Notes: CCENT – Ethernet

Notes: CCENT – Ethernet Standards and Cabling

CSMA/CD – Carrier Sense Multiple Access with Collision Detection

  • Used in old Ethernet on hubs. (It’s the year 2011, there should be no hubs sitting in production networks.) Only reason you should be using a (layer 1) hub is to temporarily capture traffic links via a packet sniffer such as WireShark. (For instance, where a router connects to a switch, you can insert the hub between the router and switch and capture all network traffic coming from the router destined for the networks, this should be done temporarily if you’re trying to troubleshoot)
  • Hosts determine transmission.
  • Used when hosts are sharing the same shared bus/bandwidth.
  • Shared Segments are collision domains (or Layer 1 domains), therefore a hub is one entire collision domain.
  • When data is sent on the same wire that is shared by other hosts (in other words, when data is sent to a layer 1 hub) all other hosts will see it. If the >FRAME< is not showing their MAC as the destination. (This causes unnecessary processing, and as well is not good for the LAN’s over head.)

Properties of shared segments:

  • The chances of collisions can occur when two hosts attempt to transmit data at the same time.
  • All duplex is half duplex. Hosts can only either transmit or receive data at one time. Full Duplex allows simultaneous transmission. (Seen on Switches (L2)

Collision Detection:

  • Hosts all listen on the wire and listen for electrical signals before attempting to transmit.
  • The hosts on the shared segment detect a voltage change on the wire from the result of data collision.
  • When a collision is detected, the source hosts that the data originated from will then send a jam signal onto the wire to tell other hosts there has been a data collision. Then they will invoke a RANDOM back-off timer until it starts transmitting again. This does does not give the hosts part of the collision priority over other hosts however, other hosts at this time will detect no signal on the wire and will transmit when the opportunity to do so arises.

Ethernet Standards – IEEE 802.3

802.3u – Fast Ethernet (Common 100BaseTX, Used over Cat5) Max 100meters
802.3ab – Gigabit Ethernet (Common 1000BaseT (copper), used over Cat5e or Cat6) Max 100meters
802.3z  – Gigabit Ethernet (1000BaseFX, used over Fiber)

  • 10Base-t – “T” stands for “twisted pair”. Max length is 100meters. “10” Refers to capacity of 10mbps. “Base” refers to base-band, which is how the signal is transmitted.
  • “10Base5” and “10Base2” -Used with coaxial cable. The “5” and “2” refer to cable length (5 is 500 meters, however 2 is rounded up from the actual 185meter length.)
  • Twisted Pair means the wires inside a cable are twisted to reduce electromagnetic interference (EMI) from other cables or outside sources.
  • Regular Cat5/6 cables use RJ-45 connectors.
  • In RJ-45, Pins 1&2 Transmit (Tx), Pins 3&6 Receive (Rx)

Crosstalk is caused by EMI when electrical signals bleed from one wire to another inside an ethernet cable.

Cable Layouts.

  • Straight Through – Standard Cable – PC to Switch or Hub, Switch to Router, end device to network. All the pins connect end to end. End to End:(White/Orange, Orange, White/Green, Blue, White/Blue, Green, White/Brown, Brown.)
  1. to 1
  2. to 2
  3. to 3
  4. to 4
  5. to 5
  6. to 6
  7. to 7
  8. to 8
  • Cross over – Connects like devices to one another. Switch to Switch, Router to Router, Host to Host. (Yes there is Auto-MDIX which lets you connect switches via straight through, but we’re not covering this.) Pins are “crossed over”
  1. to 3
  2. to 6
  3. to 1
  4. to 4
  5. to 5
  6. to 2
  7. to 7
  8. to 8
  • Roll Over: Connects RS232 (Serial) to the console port on a router or switch. Exactly as its called “roll over”:
  • If you’re looking for a roll over, get an adapter, these days laptops don’t even have serial ports, you can find USB to RS232 for like $5 on eBay.
  1. to 8
  2. to 7
  3. to 6
  4. to 5
  5. to 4
  6. to 3
  7. to 2
  8. to 1

Layer 2 Addressing:

Mac Addresses:

  • Also known as the NIC, Physical, BIA, or Ethernet Address
  • Used at Layer 2 by switchies to send frames to their destination. (LAN Switching)
  • MAC Adds are in Hexadecimal values: aa-bb-cc-11-22-33 or aabb.cc11.2233 when viewing on devices.
  • aa-bb-cc-11-22-33 is an example of a unicast mac.
  • aa-bb-cc is the OUI portion – used to identify the organization (Organizational Unique Identifier) its made by.
  • 11-22-33  – Unique Hardware Identifier.
  • ff-ff-ff-ff-ff-ff : This is a broadcast MAC address, destination for all hosts in a layer2 broadcast domain.
  • MAC address starting with 01-00-5e are the beginning of multicast MACs. They will end from 00-00-00 through 7F-FF-FF. An Example of a multicast: 0100.5e7f.ffff

Wan Cables:

  • Cisco routers can connect to each other via Serial Cables using a DTE/DCE cable.
  • One end of a serial cable is the DTE side (Data Terminal Equipment) and the DCE side (Data Circuit Equipment). Its called DCE because this is where the clock rate is supplied FROM.  Router DCE(Clockrate)< —- > DTE Router


DTE/DCE cable:

Straight-Through Cable:

Crossover Cable:

RollOver Cable:

RollOver Cable:

USB to RS232: