Understanding Modern Digital Modulation Techniques

Hi Friends,

Today i will share my learning experience about QAM and BW. I used to think initially how BW increases in Tems link planner when QAM increase and quality decreases and interference increase. :).

QAM, Quadrature amplitude modulation is widely used in many digital data radio communications and data communications applications. A variety of forms of QAM are available and some of the more common forms include 16 QAM, 32 QAM, 64 QAM, 128 QAM, and 256 QAM.

The various flavours of QAM may be used when data-rates beyond those offered by 8-PSK are required by a radio communications system. This is because QAM achieves a greater distance between adjacent points in the I-Q plane by distributing the points more evenly. And in this way the points on the constellation are more distinct and data errors are reduced. While it is possible to transmit more bits per symbol, if the energy of the constellation is to remain the same, the points on the constellation must be closer together and the transmission becomes more susceptible to noise. This results in a higher bit error rate than for the lower order QAM variants. In this way there is a balance between obtaining the higher data rates and maintaining an acceptable bit error rate for any radio communications system.

QAM applications

QAM is in many radio communications and data delivery applications. However some specific variants of QAM are used in some specific applications and standards.
For domestic broadcast applications for example, 64 QAM and 256 QAM are often used in digital cable television and cable modem applications. In the UK, 16 QAM and 64 QAM are currently used for digital terrestrial television using DVB - Digital Video Broadcasting.
In addition to this, variants of QAM are also used for many wireless and cellular technology applications.  
                           

Constellation diagrams for QAM

The constellation diagrams show the different positions for the states within different forms of QAM, quadrature amplitude modulation. As the order of the modulation increases, so does the number of points on the QAM constellation diagram.
The diagrams below show constellation diagrams for a variety of formats of modulation:

QAM bits per symbol

The advantage of using QAM is that it is a higher order form of modulation and as a result it is able to carry more bits of information per symbol. By selecting a higher order format of QAM, the data rate of a link can be increased.
The table below gives a summary of the bit rates of different forms of QAM and PSK.
                     

QAM noise margin

While higher order modulation rates are able to offer much faster data rates and higher levels of spectral efficiency for the radio communications system, this comes at a price. The higher order modulation schemes are considerably less resilient to noise and interference.
As a result of this, many radio communications systems now use dynamic adaptive modulation techniques. They sense the channel conditions and adapt the modulation scheme to obtain the highest data rate for the given conditions. As signal to noise ratios decrease errors will increase along with re-sends of the data, thereby slowing throughput. By reverting to a lower order modulation scheme the link can be made more reliable with fewer data errors and re-sends.

L3VPN basics and Pros and cons in MPLS senerio.

Hi Friends,

Today i will try my best to clear some of your doubts about L3VPN.

What is L3VPN--  It is a VPN model, the service provider routers carry the customer data across the network, but they also participate in the customer routing. In other words, the service provider routers peer directly with the customer routers at Layer 3. The result is that one routing protocol neighborship or adjacency exists between the customer and the service provider router.

Senerio before MPLS -L3VPN model could be achieved by creating the IP routing peering between the customer and service provider routers. The VPN model also requires privateness or isolation between the different customers. You can achieve this by configuring packet filters (access lists) to control the data to and from the customer routers.
      Before MPLS came into being, the L2VPN  was deployed much more commonly than the L3VPN. The L3VPN model demanded a lot from provisioning because adding one customer site demanded many configuration changes at many sites.
 

Senerio after MPLS- The privateness in MPLS VPN networks is achieved by using the concept of virtual routing/ forwarding (VRF) and the fact that the data is forwarded in the backbone as labeled packets. The VRFs ensure that the routing information from the different customers is kept separate.

 Advantage of L3VPN model-benefit for the service provider is that it only needs to provision the link between the PE and CE routers. With the L2VPN model, the service provider needs to provision the links or virtual circuits between the sites.


Disadvantage of L3VPN model-
1. The customer must share the routing responsibility with the service provider.
2. The edge devices of the service provider have an added burden.
3.The service provider is responsible for the scalability and routing convergence of the customer networks because the PE routers must be able to carry all the routes of the many customers while providing timely routing convergence.

Br//

Rishabh Mittal.

 

what is L2VPN ??

Hi Friends,


Today i will try to clear some of your doubts about L2VPN .

L2VPN- In this the service provider supplies a service of point-to-point links or virtual circuits across his network between the routers of the customer. 
    These point-to-point services could be of Layer 1, 2, or even 3. Examples of Layer 1 are timedivisionmultiplexing (TDM), E1, E3, SONET, and SDH links. Examples of Layer 2 are virtual circuits created by X.25, ATM, or Frame Relay.

The L2VPN  service can also be provided over the IP Layer 3 protocol. Most commonly used tunnels to build the overlay network on IP are generic routing encapsulation (GRE) tunnels.

We have heard about number of services like Bank taking lease line , this is L2VPN service in which bank don't share its routes with service provider .

In next post we will try to share some points about L3VPN and pros and cons between them.

Br//

Rishabh Mittal.

Why we are going from ATM to IP?????

Hi Friends,

Today, data traffic is eclipsing voice traffic in volume, and demand by both business and consumers for a wide array of interactive services is growing.

That advantage derives from a fundamental difference between IP and ATM: IP technology can distinguish between various services and make per-hop routing, aggregation, traffic shaping and priority decisions, while ATM technology can only distinguish between virtual circuits .

