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Black Box Explains...Virtual LANs (VLANs).

True to their name, VLANs are literally “virtual“ LANs—mini subLANs that, once configured, can exist and function logically as single, secure network segments, even though they may be part of... more/see it nowa much larger physical LAN.

VLAN technology is ideal for enterprises with far-reaching networks. Instead of having to make expensive, time-consuming service calls, system administrators can configure or reconfigure workstations easily or set up secure network segments using simple point-and-click, drag-and-drop management utilities. VLANs provide a way to define dynamic new LAN pathways and create innovative virtual network segments that can range far beyond the traditional limits of geographically isolated workstation groups radiating from centralized hubs.

For instance, using VLAN switches, you can establish a secure VLAN made up of select devices located throughout your enterprise (managers’ workstations, for example) or any other device that you decide requires full access to the VLAN you’ve created.

According to Cisco, a VLAN is a switched network logically segmented by functions, project teams, or applications regardless of the physical location of users. You can assign each switch port to a different VLAN. Ports configured in the same VLAN share broadcasts; ports that don’t belong to the VLAN don’t share the data.

VLAN switches group users and ports logically across the enterprise—they don’t impose physical constraints like in a shared-hub architecture. In replacing shared hubs, VLAN switches remove the physical barriers imposed by each wiring closet.

To learn more about smart networking with VLANs, call the experts in our Local Area Network Support group at 724-746-5500, press 1, 2, 4. collapse


Black Box Explains…Energy-Efficient Ethernet.

The IEEE 802.3az Ethernet standard, ratified in 2010, provides a standardized way for some Ethernet devices to reduce power consumption. Energy-Efficient Ethernet devices have a low-power idle (LPI) mode that... more/see it nowcan cut power use by 50% or more during periods of low data activity. Because energy-efficient Ethernet devices scale down power consumption when the load is lower, they save both the energy used to power processors and the energy used to cool them.

These energy savings are currently available for 100BASE-TX, 1000BASE-T, and 10GBASE-T Ethernet as well as some backplane Ethernet. 802.3az can be found on most types of network equipment, including NICs, switches, routers, and media converters. Because these devices are totally backwards compatible with other Ethernet devices, all you need to do to reap energy savings is to swap out devices. collapse


Black Box Explains...Layer 3 switching.

In the last decade, network topologies have typically featured routers along with hubs or switches. The hub or switch acts as a central wiring point for LAN segments while the... more/see it nowrouter takes care of higher-level functions such as protocol translation, traffic between LAN segments, and wide-area access.

Layer 3 switching, which combines Layer 2 switching and Layer 3 IP routing, provides a more cost-effective way of setting up LANs by incorporating switching and routing into one device. While a traditional Layer 2 switch simply sends data along without examining it, a Layer 3 switch incorporates some features of a router in that it examines data packets before sending them on their way. The integration of switching and routing in a Layer 3 switch takes advantage of the speed of a switch and the intelligence of a router in one economical package.

There are two basic types of Layer 3 switching: packet-by-packet Layer 3 (PPL3) and cut-through Layer 3.

PPL3 switches are technically routers in that they examine all packets before forwarding them to their destinations. They achieve top speed by running protocols such as OSPF (Open Shortest Path First) and by using cache routing tables. Because these switches understand and take advantage of network topology, they can blow the doors off traditional routers with speeds of more than 7,000,000 (that’s seven million!) packets per second.

Cut-through Layer 3 switching relies on a shortcut for top speed. Cut-through Layer 3 switches, rather than examining every packet, examine only the first in a series to determine its destination. Once the destination is known, the data flow is switched at Layer 2 to achieve high speeds. collapse


Black Box Explains...Power over Ethernet (PoE).

What is PoE?
The seemingly universal network connection, twisted-pair Ethernet cable, has another role to play, providing electrical power to low-wattage electrical devices. Power over Ethernet (PoE) was ratified by the... more/see it nowInstitute of Electrical and Electronic Engineers (IEEE) in June 2000 as the 802.3af-2003 standard. It defines the specifications for low-level power delivery—roughly 13 watts at 48 VDC—over twisted-pair Ethernet cable to PoE-enabled devices such as IP telephones, wireless access points, Web cameras, and audio speakers.

