LAN

A privately owned network interconnecting stations over short distances (few meters to several hundred meters). Operates at high bit rates ($1$ Mbps to $100$ Mbps). Error rate: $10^{-8}$, compared to WAN at $10^{-5}$ or worse.

IEEE 802 standards split the Data Link layer into LLC and MAC sublayers.

Transmission Modes

Baseband

Digital technology. Driver introduces voltage levels. Supports TDM for multiple access.

Broadband

Analogue technology. Uses modems. Uses FDM with multiple carriers and sub-channels on a single path.

IEEE 802 Standards

Logical Link Control

Aka. LLC. Upper sublayer. Subset of HDLC. Common across 802.x variants.

LLC Classes of Service

• Type 1
  Connectionless. No ACKs, no flow control, no error control. Higher-level protocols handle these.
• Type 2
  Connection-oriented. Provides ACKs, flow control, and error control. Optional.

LLC Protocol Data Unit

| DSAP Address | SSAP Address | Control | Information |

• DSAP
  Destination Service Access Point.
• SSAP
  Source Service Access Point.
• Control
  Carries HDLC commands and responses. Differs based on service type.

Media Access Control

Aka. MAC. Lower sublayer. Media-specific. Encompasses 802.3, 802.4, 802.5.

MAC Protocol Data Unit

| Preamble | Starting Frame Delimiter | Dest. Address | Source Address | LLC PDU or PAD | FCS | Ending Delimiter |

Token Ring

Priority-based scheme. A token circulates the ring indicating free/busy status. Standardized as IEEE 802.5.

Operation:

1. Station wishing to transmit captures an empty token.
2. Modifies token to include start-of-frame identifier plus data and control fields.
3. Sends to next station.
4. Each intermediate station regenerates the token. Copies data if addressed to it.
5. Originating station recognizes its own frame by source address, removes it from the ring, and frees the token.

Ring formats:

• Token
• Abort Token
• Token-and-Data

Token Bus

Physical bus topology with a logical ring determined by numeric address order. Standardized as IEEE 802.4.

MAC machines:

• IFM
  Interface machine.
• ACM
  Access control machine.
• RxM
  Receive machine.
• TxM
  Transmit machine.
• RRM
  Regenerative repeater machine. Optional.

Frame structure matches 802.5. Access control and frame status fields are absent. Token sender waits for the successor to transmit the next frame as confirmation. If no confirmation, a new successor is established.