Ethernet

Sending data via Ethernet

There are three main types of data transmission.

Unicast

Sending of messages to a single network destination identified by a unique address

Broadcast

Transmitting the same data to all possible destinations

Multicasting

Sends data only to interested destinations by using special address assignments

Collision Handling

Whenever two or more nodes transmit at the same time, the signals would collide and interfere with one another. All transmissions involved would fail as a result.

Collision Avoidance

  1. Before transmission, wait for the line to become quiet
  2. While transmitting, continually monitor the line for signs that a collision has occured
  3. If a collision has occured, cease transmissing and use a backoff-and-retransmit strategy

Backoff algorithm

If a collision occurs, the nodes will try to send the transmission again after a certain period of time.
After the first collision, there are two different back-off times available. One is chosen at random by each of the nodes involved in the collision. Transmission probability is now 50%.

After a second consecutive collision, 4 different back off times are made available, from which one is chosen at random again. This makes the transmission probability 75%

Ethernet

Ethernet Address

Also called a MAC address. This address is globally unique for every device as it is burnt into ROM at the time of manufacture, and cannot be modified. (It can be spoofed though).

A MAC address is 6-bytes in length - 12 hex digits - 48 bits

Ethernet Data

There is preamble of 56 bits of alternating 1's and 0's at the beginning of the data packet.The next byte is the start frame delimiter flag (SFD)

The next 6 bytes is the destination address. The next 6 is the source address. The next 2 bytes is the length or type of the data, the next section is the data itself (with padding). The length is stored in the section previous. The last 4 bytes of the packet is CRC (redundancy checks)

The minimum payload of data for a packet is 46 bytes, the maximum is 1500 bytes. Making the overall packet minimum frame length 64 bytes, or a maximum of 1518 bytes

Physical Ethernet

Four generations of Ethernet - All are backwards compatible

  1. Standard - 10Mbps - coaxial or UTP cat 3
    1. 10Base5 - Bus, thick coaxial
    2. 10Base2 - Bus, thin coaxial
    3. 10Base-T - Star, UTP (unshielded twisted pair)
    4. 10Base-F - Star, fiber
  2. Fast - 100Mbps - UTP cat 5 or optical fibres
    1. 100Base-TX - Two wires, cat 5 UTP
    2. 100Base-FX - Two wires, optical fibre
    3. 100Base-T4 - Four wires, cat3 UTP
  3. Gigabit - 1Gbps - UTP cat 5e or optical fibres
    1. 1000Base-SX - Two wire short wave optical fibre
    2. 1000Base-LX - Two wire, long wave optical fibre
    3. 1000Base-CX - Two wire, copper (STP)
    4. 1000Base-T - Four wire UTP
  4. Ten-Gigabit - 10Gbps - UTP cat 5e or optical fibres

Repeaters and hubs

A repeater can join cables to extend the length of Ethernet reach. A hub splits the signal/bandwidth to reach many hosts. Like a broadcaster