Recently, the basic 802.3af standard was joined by the IEEE 802.3at PoE standard (also called PoE+ or PoE plus), ratified on September 11, 2009, which supplies up to 25 watts to larger, more power-hungry devices. 802.3at is backwards compatible with 802.3af.

How does PoE work?
The way it works is simple. Ethernet cable that meets CAT5 (or better) standards consists of four twisted pairs of cable, and PoE sends power over these pairs to PoE-enabled devices. In one method, two wire pairs are used to transmit data, and the remaining two pairs are used for power. In the other method, power and data are sent over the same pair.

When the same pair is used for both power and data, the power and data transmissions don’t interfere with each other. Because electricity and data function at opposite ends of the frequency spectrum, they can travel over the same cable. Electricity has a low frequency of 60 Hz or less, and data transmissions have frequencies that can range from 10 million to 100 million Hz.

Basic structure.
There are two types of devices involved in PoE configurations: Power Sourcing Equipment (PSE) and Powered Devices (PD).

PSEs, which include end-span and mid-span devices, provide power to PDs over the Ethernet cable. An end-span device is often a PoE-enabled network switch that’s designed to supply power directly to the cable from each port. The setup would look something like this:

End-span device → Ethernet with power

A mid-span device is inserted between a non-PoE device and the network, and it supplies power from that juncture. Here is a rough schematic of that setup:

Non-PoE switch → Ethernet without PoE → Mid-span device → Ethernet with power

Power injectors, a third type of PSE, supply power to a specific point on the network while the other network segments remain without power.

PDs are pieces of equipment like surveillance cameras, sensors, wireless access points, and any other devices that operate on PoE.

PoE applications and benefits.

  • Use one set of twisted-pair wires for both data and low-wattage appliances.
  • In addition to the applications noted above, PoE also works well for video surveillance, building management, retail video kiosks, smart signs, vending machines, and retail point-of-information systems.
  • Save money by eliminating the need to run electrical wiring.
  • Easily move an appliance with minimal disruption.
  • If your LAN is protected from power failure by a UPS, the PoE devices connected to your LAN are also protected from power failure.

  • Converters and Scalers Selector
    PoE Standards PoE
    IEEE 802.3 af
    PoE IEEE 802.3 at
    Power available at powered device 12.95 W 25.5
    Maximum power delivered 15.40 W 34.20
    Voltage range at powred source 44.0-57.0 V 50.0-57.0 V
    Voltage range at powred device 37.0-57.0 42.5-57.0 V
    Maximum current 350 mA 600 mA
    Maximum cable resistance 20 ohms 12.5 ohms
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    Black Box Explains…SFP compatibility.

    Standards for SFP fiber optic media are published in the SFP Multi-Source Agreement, which specifies size, connectors, and signaling for SFPs, with the idea that all SFPs are compatible with... more/see it nowdevices that have appropriate SFP slots. These standards, which also extend to SFP+ and XFP transceivers, enable users to mix and match components from different vendors to meet their own particular requirements.

    However, some major manufacturers, notably Cisco®, HP®, and 3Com®, sell network devices with SFP slots that lock out transceivers from other vendors. Because the price of SFPs—especially Gigabit SFPs and 10GBASE SFP+ and XFP transceivers—can add significantly to the price of a switch, this lock-out scheme raises hardware costs and limits transceiver choices.

    Many vendors don’t advertise that SFP slots on their devices don’t accept standard SFPs from other vendors. This can lead to unpleasant surprises when a device simply refuses to communicate with an SFP.

    Another game that some vendors play is to build devices that accept open-standard SFPs, but refuse to support those devices when SFPs from another vendor are used with them.

    The only way around this “lock-in” practice is to only buy network devices that accept standard SFPs from all vendors and to buy from vendors that support their devices no matter whose SFPs are used with them. Questions? Call our FREE Tech Support at 724-746-5500. collapse


    Black Box Explains...Ethernet.



    If you have an existing network, there’s a 90% chance it’s Ethernet. If you’re installing a new network, there’s a 98% chance it’s Ethernet—the Ethernet standard is... more/see it nowthe overwhelming favorite network standard today.


