Saturday, December 31, 2011

How To Make Comment in SQL

In SQL (Structured Query Language) we can write comment by using double dash (--) before the text

--this is the comment

Sunday, December 25, 2011


ADSL Equipment

Two groups of equipment
  •  Central Office - DSL Access Multiplexer (DSLAM), Repeaters
  • Customer Premise - DSL Modems, Gateways, Network Interface Card (NIC), splitters and filter


  • DSLAM is usually found in a Central Office
  • xDSL line cards are installed in a DSLAM to terminate incoming xDSL signals
  • The DSLAM then combines multiple xDSL access lines into one high speed line
  • The muxed traffic is converted into ATM cells which gets sent over an ATM backbone

 Generic DSL Line Card for DSLAM Applications

 DSL Modem/Gateway

  • A xDSL modem is the device found at the customer’s premise which is used to transmit & receive xDSL signals
  •  Could be an external “box” or a network interface card placed inside a computer
  •  An xDSL Gateway combines the functionality of a modem and router 

 DSL Modem Residential Gateway

Friday, December 9, 2011



CSMA /CD Carrier sense multiple access/collision detection (CSMA /CD) is one of the most popular access methods in use today. With CSMA /CD, every host has equal access to the wire and can place data on the wire when the wire is free from traffic. If a host wishes to place data on the wire, it will “sense” the wire and determine whether there is a signal already on the wire. If there is, the host will wait to transmit the data; if the wire is free, the host will send the data, as shown in Figure .

The problem with the process just described is that, if there are two systems on the wire that “sense” the wire at the same time to see if the wire is free, they will both send data out at the same time if the wire is free. When the two pieces of data are sent out on the wire at the same time, they will collide with one another, and the data will be destroyed. If the data is destroyed in transit, the data will need to be retransmitted. Consequently, after a collision, each host will wait a variable length of time before retransmitting the data (they don’t want the data to collide again) thereby preventing a collision the second time. When a system determines that the data has collided and then retransmits the data, it is known as collision detection. 

To summarize, CSMA /CD provides that before a host sends data on the network it will “sense” (CS) the wire to ensure that the wire is free of traffic. Multiple systems have equal access to the wire (MA), and if there is a collision, a host will detect that collision (CD) and retransmit the data


Carrier sense multiple access/collision avoidance (CSMA /CA) is not as popular as CSMA /CD and for good reason. With CSMA /CA, before a host sends data on the wire it will “sense” the wire as well to see if the wire is free of signals. If the wire is free, it will try to “avoid” a collision by sending a piece of “dummy” data on the wire first to see whether it collides with any other data. If it does not collide, the host in effect assumes “If my dummy data did not collide, then the real data will not collide,” and it submits the real data on the wire. 

Token Passing

With both CSMA /CD and CSMA /CA, the possibility of collisions is always there, and the more hosts that are placed on the wire, the greater the chances of collisions, because you have more systems “waiting”’ for the wire to become free so that they can send their data.
Token passing takes a totally different approach to deciding on how a system can place data on the wire. With token passing, there is an empty packet running around on the wire — the “token.” In order to place data on the wire, you need to wait for the token; once you have the token and it is free of data, you can place your data on the wire. Since there is only one token and a host needs to have the token to “talk,” it is impossible to have collisions in a token-passing environment. 

For example, if Workstation 1 wants to send data on the wire, the workstation would wait for the token, which is circling the network millions of times per second. Once the token has reached Workstation 1, the workstation would take the token off the network, fill it with data, mark the token as being used so that no other systems try to fill the token with data, and then place the token back on the wire heading for the destination host.

Saturday, December 3, 2011


Network architecture is something that came about one day when someone sat down and said “We are going to design a network architecture; let’s use CAT 3 cabling, a star topology, and CSMA /CD as an access method. Let’s call this architecture 10BaseT” In this example, 10BaseT was the name assigned to the architecture because 10 Mbps is the transfer rate of the network, Baseband communication is the technique used to transmit the signal, and the T means our cable type —in this case twisted pair. We know different types of twisted-pair cabling, but CAT 3 is the one that runs at 10 Mbps, so it is the cable used in 10BaseT.
The first types of network architecture to look at are the different Ethernet architectures. When designing networks, one of the first decisions we usually make is “Do we want to use Ethernet or the competing network architecture called Token Ring? we want to use Ethernet. What flavor of Ethernet?” In this discussion you will understand what the different flavors of Ethernet are. Ethernet is defined as the IEEE 802.3 standard.

