What is a VLAN?
Posted by Joseph Brochin
by Roberto Bell
A Virtual Local Area Network (VLAN) is a group of hosts that feature a like set of requirements and can communicate as though they were all connected via the same wires. However, unlike a traditional LAN, a VLAN is not necessarily all in the same place, and the hosts involved in the network can be located practically anywhere. All VLANs have the exact same attributes as a traditional physical local area network, though they can be configured via software instead of having to actually relocate the hosts.
Many VLANs are designed as replacements to the services that are traditionally provided by routers on physical LANs. VLANs are able to tackle issues like network management, security, and scalability with ease.
VLANs are known as Layer 2, or Data Link layer, constructs which transfer data between network nodes. Because they are Layer 2 constructs, they can interface with Layer 3 IP subnets with a one to one relationship.
How are VLANs designed?
The IEEE assigned IEEE 802.1Q as the protocol by which to configure virtual LANs. IEEE 802.1Q provides for a header with a 2-byte protocol identifier and another 2-byte tag control information. These are known as TPID and TCI, respectively. The TPID is fixed at 0×8100 while the TCI contains information pertaining to the user priority, canonical format indicator, and VLAN identifier.
What are the two types of VLAN memberships?
VLAN membership can be assigned in one of two ways. Static or Dynamic.
A Static VLAN is also known as a port based VLAN. These types of VLANs are created by assigning specific ports to a VLAN. As new devices enter into the VLAN network, the device assumes a specific port. If end users change ports but require access to the same VLAN, network administrators are required to assign new port to VLAN assignments for the new port.
A Dynamic VLAN is designed to be easily configured using software. The most common software for designing dynamic VLANs is CiscoWorks, a program developed by Cisco. A VLAN Management Policy Server (VMPS) must be set up first for a dynamic VLAN. This server will allow the VLAN to automatically switch ports whenever new devices connect to the VLAN. It uses information such as the device’s ports, unique usernames, and MAC addresses to determine whether or not to change ports to fit the needs of the incoming device.
Tom Paine writes for The Tech FAQ and is the author of articles such as VLAN, VLAN Hopping, and VLAN.
Article Source: Rhino Articles
Introduction to New Type of Memory Cards - SDHC - Secure Digital High-Capacity
Posted by Joseph Brochin
SDHC (Secure Digital High-Capacity) are new Flash memory cards based on the SDA (Secure Digital Association) 2.00 specification. SDHC provides removable memory storage for compatible digital devices, including cameras, camcorders, PDAs, MP3 players and more.SDHC cards guarantee a minimum sustained data transfer speed, not a maximum speed. There are three available speeds - Class 2, Class 4 and Class 6.There is difference of Data Transfer Rate (DTR) between these classes.
Class 2 devices guaranty a minimum DTR of 2 MB/sec
Class 4 devices guaranty a minimum DTR of 4 MB/secClass 6 devices guaranty a minimum DTR of 6 MB/sec
The device class is specified on the device itself.
The previous versions of SDA specifications did not provide such common standards for sustained data transfer speeds to be shared by all manufacturers. The new speed classification allows users to select card performance based on the needs of particular applications, such as HD video, and other MPEG applications, rather than just a maximum speed rating.
SDHC Cards comes in different capacities ranging from 4 GB-32 GB. Please see the highest capacity supported by your device. SDHC logo on your device does not neccesarily means that your device will support all the capacities that are provided under SDHC cards. There is probabilty that your camera may support only 4 GB of capacity which is also SDHC. Please contact your device manufacturer or refer to your manual for further clarifications.
SDHC cards has the same physical size as SD cards but with higher capacities which cannot be used with SD host devices that do not comply with SDA Specifications Version 2.00.It makes sense,since the same size card with increased capacity would have a different circuit design all together. Also, there is a Sandisk Card Reader that can start supporting SDHC cards after a small firmware upgrade. But only few device versions of that particular model can be upgraded to support SDHC Cards.Host devices complying with SDA Version 2.00 can use both SD Memory Cards and the new SDHC Memory Cards.
Current SD cards complying with Version 1.01 and Version 1.10 use either the FAT16 file system, whereas Version 2.00 uses FAT32. This latest specification was established to support SDHC card applications with higher capacity needs.Remember, only if the approved SDHC logo appears on the card or package, the product is compliant with SDA 2.00 specifications.
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