sequence I just referred to. We would borrow bits out of the certain octets in order to determine on hook – off hook and other signal information but would turn that turns out to be not robust enough and if you remember saying super frames and extended super framing and things like that, then you see that kept trying to push the envelope and realized this is just limited in capability, still in use in some areas but most major metropolitan areas around the world it is being removed or has been removed from service for quite some time.

Well, what do we do now? This is called Common Channel Signaling. With all the features that we want to have associated with our datacom and telecom systems, then we all have separate signaling network in effect and they separate signaling system. So, I have a separate channel assigned for signaling and for us this will be SS7, Signaling System 7, and the international telecommunication union standard, Q.700. So, in essence then, the telephone network is split. We have a signaling network and the communications network. But this particular network, signaling and control it complex but very fortunately it’s capable and supports a wide range of features and this is how we’re going to get caller ID, call forwarding and all the other features which we now take for granted in our landlines.

Let’s take a look at more SS7 that’s used in public networks. In SS7, we get the call forwarding I just talked about, we can do mobile roaming, routing for toll free numbers and credit card verification and things like that for a credit card calls and so forth.

QSIG is – I have no acronym that I’m aware of here, is used for signaling in private networks, perform a similar function such as Inter-PBX (Private Branch Exchange) type of signaling and enterprise network. QSIG was developed by European Technical Standards Institute or ETSI.

Let’s take a look now, SS7 and QSIG, we’re looking now at inside the network signaling processes or what about the signaling at the edge of the network. And remember, we’ve seen what we have for most of us if we still have a dialogue phone at home. But, suppose we have a digital connection at home to go along with our graphics, this will involve ISDN signaling, and that is defined by another ITU standard of Q.931 and a lot of the standards have been built upon a very well bust, very well reliable…and used by many telecommunications expert witness consultants in court. So, Q.931 defines digital local loop signaling, not analog local loop signaling. It’s handled in D-separate channels in the ISDN channels, all the D channels. D-channels stands for Data channels, but the data here is not information being transported but a control channel, supported by LAPD, so this is the Link Access Procedure for D-channels, provides error correction, want to make sure this works correctly and we have different processes for the different interface associated with ISDN. The different interfaces are the Primary Right and Basic Right Interfaces.

Telecommunications Essentials Pt 2

Telecommunications Essentials Pt 1

Welcome to the discussion of circuit switched wide area networks. My name is Dave and I’ll be instructor in this module. If you have questions, use the question/comment box on the Gogo website. Let’s get started.

In this module, we’re going to start with an overview of telecom and telecom circuits. We’ll take a look at some of the old technology that I guess many of us had hoped would drift into oblivion but we still need to do with the fact that a lot of modem type of technologies out there not just the type of modems that we used to use for low speed dial-up internet access. And we’ll talk about another type of circuits which technology and that is integrated services digital network.

So, in telecom, what do we need to – we’re talking about telecommunications here, but in circuit switching, what do we have is some basic building blocks to work from. Circuit switches and circuits are formed as bi-directional physical connections and they carry information at whatever the speed of propagation is on the medium, let’s say at light speed, I guess that’s true, but it’s not same as a speed and free-space but it’s limited to speed that can be transferred along the medium and turns out that what you need is the plastics that we encountered in wired transmission, lines and the refractor industry associated with glass optical fiber is the actual propagation velocity surround two thirds of the speed of light into free space. So, we can use copper or we can use glass for communications, methods if we’re going to a wired environment or we can actually use a wireless approaches we discussed in a previous module.

In our circuit switching here, this is an analogous to the PHY layer of datacom protocols. So, let’s take a look at what we actually are going to do here. We create and end-end connection and remember that online package switching and circuit switching, we formed a dedicated path from one in system to another in system and we patched through the switching and relaying devices internal to the networks. The actual in system connection can call on lines and the interconnections are groups of lines called trunks. The actual service providers in circuits switch technologies form meshes of circuits right matter there are redundant measures of interconnections and this makes them to the ability to provide reliable wide area connections at all times. The connection between the premises equipment for the customers through the circuits switches is part of the services provided and then the other part of the job of the service provider is to interconnect circuit switches within the network core. So, this I established connections from my device to the network and the network does the interconnection and then forms a connection from the other into the network out to the customer. So, the interconnection lines remember called trunks.

