The Physical Network
Ethernet Basics
- What is Ethernet?
- The Ethernet frame (of data) never really changes!
- Max frame is 1,500 bytes.
- Ethernet is defined by the IEEE 802.3 standard.
- The IEEE has defined many versions of Ethernet.
- For the test, be able to recognize the Ethernet naming syntax.
- 802.3 standards:
- 802.3a
- 802.3b
- 802.3i
- 802.3ae
- and more
- Standard Nomenclature (for Ethernet):
- 10 Base 5
- ’10’ is the speed in Mbps
- ‘5’ is the length of the cable (5 for 500m, But today the only value we’ll see here is ‘T’.)
- ‘Base’–only one channel on the entire bandwidth.
- or, ‘Broad’–Broadband is ethernet that runs like cable TV (with lots of channels or ‘conversations’ at one time).
- Ethernet Frames (Remember! This is an Ethernet frame, NOT an IP packet!)
- Ethernet frames consist of a preamble, destination MAC, source MAC, data type, data, pad, & FCS.
- A jumbo frame can carry 9,000 bytes.
- FCS is used for error detection.
- Other types of data have other frames, but since MOST traffic flows through ethernet, ethernet frame is the most popular.
- Ethernet Frame (in reality all these “chunks” are a long string of 1’s & 0’s)
- Preamble–tells the network card that a frame is coming.
- Destination MAC–addressing of where it’s going; Remember, the MAC address is built into every network card in the universe!
- Source MAC–return MAC address;
- Data Type–Data type =’s Ether type; Specifies what kind of data is coming.
- Data–the data itself; Minimum-64 bytes; Maximum-1522 bytes. (Note: If the data is less than 64mb then a “pad” block is added to “fill in” the unused data space!)
- Jumbo Frame–unique for really high-speed stuff; 1 frame can max out at 9,000 bytes.
- MTU–Maximum Transmission Unit; the maximum amount of data that can be hauled!
- FCS–Frame Check Sequence–a 32-bit cycle redundancy check; Does some computations to ensure the frame is complete & in order.
- Early Ethernet:
- CSMA/CD stands for ‘Carrier Sense Multiple Access/Collision Detection’.
- 10Base5 and 10Base2 require terminating resistors at both ends of a segment (cable).
- When connecting to 10Base2 always use a “T” connector.
- Segmented Ethernet–no switch/router; just one big cable in the ceiling.
- BNC connector– with terminating end; –> ‘T’ connectors
- The Death of Ethernet: 10BaseT
- 10BaseT runs at 10Mbps over Cat3 or better UTP.
- 10BaseT can have up to 1024 nodes per switch.
- 10BaseT cable runs are a maximum of 100 meters between the switch & node.
- Token Ring (made by IBM)
- MSAU–Multi-station access unit
- Terminating Twisted Pair
- An RJ-45 (aka-8P8C) connector is used to connect to most network cards.
- Watch the position of the wires when crimping to follow 568A or 568B standards.
- Straight-through cables are the most commonly used in networks.
- Hubs vs. Switches
- Switches forward frames based on MAC addresses
- Hubs use CSMA/CD to avoid collisions
- Switches create and use MAC address tables to map ports & host devices.
- Hub –> nothing more than a multi-port repeater.
- Switch –> also a multi-part repeater, but different. A switch looks at MAC addresses of the individual hosts. Using the MAC address, a switch creates a ‘MAC’ table for itself to reference what goes where.
- Broadcast Domain–Typically “collision domain” refers to hubs, & “broadcast domains” we mean switches.
- Advantages of Switches over Hubs
- Switches can use cross-over or straight-through cables to connect to other switches, BUT hubs require crossover cables to connect to other hubs.
- Switches reduce collision domains to the two ports involved in communication by isolating them from the other switch ports for the duration of the communication. All ports of a hub are on the same collision domain and can be involved in a broad spectrum of collisions.
- Switches can auto-detect connections to other switches, & internally configure the connection port to be a crossover or straight-through, as necessary.
