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Monthly Archives: November 2010

Bridges In My Network- What Are They Good For

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    There is confusion in the networking world regarding the purpose of a bridge and its expected behavior. Bridges were created to allow network administrators to segment their networks transparently. This is important because individual stations don’t need to know whether there is a bridge separating them or not. Therefore, it is up to the bridge to make sure that packets get properly forwarded to their destinations. Segmenting a large network with a bridge has numerous benefits including reduced collisions (in an Ethernet network), contained bandwidth utilization, and the ability to filter out unwanted packets.

    The are two main types of bridges:

    1. Transparent Bridges
    A transparent bridge examines a packet’s Destination Data Link Address and looks up the Address in its internal tables to determine to which of its ports, if any, to forward the packet. Promiscuous listening is the key to the bridge’s transparent operation. Since the bridge effectively “hears” all packets that are transmitted, it can decide whether forwarding is necessary without any special behavior from the individual stations.

    2. Learning Bridges
    A learning bridge can be added to a network, and it will learn the network topology without help from humans. If a station is moved, the bridge will realize it and update its tables appropriately.

    Originally, there were some transparent bridges that did not have learning capability, but today, the terms “transparent bridge” and “learning bridge” are used interchangeably.

    Beware of bridge loops.
    It’s also important to consider bridge loops, which occur in a network and are often unpredictable. In a nutshell, bridging loops cause trouble because of the very feature that makes bridges so desirable: their transparency. Because the bridges don’t realize that packets are being forwarded from another bridge, they continually flood the packets onto their ports, creating more packets, which are flooded, creating more packets, etc.

    For more information about bridges, routers, and network connecters we recommend Radia Perlman’s book “Interconnections – Bridges and Routers.”

      Troubleshooting Your Ethernet? Look for Physical Frame Corruption!

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        When troubleshooting your Ethernet network, the first thing to look for is physical frame corruption. Provided an organization is using coaxial Ethernet, below are four possible causes of physical frame corruption in an Ethernet network, each one different in the way it corrupts the frame and therefore recognizable (Note: Twisted-pair Ethernet implementations will not manifest these types of corruption patterns).

        1) Collisions

        Generally when a collision occurs, several bytes of the preamble of the colliding frame will be read into your analyzer’s buffer before the signal is completely destroyed. You will see these bytes in the hexadecimal decode of the packet as either several bytes of AA’s or several bytes of 55′s at the very end of the frame (remember, AAh=1010b, 55h=0101b. Depending on where the collision occurred, the preamble could be perceived as either of these). If you see more than 8 bytes of AA or 55, then the corruption was not caused by a collision and more investigation is necessary.

        2) Signal Reflections

        One cause of signal reflection is an un-terminated cable. Electrons travel down the wire until they reach the cable’s end, where, with no resistor to absorb the voltage potential, they reflect back from the open end of the cable. Another cause of signal reflections is mixing cables with different impedances. Impedance can be thought of as the “rate of flow” of the wire. When electrons from the higher impedance wire attempt to travel through the lower impedance wire, some of them can’t make it and are reflected back, destroying the signal. The final cause of signal reflections is exceeding the maximum allowable bend radius of the cable. The copper media is deformed, causing reflections.

        3) Electrical Noise

        Physical frame corruption caused by electrical noise is similar in appearance to corruption caused by reflections in that there is no preamble in the frame — the frame just seems to stop short, but is different in that the frames are generally cut off at random lengths.

        4) Malfunctioning Hardware

        Frame corruption caused by hardware malfunctions is potentially the hardest to diagnose because of the large number of ways that hardware can malfunction. Generally, hardware malfunctions will occur either randomly or constantly, but not regularly. The type of frame corruption is impossible to predict, generally manifesting as random “garbage” in the frame, but some common signs are:

          • A stream of ones or zeros. A transceiver has malfunctioned and is “jabbering” on the wire. Most transceivers have jabber detection circuitry that prevents the adapter from transmitting for longer than a certain preset time.
          • Gigantic frames (greater than 1500 bytes). Same as above.

        With these four main causes dissected above, troubleshooting an Ethernet network doesn’t have to be confusing. While these tips are universal, specific analyzer’s behavior might differ and an organization should determine what’s best in terms of troubleshooting its own specific network.

          What's the Deal With Wireless Network Adapters Lagging Behind 802.11n?

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            The increasing adoption of 802.11n has brought wireless networking to a whole new level, as wireless network speeds are now nearly comparable with many wired networks. Over the past few years there’s been significant development around 802.11n and companies have experienced several benefits including significantly higher throughput, better range, and more efficient usage of the 5-gigahertz spectrum.

            However, wireless network adapters that laptops use to connect to these upgraded networks still have a way to go. Most 802.11n wireless adapters only take advantage of a portion of the overall gains possible with 802.11n. Below are some key considerations for choosing a wireless adapter to make sure you will be able to see your most critical wireless network traffic, and make the most of the 802.11n advances :

            Take into account the “secret of NxM”

            Where N equals the number of transmit antennas and M equals the number of receive antennas. Both M and N need to be the same in the wireless adapter that you choose and the most capable access point (AP) in your network.

            Be aware of the number of streams

            For example, if your most capable APs are 3X3:2, meaning they have 3 transmit and 3 receive antenna, but only have two data streams, and you suddenly add in new APs that are 3X3:3 (3 data streams), you’ll need to upgrade your wireless capture capability and find wireless adapters that are also 3X3:3. This is critical to see all the important wireless traffic and packets.

            Support for three data streams is difficult to find

            There are lots of 3X3 wireless adapters and APs available. But 3X3:3 with support for three data streams is more difficult to find. There are more APs that fit this criterion than wireless adapters. The easiest way to find out if the wireless adapter will work is to look to see if it can support
            450Mhz.

            The truth is that as 802.11n continues to mature, the availability of related technology will increase and it will be easier to manage. Remember that the time to begin moving to this new standard is as soon as you need to upgrade any component in your existing infrastructure, in order to address the demand for additional capacity in the WLAN. By keeping an eye on these specifics, you will make sure you wind up with visibility into your entire wireless network!