Local Area Network (LAN) Protocols

• Local Area Network (LAN) data link layer protocols explained

Network Connections

• There are two types of network connections.
• Point-to-Point (PPP) connections are used for telephone connections.
• Point-to-point connections exist between the Ethernet and the host.
• Broadcast connections use a shared cable or channel.
• Traditional Ethernet, HFC in upstream, and Wireless LAN 802.11 are examples of broadcast connections.

Interference and Collision

• Connecting to a shared broadcast channel gives rise to interference
• Hundreds or even thousands of nodes can communicate directly on a broadcast channel.
• A collision occurs when nodes receive two or more frames simultaneously.

Line Delays and Collisions

• Let t be the propagation time between the furthest stations.
• The maximum time for collision detection is 2t.

Bus Sharing

• When a frame is sent on the shared medium, all connected devices receive it.
• Each NIC compares its MAC address with the destination address on the frame.
• If they match, the NIC copies the rest of the frame.
• Otherwise, it ignores the frame unless the NIC operates in promiscuous mode, in which case it passes every frame.
• But, what happens when the stations try to transmit at the same time.
• The result: A Collision !

Multiple Access Protocols

• Multiple access protocols are used for connections on a shared broadcast channel.
• Hundreds or thousands of nodes can communicate directly on a broadcast channel.
• When nodes receive two or more frames simultaneously, a collision occurs.
• These protocols determine how nodes regulate their transmissions on the shared channel.
• Communication on the shared channel must use the same channel, without an "out-of-band" channel for coordination.

Ideal Multiple Access Protocols

• In a broadcast channel with a rate of R bits/second:
• When one node needs to send data, it gets a transmission rate of R bps.
• When M nodes need to send data, they each get a transmission rate of R/M bps.
• The protocol is decentralized with no master nodes or clock synchronization.
• The protocol is simple.

Types of Multiple Access Protocols

• Multiple Access Protocols can be classified into three categories:
• Channel partitioning protocols (e.g., time or frequency division).
  • A channel divided into time slots or frequency slots
• Random access protocols (e.g., random access)
  • A channel that is not divided. Collisions may occur.
  • The involved nodes retransmit packets repeatedly
• Taking-turns protocols (collision-free protocols) - Each node has its transmission turn, nodes may have longer turns if they have more information to transmit

Channel Partitioning Protocols: TDMA

• TDMA (Time Division Multiple Access) divides the shared channel into time slots.
• Unused slots remain inactive.
• In an example, slots 1, 3, and 4 have a packet, while slots 2, 5, and 6 are inactive.

Channel Partitioning Protocols: FDMA

• FDMA (Frequency Division Multiple Access) divides the channel into frequency bands.
• Each station has an assigned frequency band.
• In an example, slots 1, 3, and 4 have a packet, while slots 2, 5, and 6 are inactive.

Random Access Protocols

• When a node needs to send a packet:
  • It transmits at the maximum allowed rate on the channel, which is R bps.
  • There is no prior coordination among nodes.
• Two or more transmitting nodes leads to a collision
• Random access protocols define how to detect and recover from collisions.

Examples of protocols

• Slotted ALOHA, ALOHA, CSMA, CSMA/CD, CSMA/CA

Slotted ALOHA Assumptions

• All packets have the same size.
• Time is divided into slots, each equivalent to a packet's transmission time.
• Nodes start transmitting packets only at the beginning of slots.
• Nodes are synchronized.
• If two or more packets collide in a slot, the nodes involved detect the event before the slot ends.

Slotted ALOHA Operations

• When a node has a new packet to send, it waits until the next slot.
• If there is no collision, the node can transmit the packet in the next slot.
• If a collision occurs, the node detects it before the slot ends and retransmits its packet with probability p during subsequent slots.

Slotted ALOHA Pros

• Allows a single node to continuously transmit packets at the maximum channel speed.
• Decentralized; each node detects collisions and decides when to retransmit independently.
• Simple protocol.

Slotted ALOHA Cons

• A fraction of slots will experience collisions.
• A fraction of unused slots remain empty and there is high latency.

Slotted ALOHA Efficiency

• Efficiency is defined as the fraction of successful slots when there are many active nodes constantly sending packets.
• Suppose N nodes each transmit with probability p.
• The probability of success for a given node is p*(1-p)^(N-1).
• The probability that any node is successful is Np*(1-p)^(N-1).
• To maximize efficiency, find p* that maximizes Np(1-p)^(N-1).
• For large N, the limit of Np*(1-p*)^(N-1) is 1/e = 0.37.
• Best Case: only 37% of slots are used.