    Ethernet was developed by Xerox®, DEC®, and Intel® in the mid-1970s as a 10-Mbps (Megabits per second) networking protocol—very fast for its day—operating over a heavy coax cable (Standard Ethernet).


    Today, although many networks have migrated to Fast Ethernet (100 Mbps) or even Gigabit Ethernet (1000 Mbps), 10-Mbps Ethernet is still in widespread use and forms the basis of most networks.


    Ethernet is defined by international standards, specifically IEEE 802.3. It enables the connection of up to 1024 nodes over coax, twisted-pair, or fiber optic cable. Most new installations today use economical, lightweight cables such as Category 5 unshielded twisted-pair cable and fiber optic cable.


    How Ethernet Works

    Ethernet signals are transmitted from a station serially, one bit at a time, to every other station on the network.


    Ethernet uses a broadcast access method called Carrier Sense Multiple Access/Collision Detection (CSMA/CD) in which every computer on the network “hears” every transmission, but each computer “listens” only to transmissions intended for it.


    Each computer can send a message anytime it likes without having to wait for network permission. The signal it sends travels to every computer on the network. Every computer hears the message, but only the computer for which the message is intended recognizes it. This computer recognizes the message because the message contains its address. The message also contains the address of the sending computer so the message can be acknowledged.


    If two computers send messages at the same moment, a “collision” occurs, interfering with the signals. A computer can tell if a collision has occurred when it doesn’t hear its own message within a given amount of time. When a collision occurs, each of the colliding computers waits a random amount of time before resending the message.


    The process of collision detection and retransmission is handled by the Ethernet adapter itself and doesn’t involve the computer. The process of collision resolution takes only a fraction of a second under most circumstances. Collisions are normal and expected events on an Ethernet network. As more computers are added to the network and the traffic level increases, more collisions occur as part of normal operation. However, if the network gets too crowded, collisions increase to the point where they slow down the network considerably.


    Standard (Thick) Ethernet (10BASE5)


    • Uses “thick” coax cable with N-type connectors for a backbone and a transceiver cable with 9-pin connectors from the transceiver to the NIC.
    • Both ends of each segment should be terminated with a 50-ohm resistor.
    • Maximum segment length is 500 meters.
    • Maximum total length is 2500 meters.
    • Maximum length of transceiver cable is 50 meters.
    • Minimum distance between transceivers is 2.5 meters.
    • No more than 100 transceiver connections per segment are allowed.
    Thin Ethernet (ThinNet) (10BASE2)


    • Uses "Thin" coax cable.
    • The maximum length of one segment is 185 meters.
    • The maximum number of segments is five.
    • The maximum total length of all segments is 925 meters.
    • The minimum distance between T-connectors is 0.5 meters.
    • No more than 30 connections per segment are allowed.
    • T-connectors must be plugged directly into each device.
    Twisted-Pair Ethernet (10BASE-T)


    • Uses 22 to 26 AWG unshielded twisted-pair cable (for best results, use Category 4 or 5 unshielded twisted pair).
    • The maximum length of one segment is 100 meters.
    • Devices are connected to a 10BASE-T hub in a star configuration.
    • Devices with standard AUI connectors may be attached via a 10BASE-T transceiver.
    Fiber Optic Ethernet (10BASE-FL, FOIRL)


    • Uses 50-, 62.5-, or 100-micron duplex multimode fiber optic cable (62.5 micron is recommended).
    • The maximum length of one 10BASE-FL (the new standard for fiber optic connections) segment is 2 kilometers.
    • The maximum length of one FOIRL (the standard that preceded the new 10BASE-FL) segment is 1 kilometer.
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    Black Box Explains...Fiber.


    Fiber versus copper.

    When planning a new or upgraded cabling infrastructure, you have two basic choices: fiber or copper. Both offer superior data transmission. The decision on which one... more/see it nowto use may be difficult. It will often depend on your current network, your future networking needs, and your particular application, including bandwidth, distances, environment, cost, and more. In some cases, copper may be a better choice; in other situations, fiber offers advantages.


    Although copper cable is currently more popular and much more predominant in structured cabling systems and networks, fiber is quickly gaining fans.