The 10Base2 Ethernet architecture is a network that runs at 10 Mbps and uses baseband transmissions. 10Base2 typically is implemented as a bus topology, but it could be a mix of a bus and a star topology. The cable type that we use is determined by the character at the end of the name of the architecture —in this case a 2. The 2 implies 200 meters. Now, what type of cable is limited to approximately 200m?  Thinnet is limited to approximately 200m (185m to be exact). The only characteristic we have not mentioned is the access method that is used.
All Ethernet environments use CSMA /CD as a way to put data on the wire.
The following list summarizes features of 10Base2:
Ø Baseband communication
Ø 10 Mbps transfer rate
Ø Maximum distance of 185 meters per network segment
Ø 30 hosts per segment
Ø 0.5 meters minimum distance between hosts

The 10Base5 Ethernet architecture runs at 10 Mbps and uses baseband transmission as well. It was also implemented as a bus topology. The cable it uses is limited to approximately 500 meters, which is thicknet, and it uses CSMA /CD as the access method. The thicker copper core in the wire allows the signal to travel further than is possible with thinnet.
The following list summarizes features of 10Base5:
Ø Baseband communication
Ø 10 Mbps transfer rate
Ø Maximum distance of 500 meters per network segment
Ø 100 hosts per segment
Ø 2.5 meter minimum distance between hosts

The 10BaseT Ethernet architecture runs at 10 Mbps and uses baseband transmission. It uses a star topology with a hub or switch at the center, allowing all systems to connect to one another. The cable it uses is CAT 3 UTP, which is the UTP cable type that runs at 10 Mbps. Keep in mind that most cable types are backward compatible, so you could have CAT 5 UTP cabling in a 10BaseT environment. But because the network cards and hubs are running at 10 Mbps, that is the maximum transfer speed you will get, even though the cable supports more.
Like all Ethernet environments, 10BaseT uses CSMA /CD as the access method.
The 10BaseFL Ethernet architecture allows for a 10-Mbps Ethernet environment that runs on fiber-optic cabling. The purpose of the fiber-optic cabling is to use it as a backbone to allow the network to reach greater distances.

Fast Ethernet (100BaseTX and 100BaseFX)

These two standards are part of the 100BaseX family, which is known as fast Ethernet. The different fast Ethernet flavors run at 100 Mbps, use a star topology, use CSMA / CD as an access method, and differ in the type of cabling used. 100BaseTX uses two pairs (four wires) in the CAT 5 cabling, whereas 100BaseFX uses two strands of fiber instead of twisted-pair cabling.

Gigabit Ethernet

Gigabit Ethernet is becoming the de facto of network architectures today. With Gigabit Ethernet we can reach transfer rates of 1000 Mbps (1 Gbps), using traditional media such as coaxial, twisted-pair, and fiber-optic cabling. There are two standards for Gigabit: IEEE 802.3z and IEEE 802.3ab.

IEEE 802.3z

The IEEE 802.3z standard defines Gigabit Ethernet that runs over fiber-optic cabling or coaxial cabling. There are three types of Gigabit Ethernet that fall under this standard:

  1000Base-SX is the Gigabit Ethernet architecture that runs at 1000 Mbps   over multimode fiber (MMF) optic cabling. This architecture is designed for   short distances of up to 550 meters.

  1000Base-LX is the Gigabit Ethernet architecture that runs at 1000 Mbps   over single-mode fiber (SMF) optic cabling. This architecture supports   distances   up to 3 kilometers. 
  1000Base-CX is the Gigabit Ethernet architecture that runs at 1000   Mbps over coaxial cable and supports distances of up to 25 meters.

IEEE 802.3ab

The IEEE 802.3ab standard, known as 1000BaseTX, defines Gigabit Ethernet that runs over twisted-pair cabling and uses characteristics of 100BaseTX networking, including the use of RJ-45 connectors and the access method of CSMA /CD. Like 100BaseTX, 1000BaseTX uses CAT 5, CAT 5e, and even CAT 6 unshielded twisted-pair, the difference being that 100BaseTX runs over two pairs (four wires) while 1000BaseTX runs over four pairs (all eight wires).

10-Gigabit Ethernet:

There are standards for 10-Gigabit Ethernet (10,000 Mbps) that have been developed that use fiber-optic cabling:

  10GbaseSR runs at 10 Gbps and uses “short-range” multimode fiber-optic   cable, which has a maximum distance of 100 meters.

  10GBaseLR runs at 10 Gbps and uses “long-range” single-mode ?ber-optic   cable, which has a maximum distance of 10 kilometers.

  10GbaseER runs at 10 Gbps and uses “extra-long-range” single-mode ?ber-  optic cable which has a maximum distance of 40 kilometers.