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Friday, January 1, 2010

Day 27: Troubleshooting and LAN versus WAN

The OSI and TCP/IP models provide an excellent framework for troubleshooting. You can isolate the network issues to a particular layer and test the protocols and configurations for that layer.
You can work top down, starting with the application layer (are other apps working?), or bottom up, are the problems caused by media connections or power to a device. Experienced troubleshooters will often begin at the layer indicated by the symptoms of a particular problem, this is called divide and conquer. In addition to this you could troubleshoot through trail and error or substitution.
  • ipconfig shows the IP configuration.
  • ping tests the network layer connectivity between devices.
  • tracert tests connectivity and displays each hop.
  • netstat shows current TCP/IP network connections to the device and protocol statistics.
  • nslookup queries the configured name serve for DNS information.
As a reminder of yesterday: a physical topology represents the location of the hardware and the logical topology represents how the devices use the network.
LANs today often represent the logical grouping of hosts for a single organization. Network administrators typically refer to the network they maintain in their building(s) as a LAN or a private intranet. LANs support high data transfer rates over Ethernet or wireless protocols in a smaller geographic area.
A wide-area-network (WAN) provides relatively lower data transfer rates over a larger geographical area. These connectivities can be symmetric or assymetric. An assymetric connection typically has a faster download speed than upload speed. Symmetric connections on the other hand provide the same upload an download speed.

These are some WAN connections:
  • A point-to-point (PPP) connection provides a specific dedicated path through the TSP network to connect two LANs over large distances.
  • A circuit-switched WAN connection allows the client to create and close connections over the TSP network (uses the entire connection), operates lke a phone call - an example is ISDN.
  • A packet-switched WAN connection allows multiple clients to share a single connection (uses a virtual circuit). An example is Frame Relay.
WANs operate at the physical and data link layers of the OSI model.  

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