    Fiber optic cable is becoming one of the fastest-growing transmission mediums for both new cabling installations and upgrades, including backbone, horizontal, and even desktop applications. Fiber optic cable is favored for applications that need high bandwidth, long distances, and complete immunity to electrical interference. It’s ideal for high data-rate systems such as Gigabit Ethernet, FDDI, multimedia, ATM, SONET, Fibre Channel, or any other network that requires the transfer of large, bandwidth-consuming data files, particularly over long distances. A common application for fiber optic cable is as a network backbone, where huge amounts of data are transmitted. To help you decide if fiber is right for your new network or if you want to migrate to fiber, take a look at the following:



    The advantages of fiber.

    Greater bandwidth-Because fiber provides far greater bandwidth than copper and has proven performance at rates up to 10 Gbps, it gives network designers future-proofing capabilities as network speeds and requirements increase. Also, fiber optic cable can carry more information with greater fidelity than copper wire. That’s why the telephone networks use fiber, and many CATV companies are converting to fiber.


    Low attenuation and greater distance-Because the fiber optic signal is made of light, very little signal loss occurs during transmission so data can move at higher speeds and greater distances. Fiber does not have the 100-meter (304.8-ft.) distance limitation of unshielded twisted-pair copper (without a booster). Fiber distances can range from 300 meters to 40 kilometers, depending on the style of cable, wavelength, and network. (Fiber distances are typically measured in metric units.) Because fiber signals need less boosting than copper ones do, the cable performs better.


    Fiber networks also enable you to put all your electronics and hardware in one central location, instead of having wiring closets with equipment throughout the building.


    Security-Your data is safe with fiber cable. It does not radiate signals and is extremely difficult to tap. If the cable is tapped, it’s very easy to monitor because the cable leaks light, causing the entire system to fail. If an attempt is made to break the security of your fiber system, you’ll know it.


    Immunity and reliability-Fiber provides extremely reliable data transmission. It’s completely immune to many environmental factors that affect copper cable. The fiber is made of glass, which is an insulator, so no electric current can flow through. It is immune to electromagnetic interference and radio-frequency interference (EMI/RFI), crosstalk, impedance problems, and more. You can run fiber cable next to industrial equipment without worry. Fiber is also less susceptible to temperature fluctuations than copper is and can be submerged in water.


    Design-Fiber is lightweight, thin, and more durable than copper cable. And, contrary to what you might think, fiber optic cable has pulling specifications that are up to ten times greater than copper cable’s. Its small size makes it easier to handle, and it takes up much less space in cabling ducts. Although fiber is still more difficult to terminate than copper is, advancements in connectors are making temination easier. In addition, fiber is actually easier to test than copper cable.


    Migration-The proliferation and lower costs of media converters are making copper to fiber migration much easier. The converters provide seamless links and enable the use of existing hardware. Fiber can be incorporated into networks in planned upgrades.


    Standards-New TIA/EIA standards are bringing fiber closer to the desktop. TIA/EIA-785, ratified in 2001, provides a cost-effective migration path from 10-Mbps Ethernet to 100-Mbps Fast Ethernet over fiber (100BASE-SX). A recent addendum to the standard eliminates limitations in transceiver designs. In addition, in June 2002, the IEEE approved a 10-Gigabit Ethernet standard.


    Costs-The cost for fiber cable, components, and hardware is steadily decreasing. Installation costs for fiber are higher than copper because of the skill needed for terminations. Overall, fiber is more expensive than copper in the short run, but it may actually be less expensive in the long run. Fiber typically costs less to maintain, has much less downtime, and requires less networking hardware. And fiber eliminates the need to recable for higher network performance.


    Multimode or single-mode, duplex or simplex?

    Multimode-Multimode fiber optic cable can be used for most general fiber applications. Use multimode fiber for bringing fiber to the desktop, for adding segments to your existing network, or in smaller applications such as alarm systems. Multimode cable comes with two different core sizes: 50 micron or 62.5 micron.


    Single-mode-Single-mode is used over distances longer than a few miles. Telcos use it for connections between switching offices. Single-mode cable features an 8.5-micron glass core.


