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Black Box Explains...LAN switches.



Rush hour-all day, every day.

Applications such as document imaging, video/multimedia production, and intranetworking are very demanding. They generate huge data files that often must be transferred... more/see it nowbetween stations based on strict timing requirements. If such traffic is not transmitted efficiently, you end up with jerky video, on-screen graphics that take forever to load, or other irritating, debilitating problems.


These problems arise because in traditional LANs, only one network node transmits data at a time while all other stations listen. This works in conventional, server-based LANs where multiple workstations share files or applications housed on a central server. But if a network has several servers, or if it supports high-bandwidth, peer-to-peer applications such as videoconferencing, the one-station-at-a-time model just doesn’t work.


Ideally, each LAN workstation should be configured with its own dedicated LAN cable segment. But that’s neither practical nor affordable. A far more reasonable solution is a network designed to provide clear paths from each workstation to its destination on demand, whether that destination is another workstation or server.


These vehicles clear the lanes.

Unlike bridges and routers, which process data packets on an individual, first-come, first-served basis, switches maintain multiple, simultaneous data conversions among attached LAN segments.


From the perspective of an end-user workstation, a switched circuit appears to be a dedicated connection-a direct, full-speed LAN link to an attached server or other remote LAN node. Although this technique is somewhat different from what a LAN bridge or router does, switching hubs are based on similar technologies.




Which route will you choose?

Switching hubs that use bridging technologies are called Layer 2 switches-a reference to Layer 2 or the Data-Link Layer of the OSI Model. These switches operate using the MAC addresses in Layer 2 and are transparent to network protocols. Switches that use routing technologies are known as Layer 3 switches, referring to Layer 3—the Network Layer—of the OSI Model. These switches, like routers, represent the next higher level of intelligence in the hardware hierarchy. Rather than passing packets based on MAC addresses, these switches look into the data structure and route it based on the network addresses found in Layer 3. They are also dependent on the network protocol.


Layer 2 switches connect different parts of the same network as determined by the network number contained with the data packet. Layer 3 switches connect LANs or LAN segments with different network numbers.


If you’re subdividing an existing LAN, obviously you’re dealing with only one network and one network number, so you can install a Layer 2 switch wherever it will segment network traffic the best, and you don’t have to reconfigure the LAN. However, if you use a Layer 3 switch, you’ll have to reconfigure the segments to ensure that each has a different network number.


Similarly, if you’re connecting existing networks, you have to examine the currently configured network numbers before adding a switch. If the network numbers are the same, you need to use a Layer 2 switch. If they’re different, you must use a Layer 3 switch.


When dealing with multiple existing networks, you’ll find they usually use different network numbers. In this case, it’s preferable to use a Layer 3 switch (or possibly even a full-featured router) to avoid reconfiguring the network.


But what if you’re designing a network from scratch and can choose either type of switch? Your decision should be based on the expected complexity of your LAN. Layer 3 routing technology is well suited for complex networks. Layer 2 switches are recommended for smaller, less complex networks.

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Black Box Explains...Gigabit Ethernet.

As workstations and servers migrated from ordinary 10-Mbps Ethernet to 100-Mbps speeds, it became clear that even greater speeds were needed. Gigabit Ethernet was developed for an even faster Ethernet... more/see it nowstandard to handle the network traffic generated on the server and backbone level by Fast Ethernet. Gigabit Ethernet delivers an incredible 1000 Mbps (or 1 Gbps), 100 times faster than 10BASE-T. At that speed, Gigabit Ethernet can handle even the traffic generated by campus network backbones. Plus it provides a smooth upgrade path from 10-Mbps Ethernet and 100-Mbps Fast Ethernet at a reasonable cost.

Compatibility
Gigabit Ethernet is a true Ethernet standard. Because it uses the same frame formats and flow control as earlier Ethernet versions, networks readily recognize it, and it’s compatible with older Ethernet standards. Other high-speed technologies (ATM, for instance) present compatibility problems such as different frame formats or different hardware requirements.

The primary difference between Gigabit Ethernet and earlier implementations of Ethernet is that Gigabit Ethernet almost always runs in full-duplex mode, rather than the half-duplex mode commonly found in 10- and 100-Mbps Ethernet.

One significant feature of Gigabit Ethernet is the improvement to the Carrier Sense Multiple Access with Collision Detection (CSMA/CD) function. In half-duplex mode, all Ethernet speeds use the CSMA/CD access method to resolve contention for shared media. For Gigabit Ethernet, CSMA/CD has been enhanced to maintain the 200-meter (656.1-ft.) collision diameter.

Affordability and adaptability
You can incorporate Gigabit Ethernet into any standard Ethernet network at a reasonable cost without having to invest in additional training, cabling, management tools, or end stations. Because Gigabit Ethernet blends so well with your other Ethernet applications, you have the flexibility to give each Ethernet segment exactly as much speed as it needs—and if your needs change, Ethernet is easily adaptable to new network requirements.

Gigabit Ethernet is the ideal high-speed technology to use between 10-/100-Mbps Ethernet switches or for connection to high-speed servers with the assurance of total compatibility with your Ethernet network.

When Gigabit Ethernet first appeared, fiber was crucial to running Gigabit Ethernet effectively. Since then, the IEEE802.3ab standard for Gigabit over Category 5 cable has been approved, enabling short stretches of Gigabit speed over existing copper cable. Today, you have many choices when implementing Gigabit Ethernet:

1000BASE-X
1000BASE-X refers collectively to the IEEE802.3z standards: 1000BASE-SX, 1000BASE-LX, and 1000BASE-CX.

1000BASE-SX
The “S“ in 1000BASE-SX stands for “short.“ It uses short wavelength lasers, operating in the 770- to 860-nanometer range, to transmit data over multimode fiber. It’s less expensive than 1000BASE-LX, but has a much shorter range of 220 meters over typical 62.5-µm multimode cable.

1000BASE-LX
The “L“ stands for “long.“ It uses long wavelength lasers operating in the wavelength range of 1270 to 1355 nanometers to transmit data over single-mode fiber optic cable. 1000BASE-LX supports up to 550 meters over multimode fiber or up to 10 kilometers over single-mode fiber.

1000BASE-CX
The “C“ stands for “copper.“ It operates over special twinax cable at distances of up to 25 meters. This standard never really caught on.

Gigabit over CAT5—1000BASE-TX
The 802.3ab specification, or 1000BASE-TX, enables you to run IEEE-compliant Gigabit Ethernet over copper twisted-pair cable at distances of up to 100 meters of CAT5 or higher cable.

Gigabit Ethernet uses all four twisted pairs within the cable, unlike 10BASE-T and 100BASE-TX, which only use two of the four pairs. It works by transmitting 250 Mbps over each of the four pairs in 4-pair cable. collapse

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