CSMA/CD

Carrier Sense Multiple Access with Collision Detection. Controls LAN access on a bus structure. Non-priority scheme. Standardized as IEEE 802.3.

Two layers:

• MAC layer
  Media independent. Handles framing and collision management.
• Physical layer
  Media dependent. Handles encoding and signal sensing.

DLL functions:

• Encapsulation and decapsulation
  Addresses, error detection fields.
• Media access management
  Frame transmission/reception, buffering, collision handling.

Physical layer functions:

• Data encoding/decoding
  Binary to Manchester code and vice versa. Preamble provides synchronization.
• Channel access
  Introduces signals onto the channel. Carrier sensing and collision detection.

History: Developed by Xerox. Jointly specified by Xerox, Intel, and DEC in 1980.

IEEE 802.3 Supplements

Media Access Control Rules

1. A signal being transmitted on the channel = carrier.
2. Station waits until channel is idle before transmitting.
3. After channel goes idle, station waits an Inter Frame Gap (IFG) then transmits.
4. If two stations transmit simultaneously, both detect the collision and reschedule.

Slot Time

The window within which a collision must be detected. $512$ bit times for $10$/$100$ Mbps.

• $10$ Mbps: $512$ bit times covers $2800$ m on coaxial.
• $100$ Mbps: $512$ bit times covers ~$205$ m. Acceptable with switches/hubs on twisted pair (max $100$ m).
• $1000$ Mbps: $512$ bit times covers only $20$ m. Slot time extended to $4096$ bit times via carrier extension.

Minimum data field = $46$ bytes (required to fill $512$ bit times).

Back-off Algorithm

IFG is $96$ bit times. Brief recovery time between frames.

• $10$ Mbps: IFG = $9.6$ µs
• $100$ Mbps: IFG = $960$ ns

Steps:

1. If no carrier and no-carrier period exceeds IFG, transmit immediately.
2. If channel busy, listen until carrier ceases, wait IFG, then transmit.
3. If collision detected, transmit 32-bit jam signal.
4. After jam, wait a random back-off period then return to Step 1.
5. Back-off duration increases with each successive collision attempt.

Back-off formula:

Here:

• $r$: randomly chosen integer back-off delay, in slot times
• $n$: number of transmission attempts

Maximum retries = $16$. After $16$ failures, transmission failure reported to higher layers. Back-off counter resets after a successful transmission.

Gigabit Ethernet Half Duplex

Signaling $10\times$ faster than Fast Ethernet. Effective network diameter shrinks to $20$ m. Slot time extended to $4096$ bits ($512$ bytes). Smaller frames padded with carrier extension (non-data symbols).

Frame bursting allows a station to send multiple smaller frames consecutively.

• First frame transmitted normally. If no collision, burst frames follow.
• IFG between burst frames filled with extension symbols. Channel held throughout.
• Improves efficiency for small frames to >90%.

Frame burst limit: $65{,}536$ bits.

Full Duplex Ethernet

Simultaneous bidirectional transmission on a point-to-point link (twisted pair or fibre optic). Standardized as IEEE 802.3x.

• Independent transmit and receive paths.
• No collision detection (or ignored).
• No multiple access. One station at each end.
• IFG still present between frames.
• Effective throughput doubled (e.g., $100$ Mbps Fast Ethernet → $200$ Mbps aggregate).
• No shared-medium timing constraints. Only media signal-carrying limits apply.
• Full bandwidth usage confined to backbone connections in practice.

Supported media: 10BASE-T, 10BASE-FL, 100BASE-T, 100BASE-FX, 1000BASE-SX/LX/CX, 1000BASE-T.

Gigabit Ethernet cable length limits:

• Twisted pair: $100$ m (same as 10/100BASE-T).
• Multi-mode fibre: up to $2$ km (full duplex). ~$412$ m on some fibres (half duplex).
• Single-mode fibre: $20$ km or more.