    Duplex-Use duplex multimode or single-mode fiber optic cable for applications that require simultaneous, bidirectional data transfer. Workstations, fiber switches and servers, fiber modems, and similar hardware require duplex cable. Duplex is available in single- and multimode.


    Simplex-Because simplex fiber optic cable consists of only one fiber link, you should use it for applications that only require one-way data transfer. For instance, an interstate trucking scale that sends the weight of the truck to a monitoring station or an oil line monitor that sends data about oil flow to a central location. Simplex fiber comes in single- and multimode types.


    50- vs. 62.5-micron cable.

    Although 50-micron fiber cable features a smaller core, which is the light-carrying portion of the fiber, both 62.5- and 50-micron cable feature the same glass cladding diameter of 125 microns. You can use both in the same types of networks, although 50-micron cable is recommended for premise applications: backbone, horizontal, and intrabuilding connections, and should be considered especially for any new construction and installations. And both can use either LED or laser light sources.


    The big difference between 50-micron and 62.5-micron cable is in bandwidth-50-micron cable features three times the bandwidth of standard 62.5-micron cable, particularly at 850 nm. The 850-nm wavelength is becoming more important as lasers are being used more frequently as a light source.


    Other differences are distance and speed. 50-micron cable provides longer link lengths and/or higher speeds in the 850-nm wavelength. See the table below.




    The ferrules: ceramic or composite?

    As a general rule, use ceramic ferrules for critical network connections such as backbone cables or for connections that will be changed frequently, like those in wiring closets. Ceramic ferrules are more precisely molded and fit closer to the fiber, which gives the fiber optic cables a lower optical loss.


    Use composite ferrules for connections that are less critical to the network’s overall operation and less frequently changed. Like their ceramic counterparts, composite ferrules are characterized by low loss, good quality, and a long life. However, they are not as precisely molded and slightly easier to damage, so they aren’t as well-suited for critical connections.


    Testing and certifying fiber optic cable.

    If you’re accustomed to certifying copper cable, you’ll be pleasantly surprised at how easy it is to certify fiber optic cable because it’s immune to electrical interference. You only need to check a few measurements.

    Attenuation (or decibel loss)-Measured in decibels/kilometer (dB/km), this is the decrease of signal strength as it travels through the fiber cable. Generally, attenuation problems are more common on multimode fiber optic cables.

    Return loss-This is the amount of light reflected from the far end of the cable back to the source. The lower the number, the better. For example, a reading of -60 decibels is better than -20 decibels. Like attenuation, return loss is usually greater with multimode cable.

    Graded refractive index-This measures how the light is sent down the fiber. This is commonly measured at wavelengths of 850 and 1300 nanometers. Compared to other operating frequencies, these two ranges yield the lowest intrinsic power loss. (NOTE: This is valid for multimode fiber only.)

    Propagation delay-This is the time it takes a signal to travel from one point to another over a transmission channel.

    Optical time-domain reflectometry (OTDR)-This enables you to isolate cable faults by transmitting high-frequency pulses onto a cable and examining their reflections along the cable. With OTDR, you can also determine the length of a fiber optic cable because the OTDR value includes the distance the optic signal travels.


    There are many fiber optic testers on the market today. Basic fiber optic testers function by shining a light down one end of the cable. At the other end, there’s a receiver calibrated to the strength of the light source. With this test, you can measure how much light is going to the other end of the cable. Generally, these testers give you the results in dB lost, which you then compare to the loss budget. If the measured loss is less than the number calculated by your loss budget, your installation is good.


    Newer fiber optic testers have a broad range of capabilities. They can test both 850- and 1300-nanometer signals at the same time and can even check your cable for compliance with specific standards.


    Fiber precautions.

    A few properties particular to fiber optic cable can cause problems if you aren’t careful during installation.

    Intrinsic power loss-As the optic signal travels through the fiber core, the signal inevitably loses some speed through absorption, reflection, and scattering. This problem is easy to manage by making sure your splices are good and your connections are clean.

    Microbending-Microbends are minute deviations in fiber caused by excessive bends, pinches, and kinks. Using cable with reinforcing fibers and other special manufacturing techniques minimizes this problem.