Pure ALOHA

• Simpler, non-synchronized protocol.
• When a packet arrives, it is transmitted immediately on the broadcast channel.
• This creates a high probability of collision.
• A packet transmitted at time t0 overlaps with other packets sent in the interval [t0-1, t0+1]

ALOHA Efficiency

• Given P(transmission with success from a given node) = P(the node transmits).
• P(no other node transmits in [t0-1, t0]) · P(no other node transmits in [t0, t0+1])
• p (1-p)^(N-1) (1-p)^(N-1)
• p · (1-p)^(2(N-1))
• Choosing the best p as n approaches infinity yields 1/(2e) = 0.18 (worse than before).

Carrier Sense Multiple Access (CSMA)

• Listens before transmitting.
• If the channel is free, transmits the entire packet.
• If the channel is busy, waits for a period of time.
• Analogy: if someone else is speaking, wait until they are done.

Carrier Sensing

• Uses Manchester encoding for signals where bits are encoded by transitions.
• IEEE 802.3 standardizes signal levels at -0.85 and +0.85.
• Absence of signal is encoded as a null signal (idle line).

CSMA with Collisions

• Collisions can still occur due to propagation delay, where two nodes may not detect each other's transmission.
• When a node detects a collision, it ceases transmission immediately.
• The distance and propagation delay significantly affect collision probability.
• A collision is detected when the received signal does not match the transmitted signal, requiring only minimal packet overlap to cause a loss.

CSMA Persistence

• A station that wants to transmit listens to the signal, before doing so, -if the channel is free, transmits the entire packet -if the channel is occupied, continues to listen to find a time that is free

CSMA 1-persistent

• Simple approach where a calculator listens on the medium when it has data to transmit.
  • If the channel is free transmits the frame -If the channel is busy it keeps listening when free it then transmits
• After a collision, the station waits a random time and restarts the process.
• Called "1-persistent" because it transmits with a probability of 1 when the channel becomes free.

CSMA 1-persistent continued

• The propagation delay of the signal impacts importance

More on delay

• Note: regardless of delay it occurs if two transmissions desire to transmit concurrently, while a third station is already transmitting.
• It does not interfere with in-flight transmissions.
• Does not require time-slot listening

CSMA Non Persistence

• Once wanting to transmit
  • listens and finds a non-occupied channel
  • Waits a casual amount of time and retries the process
• This mechanism sensibly reduces the collision but stations need to retransmit
  • if free, generally wait varied times before trying again
  • If occupied, avoid retransmission

Protocol

• This Protocol noticeably raises the efficiency for connected stations
• If highly loaded, the channel between transmission will enormously increase delay

CSMA p-persistent

• In the newer edition of the protocol the time is sectioned into aloha slots
  • continuously listens
    • transmits with probability
    • otherwise it awaits a new slot -if slot is free transmits, or awaits new slot.

Key Points

• A means to a random access is used toward the protocol
• For high uploads and fewer values, can raise efficient use, but rises transmission delay

CSMA/CD

• The protocol works in three phases:
  • carrier sense: an attempt by a device that wishes to take use of the bus, and transmits whether its available with listen before talking multiple access: despite carrier access it is likely for 2 or more networks to attempt to transmit at the same time -collision detection: occurs when a transmission has collison and attempts again

CSMA / CD

• The stations stop transmission when they detect collision.
• After a transmission, there may be a contention period if two or more stations start transmitting simultaneously.
• The duration of each contention slot depends on the propagation delay (distance between transmitting stations).
• Dead periods occur when no station has a frame to transmit.

CSMA/CD: Jamming

• After a collision, actions taken:
  • The transmitter suspends, transmits, and communicates (interference). The purpose here is notify the collisions and the transmitter -In listening mode, recognize where collisions are
• Repeat the pseudo attempt a small number of times

Collision-Free Protocols: Reservation

• Elementary bit map
  • the network will have N station. Numbered from the range 0, to N – I
• After each transmission they enter a period of contention, station that transmit value is denoted with 1 If the transmission is permitted.
• When contention is over for each station, the station must then receive transmissions at a turn
  • the stations receiving any given data they must wait for the other stations to complete.

Collision-Free Protocols: Reservation Notes

• the protocols will be cheap for higher values of N and transmission
• in these instances, the station needs to hold all n bits for other stations to not transmit

Transmission

• In an overload, over head bit are used and lowers the effectiveness

Collision Freedom: Allocation

• Effective communication = (communication in mind / transmission)
• effective data usage = the amount of bit transmitted per frame/ (carrico Complessivo)
• the higher the efficiency means more content

Protocol collision free :Token Ring

• Does not utilize broadband mass mass media, will connect the succession for topology Each Interface - uni directional ring, with stations

Token Ring Notes

• Utilizes IEEE 802.5
• Token ring will manage the utilization / ring topology
• Can have one token or multiple tokens ( multiple rings

Token Structure

• The stations are allowed to pass data when they can posses a token, (which passes a frame to the Data)
• Upon completion the transmission passes a frame through the data Can be 4-16 Mb

Token Ring Details

• The update makes possible utilization between 2 token with fibre at FDDI (fiber distributed) running about 100 Mbps

Interface token Structure- Each byte goes through a buffer