CSMA/CD algorithm works like this:
Step 1 A device with a frame to send listens until the Ethernet is not busy.
Step 2 When the Ethernet is not busy, the sender(s) begin(s) sending the frame.
Step 3 The sender(s) listen(s) to make sure that no collision occurred.
Step 4 If a collision occurs, the devices that had been sending a frame each send a jamming signal to ensure that all stations recognize the collision.
Step 5 After the jamming is complete, each sender randomizes a timer and waits that long before trying to resend the collided frame.
Step 6 When each random timer expires, the process starts over with Step 1.
CSMA/CD does not prevent collisions, but it does ensure that the Ethernet works well even
though collisions may and do occur.
*Ethernet carrier sense multiple access with collision detection (CSMA/CD) logic is that collisions should be detected within the first 64 bytes of a frame.
Collision Domain
Defines the set of devices whose frames could collide. All devices on a 10BASE2, 10BASE5, or any network using a hub risk collisions between the frames that they send, so all devices on one of these types of Ethernet networks are in the same collision domain.
To avoid collisions, and to recover when they occur, devices in the same collision domain use CSMA/CD or Buffering.
Common Types of Ethernet
Ethernet UTP Cabling
The three most common Ethernet standards used today—10BASE-T (Ethernet),
100BASE-TX (Fast Ethernet, or FE), and 1000BASE-T (Gigabit Ethernet, or GE)—use
UTP cabling.
RJ-45 connector has eight Ethernet UTP Cabling specific physical locations into which the eight wires in the cable can be inserted, called pin positions, or simply pins.
Although RJ-45 connectors and ports are popular, engineers might want to purchase Cisco
LAN switches that have a few physical ports that can be changed without having to
purchase a whole new switch. Many Cisco switches have a few interfaces that use either
Gigabit Interface Converters (GBIC) or Small-Form Pluggables (SFP). Both are small removable devices that fit into a port or slot in the switch.
Ethernet Cable Standard
Straight-through Cable
- Host to Switch or hub
- Router to Switch or hub
Crossover Cable
- Switch to switch
- Hub to hub
- Host to host
- Hub to Switch
- Router to host
Full duplex/half duplex
Full duplex means that an Ethernet card can send and receive concurrently.With only the switch and one device connected to each other, collisions cannot occur. When you implement full duplex, you disable CSMA/CD logic on the devices on both ends of the cable.first half of the address identifies the manufacturer of the card is called the organizationally unique identifier (OUI). second half of the address being assigned a number that this manufacturer has never used on another card.
Ethernet addressing (MAC, Media address control 802.3)
Ethernet LAN addressing identifies either individual devices or groups of devices on a
LAN. Each address is 6 bytes long, is usually written in hexadecimal. Unicast Ethernet addresses identify a single LAN card. Computers use unicast addresses to identify the sender and receiver of an Ethernet frame.
First half of the address identifies the manufacturer of the card is called the organizationally unique identifier (OUI).
Second half of the address being assigned a number that this manufacturer has never used on another card.
Each LAN card comes with a burned-in address (BIA, 6-byte address assigned by vendor) that is burned into the ROM chip on the card.
IEEE defines two general categories of group addresses for Ethernet:
■ Broadcast addresses: The most often used of the IEEE group MAC addresses, the
broadcast address, has a value of FFFF.FFFF.FFFF (hexadecimal notation). The
broadcast address implies that all devices on the LAN should process the frame.
■ Multicast addresses: Multicast addresses are used to allow a subset of devices on a
LAN to communicate. When IP multicasts over an Ethernet, the multicast MAC
addresses used by IP follow this format: 0100.5exx.xxxx.
LAN Header Format
*Ethernet Frame Check Sequence (FCS) field in the Ethernet trailer—the only field in
the Ethernet trailer—allows a device receiving an Ethernet frame to detect whether the bits
have changed during transmission.
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