    Connector loss-Connector loss occurs when two fiber segments are misaligned. This problem is commonly caused by poor splicing. Scratches and dirt introduced during the splicing process can also cause connector loss.

    Coupling loss-Similar to connector loss, coupling loss results in reduced signal power and is from poorly terminated connector couplings.


    Remember to be careful and use common sense when installing fiber cable. Use clean components. Keep dirt and dust to a minimum. Don’t pull the cable excessively or bend it too sharply around any corners. That way, your fiber optic installation can serve you well for many years.

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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...Layer 2, 3, and 4 switches.



    ...more/see it now
    OSI Layer Physical
    Component
    7-Application Applicaton Software

    LAN-Compatible Software
    E-Mail, Diagnostics, Word Processing, Database


    Network Applications
    6-Presentation Data-
    Conversion Utilities
    Vendor-Specific Network Shells and Gateway™ Workstation Software
    5-Session Network Operating System SPX NetBIOS DECnet™ TCP/IP AppleTalk®
    4-Transport Novell® NetWare® IPX™ PC LAN LAN Mgr DECnet PC/TCP® VINES™ NFS TOPS® Apple
    Share®
    3-Network Control
    2-Data Link Network E A TR P TR E TR E E E P E P
    1-Physical E=Ethernet; TR=Token Ring; A=ARCNET®; P=PhoneNET®

    With the rapid development of computer networks over the last decade, high-end switching has become one of the most important functions on a network for moving data efficiently and quickly from one place to another.


    Here’s how a switch works: As data passes through the switch, it examines addressing information attached to each data packet. From this information, the switch determines the packet’s destination on the network. It then creates a virtual link to the destination and sends the packet there.


    The efficiency and speed of a switch depends on its algorithms, its switching fabric, and its processor. Its complexity is determined by the layer at which the switch operates in the OSI (Open Systems Interconnection) Reference Model (see above).


    OSI is a layered network design framework that establishes a standard so that devices from different vendors work together. Network addresses are based on this OSI Model and are hierarchical. The more details that are included, the more specific the address becomes and the easier it is to find.


    The Layer at which the switch operates is determined by how much addressing detail the switch reads as data passes through.


    Switches can also be considered low end or high end. A low-end switch operates in Layer 2 of the OSI Model and can also operate in a combination of Layers 2 and 3. High-end switches operate in Layer 3, Layer 4, or a combination of the two.


    Layer 2 Switches (The Data-Link Layer)

    Layer 2 switches operate using physical network addresses. Physical addresses, also known as link-layer, hardware, or MAC-layer addresses, identify individual devices. Most hardware devices are permanently assigned this number during the manufacturing process.


    Switches operating at Layer 2 are very fast because they’re just sorting physical addresses, but they usually aren’t very smart—that is, they don’t look at the data packet very closely to learn anything more about where it’s headed.


    Layer 3 Switches (The Network Layer)

    Layer 3 switches use network or IP addresses that identify locations on the network. They read network addresses more closely than Layer 2 switches—they identify network locations as well as the physical device. A location can be a LAN workstation, a location in a computer’s memory, or even a different packet of data traveling through a network.


    Switches operating at Layer 3 are smarter than Layer 2 devices and incorporate routing functions to actively calculate the best way to send a packet to its destination. But although they’re smarter, they may not be as fast if their algorithms, fabric, and processor don’t support high speeds.


    Layer 4 Switches (The Transport Layer)

    Layer 4 of the OSI Model coordinates communications between systems. Layer 4 switches are capable of identifying which application protocols (HTTP, SNTP, FTP, and so forth) are included with each packet, and they use this information to hand off the packet to the appropriate higher-layer software. Layer 4 switches make packet-forwarding decisions based not only on the MAC address and IP address, but also on the application to which a packet belongs.


    Because Layer 4 devices enable you to establish priorities for network traffic based on application, you can assign a high priority to packets belonging to vital in-house applications such as Peoplesoft, with different forwarding rules for low-priority packets such as generic HTTP-based Internet traffic.


    Layer 4 switches also provide an effective wire-speed security shield for your network because any company- or industry-specific protocols can be confined to only authorized switched ports or users. This security feature is often reinforced with traffic filtering and forwarding features.

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