Shared link → MAC sublayer → random access (CSMA family) → controlled access → channelization.

Tip R = the channel rate, M = the number of stations. R/M is a fair equal share.

Three families of multiple-access protocols.

Broadcast link = many nodes, ONE medium. Everyone hears every transmission.

Seen again Ch 1 Part 1, slide 8: Multipoint connections

Collision = two frames on the medium at once → both destroyed, bandwidth wasted.

MAC protocols decide who sends when. They live in the MAC sublayer.

Trap In these slides framing and MAC addresses are listed under the MAC sublayer, not LLC.

Memory trick Random = free for all · Controlled = take turns · Channelization = split the channel.

Random access: no master, no turns. Each station decides for itself when to send, so collisions are possible.

CSMA = listen before you talk.

It reduces collisions but can’t eliminate them: a station far away may not hear a frame that already started.

Trap “CSMA eliminates collisions” ✗. It only reduces them.

Long cables = longer window where distant stations wrongly think the line is free.

Three persistence methods: they differ in what to do when the line is BUSY and when it is IDLE.

Non-persistent: busy → walk away for a random time, then check again.

Fewer collisions, but the line can sit idle (wasted).

1-persistent: busy → keep listening → the instant it’s idle, send (probability 1).

No idle time, but if two stations were waiting, they BOTH send → guaranteed collision.

Memory trick Non-persistent = polite (walks away). 1-persistent = selfish (stares at the line).

Best of both: fewer collisions (like non-persistent) and less idle time (like 1-persistent).

p-persistent: draw a random R in [0, 1]; if R ≤ p send, otherwise wait a slot.

Under heavy load, 1-persistent does worst (collisions); small p and non-persistent do best.

Plain CSMA keeps sending the whole (already ruined) frame. CSMA/CD listens WHILE sending and stops as soon as it detects a collision.

Used on shared Ethernet (bus or hub).

Seen again Ch 4, slide 6: CSMA/CD in classic Ethernet

Detection = the energy on the wire is higher than what I am sending → someone else is sending too.

On collision: ABORT → send a 48-bit JAM → random BACKOFF → try again with the same persistence method.

Memory trick CSMA/CD collision steps: “Abort, Jam, Back off, Retry.”

Random access: simple, decentralized, great at low load; UNFAIR and unbounded delay at high load.

Alone on the link → full rate R. Busy link → each node gets less than R/M because of collisions.

Controlled access: stations take turns → fair, no collisions, but capped at R/M.

Trap Random = can collide, unfair, R when alone. Controlled = never collides, fair, R/M.

Each cycle starts with a reservation frame of N minislots (one per station). A station sets its minislot to reserve a turn.

Centralized polling: a PRIMARY controls everything. SELECT = primary wants to send to a secondary. POLL = primary asks a secondary if it has data (data or NAK back).

Memory trick SELect = primary SEnds. POLL = primary coLLects.

Left: primary → secondary uses SEL. Right: secondary → primary starts with a POLL.

A station keeps the token only while it has data AND its allocated time hasn’t expired, then releases it.

Each station has its own frequency band, all the time.

FDMA: own band, send continuously, R/M even when alone (other bands sit empty).

Each station has its own time slot on the whole bandwidth.

TDMA: whole bandwidth, but only in your slot. Still R/M on average even when alone.

Trap FDMA, TDMA (and controlled access) cap a lone station at R/M; random access lets it use all of R.

CDMA: everyone sends at the same time on the same frequency; codes separate them.

Each station multiplies its data by its code; the channel carries the SUM; a receiver multiplies by the wanted code to pull its data out.

FDMA = frequency slices. TDMA = time slices. CDMA = code slices (everyone uses all frequencies all the time).

Memory trick F = Frequency, T = Time, C = Code.