Two types of Links: -> Point-To-Point (link between Ethernet switch, host) -> Broadcast (shared wired or medium)

collsions if node receives two or more signals at the same time

Multiple access protocol: → distributed algorithms that determines when nodes can transmit → fundamental component of the network

given: multiple access channel (MAC) of rate R bps desiderata:

1. when one node wants to transmit, it can send at rate R.
2. when M nodes want to transmit, each can send at average rate R/M
3. fully decentralized: • no special node to coordinate transmissions • no synchronization of clocks, slots
4. simple

MAC protocols: taxomony -> channel partitioning - devides channel into smaller exclusive use pieces -> random access - channel not divided, causing collisions to be solved -> taking turns - take turns, but nodes with more to send can take longer turns

TDMA: time division multiple access

FDMA : Frequency Division Multiple Access

Random access protocols

• when node has packet to send
  -> transmit at full channel data rate R
  -> no a priori coordination among nodes
  - two or more transmitting nodes: “collision”
• random access protocol specifies:
  -> how to detect collisions
  -> how to recover from collisions (e.g., via delayed retransmissions)
• examples of random access MAC protocols:
  -> ALOHA, slotted ALOHA
  -> CSMA, CSMA/CD, CSMA/CA

Slotted ALOHA

• all frames same size
• time divided into equal size slots
• nodes start to transmit only at slot beginning
• nodes are synchronized
• if 2 or more nodes transmit in slot, all nodes detect collisions

When a node obtains fresh slot, transmits in next slot → if no collision - node can send new frame in next slot -> if collision - node re transmits frame in each subsequent slot with prob p until success

Pure (unslotted) ALOHA

• Simpler, no sync
• when frame first arrives transmits immediately
• collisions probability increases
• worse than slotted ALOHA

CSMA (carrier sense multiple access)

• listen before transmit

• If channel sensed idle: transmit entire frame

• if channel sensed busy: defer transmission

• collisons can still occur: two nodes may not hear each other causing delay

• distance and propagation delay play role in determining collision probability → human analogy: donʼt interrupt others!

CSMA/CD (CSMA with colission detection)

• CSMA/CD reduces the amount of time wasted in collisions
• transmission aborted on collision detection

CSMA/CD: CSMA with collision detection

• collisions detected within short time
• colliding transmissions aborted, reducing channel wastage
• collision detection easy in wired, difficult with wireless -> human analogy: the polite conversationalist → better performance than ALOHA and cheaper and simples to implement

CSMA/CA

• Used in wireless2

• Listens to the channel (carrier sense)

• Waits if the channel is busy (multiple access)

• Random backoff time before sending (collision avoidance)

• May use RTS/CTS (Request to Send / Clear to Send) to reduce risk of collision in hidden node scenarios

• It does NOT detect collisions after they happen.

• Unlike CSMA/CD (used in Ethernet), Wi-Fi devices cannot detect collisions while transmitting (half-duplex radios).

• So, CSMA/CA tries to prevent them, but if a collision happens, the devices just don’t get an ACK and assume a failure.

• O tamanho mínimo da trama Ethernet é 64 bytes (512 bits).

• Esse valor foi definido para garantir que a colisão seja detectada mesmo na maior distância possível entre nós na rede

if a collision still occurs?

• No acknowledgment (ACK) is received.

• The sender waits and retries after a random backoff.

• This is how Wi-Fi handles retries — not through detection, but through lack of ACK and retry logic.

• CSMA/CA does not detect collisions, it avoids them

Taking Turns MAC

Polling:

• master node "invites" slave nodes transmit in turn
• typically used with "dumb" devices
• Cons: -> polling overhead -> latency -> single point of failure(master)
• Bluetooth uses polling

Token Passing:

• control token passed from one node to the other sequencially
• token message
• Cons: -> token overhead -> latency -> single point failure(token)

Summary of MAC protocols

• channel partitioning, by time, frequency or code -> Time Division, Frequency Division
• random access (dynamic), -> ALOHA, S-ALOHA, CSMA, CSMA/CD -> carrier sensing: easy in some technologies (wire), hard in others (wireless) -> CSMA/CD used in Ethernet -> CSMA/CA used in IEEE 802.11 (Wi-Fi)
• taking turns -> polling from central site, token passing -> Bluetooth, FDDI, token ring