Network congestion and slowdowns--whether caused by faulty hardware, negligent users, viruses or spyware applications gone wild, or other factors--lead to serious headaches for network administrators and support personnel. By keeping a wary eye tuned for the following 10 items, IT professionals can help prevent the most common causes of network slowdowns.
1. Bad NICs
Intermittent network errors, particularly those isolated to a specific workstation or server, can often be traced to a failing network interface card. When you believe a network adapter may be failing, visually inspect the card's LED link lights.
A solid green (or amber) LED indicates the NIC has a good active physical connection with another network device, such as a network switch or router (blinking LEDs typically indicate the NIC possesses an active connection and is processing network traffic). If the LED is not lit green, it's likely the network adapter is disabled within Windows or doesn't have an active connection to the network. It's also possible the cable plugged into the NIC is connected to a nonfunctioning wall-jack or faulty network port.
If you can rule out nonfunctioning wall-jacks and faulty network ports (the easiest method of doing so is to connect the same network connection to a laptop known to have a properly functioning network adapter), and if the network adapter is properly enabled and configured in Windows, it's likely the NIC is bad and requires replacement.
2. Failing switches/routers
Many network slowdowns are foreshadowed by strange occurrences. For example, regular Web traffic may work properly, but e-mail may stop functioning. Or, regular Web traffic may work properly but attempts to connect to any secure (HTTPS) sites may fail. In other cases, Internet access simply ceases across the board.
Often the best remedy for inconsistent network outages and/or slowdowns is to reboot or power cycle the network's routers/switches. If local network connectivity exists (if users can view and access network shares) but they are not receiving e-mail from external users or cannot access the Internet, rebooting or power cycling the WAN modem can often return the network to proper operation.
If you're having to reboot or power cycle a piece of network equipment consistently, make sure that it's connected to a quality uninterruptible power supply. Power fluctuations often result in confused switches and routers. If a network device is connected to a good UPS and still frequently experiences trouble, it may be necessary to replace the failing switch, router, or modem.
3. Daisy chaining
As organizations grow, particularly small businesses, outside IT contractors often implement simple solutions. Many consultants choose to simply add a five-port router to an existing four-port router/firewall. Small businesses everywhere boast just such a setup.
However, as switches are added to a network, data packets must navigate additional hops to reach their destination. Each hop complicates network routing. Depending upon the amount of traffic a network must support--and even a small dentist's or doctor's office can easily stress 10/100 Mbps systems due to X-ray imagery, patient file information, and other data--the addition of an extra hop or two can spell the difference between a smooth running network and one that frequently slows employee productivity to unacceptable levels.
Resist the urge to daisy chain multiple network switches and routers. Instead, plan for capacity. Or if unforeseen growth has resulted in successive connected switches, eliminate as many devices as possible through consolidation to a more potent and scalable unit.
4. NetBIOS conflicts
NetBIOS, still in use on many Windows NT 4.0 networks in particular, contains many built-in processes to catch and manage conflicts. Occasionally, however, those processes don't handle conflicts properly. The result can be inaccessible file shares, increased network congestion, and even outages.
Guard against NetBIOS conflicts by ensuring older Windows systems all receive the most recent service packs. In some cases, Windows NT 4.0 systems having different service packs will generate telltale NetBT (ID 4320) errors.
Strange network behavior can also occur when two systems are given the same computer name or when two systems both believe they serve the master browser role. Sometimes the error will log itself as Event ID 8003 in a server's system log. Disabling WINS/NetBT name resolution (only if it's not required) can eliminate such issues.
If disabling NetBT is not an option, such errors can often be eliminated by identifying the second system that has the same computer name within the same domain and giving it a new name or by restarting the Netlogon Service on the domain controller. Yet another option for eliminating legacy NetBT issues is to search a system's LMHOSTS file for inaccurate or outdated entries. Some IT professionals claim they've solved similar errors by disabling and re-enabling the NIC on the offending system.
5. IP conflicts
Windows typically prevents two devices with the same IP address from logging on to the same network (when using DHCP). But occasionally, two systems with the same address wind up on the same network. For example, one system could receive an address automatically, while another computer logs on using a static address specified by a user. When such conflicts occur, network slowdowns result (and the systems sharing the same address frequently experience outages).
Troubleshoot IP address conflicts by ensuring you don't have a rogue DHCP server on the network. Confirm, too, that configured DHCP scopes don't contain overlapping or duplicate entries and that any systems (such as servers and routers) that have been assigned static IP addresses have been excluded from the DHCP pools.
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