ATM transports voice and data traffic in fixed-size, 53-byte cells. In the ATM world, all cells, and therefore all traffic, look the same, and an ATM-optimized DSLAM cannot discriminate between one kind of traffic and another. IP works with Internet packets, which are larger than ATM cells and distinguishable by type of service and destination. An IP-based concentrator can see these packets in their entirety, and discriminate between different kinds of traffic and between traffic from different subscribers.

Like, for example, a DSL service provider that signs up 100 subscribers for a full menu of services, including QoS VoDSL. To provision 10 voice calls for 10 subscribers with an ATM-optimized DSLAM, this provider must set up and manage 100 PVCs. In contrast, a DSL service provider that standardizes on DSLAMs with IP service intelligence can aggregate all voice subscribers into one low-delay path through the entire network. Where the ATM-centric provider must set up 100 PVCs to provision 10 calls for 10 subscribers, the IP-based provider can provision the same number of calls with only one network path.

Br.//

Rishabh Mittal.

Why TDM technology lost its value after the evolution of data traffic???

Hi Friends,

As we all know TDM was a great hit since the launch of mobile services in any country for any operator. Now a days as we are going towards data  traffic at good pace but TDM  technology has not been Optimized for efficient transmission of bursty traffic , i will try to explain why TDM not able to handle data traffic at such good rate.

PROBLEM--Due to Time division Multiplexing used in TDM it send 64K time slot every time whether data is there to send or not , it occupy full capacity assigned all the time and this capacity cant be assigned to other traffic or any other service. In case of bursty traffic this becomes a big pain as data may not be available all the time but we have to assign the BW , or for sudden data huge limitation and choking as BW is always constant.This is the main pain associated with the TDM technology for handling data and busty traffic.

However to handle the problem to some extend two technologies are developed which can provide some improved performance for TDM but not the sure shot solution.

1. Abis Optimization- Abis Optimization, which is a  solution for delivering Packet Abis over TDM transport networks, saves bandwidth by solely transferring bits that contain data. All other bits (for example, those that were
previously used to maintain a constant bit rate to fit the PDH channel format) are no longer inserted.

 2. Quadrupling the performance of EDGE -  Many operators who deploy EDGE have deemed it necessary to constrain throughput to match the limitations of available TDM transmission capacity. The classic TDM- based Abis interface has a fixed allocation of transmission timeslots to basic radio channels.
Therefore, the subordinate 16kbps timeslot on the Abis interface permanently allocated to a traffic channel (TCH) for voice service will never be available to carry EDGE data. With packet Abis the transport resources make up a common pool that is used by the traffic offered at each moment in time.

:)

Br//

Rishabh 

Main Difference between OTN and SDH (Sonet)

Hi Friends,

Today i will try to share the main difference between the OTN( Optical Transport network) and SDH .

From the starting phase of my career  i have seen SDH and SONET deployment in huge numbers in with lots of single parenting and double parenting Ethernet rings in operator networks.

The main difference between them apart from the OSI layer compatibilty  is in the framing. ( Frame we can assume a payload unit here ).

Usually we have three basis things in terms of the speed and channel capacity enhancement.

1. Frames/sec.

2 Frame period .

3. Frame size.

So capacity depend on as = No of frames/sec * Frame Size ( will give data transmitted).

In case of SDH we cant change the NO of frames /sec as this is TDM technology but we can change Frame size ( Increase frame size increase capacity).

But in case of OTN we cant change the Frame size but we can change the No of frames per second ( Increased capacity as no of frames in a sec increases).

Optical and Electrical Interface Pros and cons!!!!

Hi Friends,

Today i will try to share my experience and learning about optical and electrical interfaces. As in transport we have number of equipments from IP RAN to LRAN and from HRAN to Core which have optical and electrical interfaces. I will try to clear some of the basics doubts regarding the basis difference between them and channel capacity requirement and cost associated with each of them.

1 Basic difference-  As shared in last post when electrical pulses to represent binary 1s and 0s then the interface is electrical interface. Electrical signals need medium to travel like any other pulse . And optical interfaces use pulses of light to represnt the signals as one and zero.

                                       

2. Channel Capacity- As we all know from our engineering subject , when we have less attenuation then S/N ratio is much better , Optical always have good S/N ratio as compared with electrical medium so channel capacity is much larger in case of optical as compared to electrical.

     

3. Cost - Opticals interfaces are costly as compaired with electrical interfaces.

Generally we can say these are the basic differences between the optical and electrical interfaces in terms of very basic terms.

I will share more observation and finding from my experience in transport and main terminology from the RAN and IP networks part.

Br//

Rishabh  

What is an electrical Interface?

Hi,

We as transport engineers have this first things to deal with , optical electrical interfaces, I myself had number of doubts regarding the the actual difference between them.
After my work experience in RAN domain i tried to find the difference between them and there impact in terms of signal to noise ratio and cost requirements.

Electrical Interface- When electrical pulses are used to represent binary 1s and 0s then the interface is electrical interface. Electrical signals need medium to travel like any other pulse, but i case of electrcal from transport part we have medium generally as CAT6 , old thicknet/thinnet coax cables. These cables or mediium has high attenuation as compaired to fibre so channel capacity is gerenally less in terms of electrical signals.As ethernet isn't actually a cable as it's often referred to, it's a data link layer protocol, ethernet networks can be created over different physical mediums.