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  • Network Basics-Part 1


    Basic Network Principles Sermon

    A computer on your network is connected via a NIC, or Network Interface Controller. The NIC is the part of the computer where the network cable plugs in to. When your PC wants to send data, the NIC converts that data to a FRAME and sends the data through the cable. A frame consists of a Destination Address, a Source Address and then the data. In data networking, the concept of Media Access Control protocol, or MAC, provides a unique ID (or address) for each device. The address is 48-bit and symbolized by three, four hexadecimal groups. This address does not tell the actual location of your device, it just tells the unique MAC ID.




    With these MAC addresses our NIC can create a frame. Once the frame is sent another NIC will see its own MAC address on the frame and then accept the frame also seeing the address of the sender. This is a very basic description but it suits us for the show.


    If you are sending data to another network your NIC makes a packet. A packet is different than a frame in that it also includes the Destination and Source Network Address in addition to the MAC addresses. The one thing to note is that the MAC address will change as it goes across networks. The network address will stay the same.

    Fast Ethernet

    The Fast Ethernet was introduced in 1995. This standard is at 100 Mbit/s. Remember that when we talk Mbit/s that the number needs to be divided by 8 to get megabytes. For example, I get 30 Mbit/s download from my ISP which equates to 3.75 megabytes per second download. This conversion means that the highest theoretical speed in a 100 Mbit/s network is 12.5 megabytes per second. In reality you would be lucky to get 75% of that.

    Gigabit Ethernet

    Finalized in 1998, Gigabit Ethernet is the now becoming mainstream for desktop PC manufacturers and for home enthusiasts around the world. 1000BASE-T, as the copper wiring spec is known, actually uses all four cable pairs to transmit data in both directions. The speed of Gigabit is 1,000 Mbit/s which converts to 125 megabytes per second. Again, you will never get that max number and will settle for up to 80% or so.



    Network cabling (non-fiber)

    Cat 5e cable is used for Fast Ethernet although you can get away with using it for Gigabit. Cabling for Cat 5e can go 328 feet.


    Cat 6 Cable is used for Fast, Gigabit and even 10-Gigabit Ethernet. Cabling for Cat 6 can go 300 feet while 10-Gigabit lengths are limited to 180 feet. This is more than enough for any enthusiast listening to this show.


    The switch is a multiport complex device that acts as a bridge at the MAC address level. Switches allow you to segment the network and allow for greater network capacity.


    When the packet enters the switch the destination mac address the switch looks within the broadcast domain.


    As I start talking about this keep in mind that there are two addresses. There is a Source MAC address and a Destination MAC address. Think of an eight port switch you can buy at Fry’s or BestBuy. You connect each device into a port. The switch keeps a table of the MAC address of each device. When you send a frame, the switch can do one of three things: Flood, filter and/or forward.


    If the destination MAC is a broadcast address then the frame is sent to all ports except the sending port and this is called Flooding. Flooding also happens if the destination MAC address is NOT in the table.


    If the MAC address is the same as the sender then there is no reason to forward it and this is called Filtering.


    If the Destination MAC address is in the table the frame is sent to the proper port and this is called Forwarding.


    Unicast is sent and the CAM table finds the PC and just forwards to the port of the MAC address. When PC two replies the switch sends the data

    Switching Methods

    Store and Forward – Receives entire frame before switching and verifies addresses and error checks. Latency is an issue at times but this is going to be the most common for Linksys, Netgear and D-Link switches.


    Cut Through – Starts as soon as it sees the MAC address. No error check so end device has to check


    Fragment Free - Receives first 64 bytes before forwarding, this prevents collisions.


    The switch allows many workstations to transmit at the same time.


    Cisco offers a good set of training documents for anyone looking for more information. The link will be in the show post.


    Expensive and setup intensive a router connects two or more devices on separate networks. Your router is what is needed to communicate with your ISP and determines the path when transferring data from one network to another. Your router will control access, port forwarding and other network specific settings. You can also have your router do policy-based routing which sets some rules for things like QOS for applications like VOIP and gaming apps. The main purpose of a router though is to connect your network to another network (internet) and forward packets to their destination. Routers do not care about the actual data in the packet and only look at the address. We heard last week though that you can build your own router and use software to create your own forwarding decision structure.




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    Great Network Basics Part 1 sermon, looking forward to the following sermons. Great addition to my Know How library. Would be great to know what software or hardware you recommend to check the LAN speed of a small home network?
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    I recommend LAN Speed Test, (LAN_SpeedTest.exe,) It is about 60KB in size and creates a new file each time to test upload and the read speeds. Default file size is 100MB, but you can make it whatever size you wish.
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