In circuit switching we have on-demand physical connections between circuits. The in system decide when to create a connection and as you’d expect, there is a set up delay but unlike packet switching there’s no store-and-forward mechanism and there is no error correction mechanism involve. The digital bits just move through the switches from insistent incest. So, we build and for this communications to take place. We obviously have to build the circuits’ end-to-end. And so, the circuits within this can be classified as dial-up or there will be a set up and tear down time or lease circuits and lease circuits are analogous to the premise virtual circuits we talked about. The lease circuits are permanent and yet they are set up for long periods of time based on a subscription contract or service level agreement.

With circuit switching, the connections are acting as bit-pipes and as such provide or introduce a minimal amount of delay and minimal jitter. In theory, we say there’s no jitter but no electronic devices perfect soldiers, perhaps there are some but very, very little. There is no attempt to quantitize the groups of ones and zeros. There’s no attempt to buy the system to require a frame. So, I can just pure an ongoing stream of digital information from one in system to another in system.

Now, in a lot of traditional phone systems that we have today, the ones and zeros not begin until we get to the local exchange which would be in the network lab so we would actually have an analog signal going back to an interface card at the local exchange. But, the important part is minimum possible delay in jitter, but there is no error checking.  And for real-time applications are error checking and error correction don’t make any sense. You don’t have the opportunity to retransmit or correct the error. You just have to attempt to deal with it by hiding it or ignoring it. So, this is ideal for a lot of real-time transmission such as voice.

Telecommunications Essentials Pt 1

A Review Of Telecommunications Expert Witnesses

post by Grayserr G.

The breakthrough of the cellular phone age was one of the most advance evolutions in human history. Imagine being able to communicate with other people who can be located in other parts of the world without leaving the comforts of your own home, it brings convenience to everyone.

The cellular phone is one of the most important tools in everyone’s daily life. We use our cell phone primarily as a tool to communicate to our family, friends and acquaintances.

Years back, we use our cell phones to call, and then later on as the cell phone upgraded we started sending text messages and now we can send pictures and videos already using our mobile phones. The latest mobile phones are also capable of some superb capabilities, for instance, aside from calling and sending messages, nowadays we also use our cell phones to capture pictures and videos, as well as to record and play music. Due to the advancement in technology of most cell phones crimes can also be committed using nowadays using cell phones.

A telecommunication expert is a person who can help you when you are faced with legal cases that is related to the use of cell phone such as telecommunication engineering and telecommunication services. Telecommunication expert witness can testify regarding telecommunications networks, telecommunications facilities, and telecommunications equipment, as well as related issues.

Hence, how can the telecommunication expert witness use the cell phone as evidence in court? There are several materials that can be used in a case using the cell phone like the Logged Incoming & Last Dialled numbers, Text & Multimedia messages, System Settings (including date/time/volume), Stored audio/visual materials, saved computer and data files, Calendar and Alarm notifications and the Internet settings and websites accessed.

One of the most common question that is where will the evidence be found, is it in the phone itself or in the SIM card? Well the answer is both that is the reason why comprehensive analysis of both the cell phone and the SIM card must be done. Some of the network records like text messages and call logs as well as the phone directory can be found in the SIM card while important photos, videos or sound recordings can be found in the cell phone.

Another question that is commonly asked is can scandalous and explicit photos are kept and stored in the phone? Yes. That is the primarily due to the prevalence of cell phones with very good photo resolution these days. The photos that can be found in the handset can be used by the Telecommunication Expert in the case that will be filed. The cell phone unit can also be used by the Telecommunication Expert Witness using the DNA traces on the handset itself to link a user to the device.

Telecommunication Expert Witness can be found in all over the internet. There are numerous ways to find them. It will be better if you will be able to find someone who was recommended by a friend or a fellow professional.

A Review Of Telecommunications Expert Witnesses