COIT11238 Networked Infrastructure Foundations · Week 2

Networking resources

Lesson 2 - Follow the Frame

Send a message across one cable, race a file across a link, watch a switch learn where everyone lives, wire a connector, then build and repair a two-computer network. Work through the lessons in order, or open any lesson for revision.

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Lesson 1 of 8

Across one link

  • Name the parts a message passes through
  • Separate bits from the physical signal

Send a message across one Ethernet cable

Type a short message, then step through its journey from an application on Computer A to an application on Computer B. Keep it short so the bytes stay readable.

Computer A Application Operating system NIC Computer B Application Operating system NIC One Ethernet cable (the link) first byte, sent as a changing signal

    Lesson 2 of 8

    Speed and delay

    • Use data rate, throughput, delay and RTT correctly
    • Explain why a transfer rarely reaches the advertised rate

    Transfer-time explorer

    A link’s advertised data rate is a capacity, not a promise. Move the sliders and watch how file size, link rate, congestion and distance change the result.

    –Transmission time (size ÷ available rate)
    –Propagation delay (distance only, one way)
    –Simplified one-way total. Real delay also includes processing, queuing and every other link on the path.
    Speed-test tip. On speedtest.net, download and upload are throughput measurements between your device and a test server, and ping is a round trip time. A 1 Gb/s laptop reporting 310 Mb/s is normal, not automatically a fault.

    Lesson 3 of 8

    Why use a switch?

    • Compare direct links with a switched LAN
    • Count cables and NICs for each design

    Grow the network and watch the cabling

    Two computers connect with one cable. Connecting every computer directly to every other computer stops scaling very quickly. Add computers and compare.

    6 cables3 NICs in every computer · a separate link to every other computer
    4 cables1 NIC in every computer · a switch with 4 ports

    Direct links need n(n − 1) ÷ 2 cables. A switched LAN needs one cable for each computer. That difference is why the switch is the standard building block of the local area network.

    Sizes of networks. A LAN covers a small area such as a home, office or building. A WAN joins networks across larger distances and is usually run by a telecommunications provider. Routers join LANs and WANs into an internetwork; the Internet is the largest example.

    Lesson 4 of 8

    Follow the frame

    • Read source and destination MAC addresses
    • Explain learning, forwarding and flooding

    Teach the switch where everyone lives

    Five hosts share one Ethernet switch. The switching table starts empty. Choose a source and destination, send a frame, and watch what the switch does. Challenge: fill the whole table using as few frames as you can.

    Frames sent: 0

    Switching table

    MAC addressPort
    Empty. The switch has learnt nothing yet.

    What just happened

    No frame has been sent yet.
    Two different jobs for two addresses. The switch learns from the source MAC address, because the source must be reachable through the port the frame arrived on. The switch forwards using the destination MAC address. When the destination is unknown, or the destination is the broadcast address FF-FF-FF-FF-FF-FF, the frame is sent out every other port. That is called flooding.

    Lesson 5 of 8

    Protocols and layers

    • Describe a protocol as agreed rules
    • Build and unwrap an encapsulated message

    Why rules matter

    A protocol is a set of agreed rules covering message format, meaning, order and the actions to take. Toggle between a message with agreed rules and one without.

    Choose one of the two messages above.

    Wrap it up: encapsulation builder

    Layered networking places one protocol’s message inside another. Add the headers in the correct order, then deliver the frame and unwrap it exactly as Computer B would.

    Lesson 6 of 8

    Ethernet and MAC addresses

    • Wire an RJ45 connector to T568B
    • Split a 48-bit MAC address into its two halves

    Wire the connector: T568B

    A LAN cable holds four twisted pairs, eight wires in total. Hold the connector with the clip away from you; pin 1 is on the left. Select a wire, then select the pin it belongs in. Use the same standard at both ends for a straight-through cable.

    Blue Category 5 Ethernet cable with clear 8P8C RJ45 plugs on each end
    Finished cableThe eight coloured conductors are pushed into a clear 8P8C plug, then crimped so the gold contacts bite into the wires. Photo: Fo0bar, CC BY-SA 3.0/GFDL.
    Close-up of a transparent uncrimped RJ45 plug showing the slots where wires slide in
    Before crimpingThe plug is transparent so you can see where each wire travels. In the game below, pin 1 is the left slot when the clip is facing away from you. Photo: Mike1024, public domain.
    • Keep the wire colours in order all the way to the front of the plug.
    • The crimp tool pushes the gold contacts down through the insulation to touch each copper conductor.
    • For a straight-through cable, use the same wiring standard at both ends. This exercise uses T568B.

    Decode a MAC address

    A MAC address is 48 bits, written as six pairs of hexadecimal digits. Each pair is one byte. Type any MAC address, or use the one from the lecture’s PowerShell output.

    Read the PowerShell output

    PS C:\Users\user> Get-NetAdapter -Physical Name InterfaceDescription Status MacAddress LinkSpeed ---- -------------------- ------ ---------- --------- Ethernet Intel(R) Ethernet Connection (10) Disconnected CC-48-3A-3F-92-45 0 bps Ethernet 3 Realtek USB GbE Family Controller Up CC-48-3A-86-3F-62 1 Gbps Wi-Fi Qualcomm QCA61x4A 802.11ac Wireless Disconnected 28-CD-C4-9B-7D-0F 144.4 Mbps

    Lesson 7 of 8

    Build and troubleshoot

    • Follow design, build, test and document
    • Diagnose cable, addressing and firewall faults

    The virtual bench: connect two computers directly

    Configure a direct Ethernet connection, then test it with a simulated Test-NetConnection (ping). For this Week 2 exercise, both addresses use a simplified /24 rule: the first three decimal sections must match and the final section must differ. Formal subnetting comes later in the unit.

    Computer A (Windows)

    MAC 01-23-45-67-89-AB

    Computer B

    MAC CC-DD-EE-FF-00-11

    The bench is ready. Set the addresses, then run the test.

    Fix-the-fault challenges

    Each challenge loads a broken bench. Find the fault, fix it, and make the ping succeed.

    Troubleshooting order. Check the physical layer first: is the cable in, and are the link lights on? Then the addresses: valid, unique, and in the same /24? Then the test itself: is the correct destination address being used? Only then suspect software such as the firewall.

    Lesson 8 of 8

    Check understanding

    • Review the central Week 2 ideas without notes

    Eight questions, instant feedback

    Answer without returning to the earlier lessons. Feedback appears immediately; treat a wrong answer as a pointer to the lesson worth revisiting.

    0 / 8Answer every question to finish the review.
    Week 2 tutorial complete.