MAC Sublayer Protocols: ALOHA & Channel Allocation

MAC Sublayer Introduction

• Networks are categorized as point-to-point (direct connections) or broadcast (shared channels)
• Multiple access control is a challenge in broadcast networks, determining channel use when multiple devices compete
• Protocols addressing this operate in the MAC sublayer of the Data Link Layer

Static Channel Allocation

• Frequency Division Multiplexing (FDM) divides bandwidth into N equal portions, assigning each to a user
  • FDM is efficient with few users, constant traffic, and high traffic loads
  • FDM becomes inefficient with many users or if some users remain idle
• Time Division Multiplexing (TDM) divides time into slots, each for a specific user
  • TDM is wasteful if a user doesn't need their allocated time slot

ALOHA Protocol

• Developed by Norman Abramson at the University of Hawaii
• Users transmit data whenever they have a frame
• After collisions, the sender waits a random time before retransmitting

Pure ALOHA

• Frames can be sent at any time
• After collisions, retransmission occurs after a random delay
• Throughput is defined as S=Ge−2GS = G e^{-2G}
• Maximum efficiency is 18% when G = 0.5

Slotted ALOHA

• Time is divided into discrete slots
• Devices transmit only at the start of a time slot
• A random backoff time is applied after collisions
• The throughput is defined as S=Ge−GS = G e^{-G}
• Maximum efficiency hits 37% when G = 1

Carrier Sense Multiple Access (CSMA)

• Aloha suffers from low efficiency because channels are not checked resulting in collisions
• CSMA improves efficiency by requiring stations to check the channel before sending data

1-Persistent CSMA

• If the channel is idle, it transmits immediately
• If it is busy, the station listens and sends immediately when free
• Problem: multiple stations waiting can cause collisions

Non-Persistent CSMA

• The station waits a random amount of time instead of continuously sensing if the channel is busy
• This reduces, collisions but increases delays

P-Persistent CSMA

• In slotted channels, if idle, it transmits with p probability, or waits with (1 - p) probability
• It waits for the next slot, if busy

CSMA with Collision Detection (CSMA/CD)

• Stations listen to the channel while sending data
• Sending is immediately halted and retried after a random backoff time if a collision is detected
• Utilized in Ethernet networks

Wireless LAN and MAC Protocols

• Wireless networks utilize radio signals
• Hidden Station Problem: A device can't detect another device transmitting to the same receiver because it is out of range
• Exposed Station Problem: A station refrains from transmitting because it detects another transmission that wouldn't interfere

Multiple Access with Collision Avoidance (MACA)

• Request to Send (RTS) and Clear to Send (CTS) signals are used to prevent collisions
• Stations that hear RTS/CTS remain silent
• Collisions are still possible if multiple stations send RTS at the same time

Ethernet (IEEE 802.3 Standard)

• Classic Ethernet uses coaxial cable (up to 2.5 km long) and CSMA/CD
  • Standardized to 10 Mbps by DEC, Intel, and Xerox in 1978
  • Modern Ethernet supports 100 Mbps and 1 Gbps
• Switched Ethernet uses hubs or switches instead of shared cables

Network Devices

• Repeaters amplify and retransmit signals at the physical layer
  • They do not process data or addresses
• Hubs connect multiple devices, retransmitting to all ports
  • They operate in the physical layer and create collision domains
• Switches read MAC addresses and forward frames to the correct port
  • They operate at the data link layer
• Bridges connect LAN segments and learn which devices are connected to which ports
  • Forwarding tables are maintained to filter traffic
• Routers operate at the network layer, routing based on IP Addresses
• Gateways convert data formats between different network architectures or protocols

Ethernet Frame Format

• Preamble: Synchronization pattern
• Destination & Source MAC Address: 48-bit unique identifiers
• Type/Length: Protocol type specified
• Data: Payload (up to 1500 bytes)
• Checksum (CRC): Error detection mechanism