In case the medium is busy (another station in the network is currently transmitting information) then the station will delay its transmission requests to a later time. Otherwise, the medium is free and the station is allowed to transmit the desired information. The problem with the CSMA protocol is the case where a collision occurs. A collision may occur if two different stations sense the medium as free and begin to transmit information. For this purpose, we use a collision avoidance mechanism: a station willing to transmit senses the medium.
If the medium is busy, it defers its request. Otherwise, it sends an RTS (Request To Transmit) which includes the source, destination and the duration of the transaction. If the medium is free then the destination station will respond with a packet called CTS (Clear To Send) which will include the same duration information and once the source station receives this packet, it starts to transmit. The destination station checks the CRC (Cyclic Redundancy Check) and sends an acknowledgment package (ACK).
Receiving the ACK informs the source station that no collision had occurred. If the source station does not receive the ACK it will keep resending the data or will throw it away after a given number of retransmissions. When a station wishes to join an existing BSS, it needs to receive synchronization information from the BSSs AP. First, the station needs to go through the APs authentication process. During this process, the station and the AP exchange information proving to each other that each side is familiar with a specific password.
Only after the station is authenticated, the association process begins. During this process, the station receives information about the different stations in the BSS and of the BSS capabilities and the set of APs that belong to the BSS receive information about the current location of the station. Only after the association process is completed, the station is capable to transmit and receive information through the BSS. 2. 2. AVAYA access point 2. 2. 1 the access point properties
The access point is a wired connection to a wireless bridge that can be used to connect wireless cells to one another or to a wired LAN. The access point can serve mobile wireless stations roaming (moving) between various locations within a network. In order to initialize the AP, we need to set a unique IP address using a DHCP server (Dynamic Host Configuration Protocol). By doing this we reserve an IP address for each unit based on its MAC Address. Each time the unit reboots, the DHCP Server assigns the same IP address.
This method helps track network devices and facilitates the AP configuration. After setting the IP Address, we configure the AP unit using the HTTP Interface or the CLI (Command Line Interface a set of keyboard commands and parameters used for configuring and managing the AP). Wireless clients use Client Manager software for network access. Once connected, users may roam from one coverage cell to another while maintaining connection. To operate the AP we need at least one standard Avaya Wireless PC Card.
The standard Avaya Wireless PC Card is a wireless network card with integrated radio modules and antennas (2. 4 GHz). 2. 2. AVAYA access point 2. 2. 2 SNMP protocol The Simple Network Management Protocol is the standard operations and maintenance protocol for the Internet. It is a communication protocol designed to manage TCP/IP networks and devices. SNMP was developed by the IETF (Internet Engineering Task Force) and adopted as a TCP/IP-based management standard in 1989. SNMP refers to a set of standards for network management including a protocol, a database structure specification, and a set of data objects.
The SNMP architecture defines a Client/Server relationship. The server (Network Management Station) executes management applications or directives which manage, control and monitor network elements. Clients (Network elements) are devices such as hosts, gateways, terminal servers etc. which have management agents responsible for performing the network management functions requested by the Server. The SNMP is used to transfer management information between the network management stations and the agents in the network elements.
The SNMP supplies all management agent functions such as alterations (set functions) or inspections of variables (get functions). The database, controlled by the SNMP agent, is referred to as the MIB (Management Information Base). The MIB is a collection of objects which describe an SNMP manageable entity. It is not a database or a data store since it does not contain data nor does it retrieve data from a monitored product. When a network manager wants to learn about a specific node, whether it is hardware or software, he must have some way of determining what information is available to him, and what it means.
This is where the MIB comes in. It is a way of logically grouping data so that it is easily understood by all. The MIB holds definitions and descriptions of the components of a specific product. It also holds information about the data-objects which the network manager would be interested in. A network manager could look at the MIB and gain a basic understanding of the product and also determine what specific data-objects he would like to query on. 3. Microsoft . NET developing tool 3. 1 SQL structured query language
SQL allows users to access data in relational database management systems, such as Oracle, Sybase, Informix, Microsoft SQL Server, Access, and others, by allowing users to describe the data they wish to see. SQL also allows users to define the data in a database, and manipulate that data. This chapter will describe how to use SQL, and will give examples of usage. Basic use of the SELECT statement In a relational database, data is stored in tables. An example table would relate Social Security Number, Name, and Address.
A possible query for this database would be to see the address of each employee. Use the SELECT statement in the following manner to achieve this goal: SELECT FirstName, LastName, Address, City, State FROM EmployeeAddressTable; In the above statement we asked for the all of data in the EmployeeAddressTable, and specifically, we asked to view the columns called FirstName, LastName, Address, City, and State. The general form for a SELECT statement, retrieving all of the rows in the table is: SELECT ColumnName, ColumnName, ¦ FROM TableName;
To get all columns of a table without typing all column names, use: SELECT * FROM TableName; Conditional Selection If we wanted to see the EmployeeIdNum of those employees that are making $50,000 or more, we would use the following statement: SELECT EmployeeIdNum FROM EmployeeStatisticsTable WHERE Salary >= 50000; The same can be done for columns containing text: SELECT EmployeeIdNum FROM EmployeeStatisticsTable WHERE Position = Manager; As we can see, the general form for a Conditional SELECT statement is: SELECT ColumnName, ColumnName, ¦
FROM TableName WHERE condition; Creating New Tables CREATE TABLE Orders (OwnerId INTEGER NOT NULL, ItemDesired CHAR(40) NOT NULL); This statement gives the table name and tells the DBMS about each column in the table. Some common generic data types are: Char(x), Integer, Decimal(x, y), Date, Logical. Altering Tables Let us examine the case in which we would want to add a column to the Antiques table to allow the entry of the price of a given Item (Parentheses optional): ALTER TABLE Antiques ADD (Price DECIMAL(8,2) NULL); Adding Data.
In order to insert new rows into a table, we need to perform the following: INSERT INTO Antiques VALUES (21, 01, Ottoman, 200. 00); This inserts the data into the table, as a new row, column-by-column, in the pre-defined order. Deleting Data Now, we would like to delete this new row from the database: DELETE FROM Antiques WHERE Item = Ottoman; Updating Data In order to update a Price into a row whose product does not have a price attached to it yet we need to write the following statement: UPDATE Antiques SET Price = 500. 00 WHE.