Slotted ALOHA Protocol Quiz

Podcast

Play an AI-generated podcast conversation about this lesson

Questions and Answers

What is a characteristic of the Slotted ALOHA protocol?

It allows multiple nodes to transmit simultaneously without any collisions.

Only one node can be active at a time.

Nodes can transmit at any time without synchronization.

Time is divided into equal-sized slots for transmission. (correct)

Which of the following protocols is categorized as a random access MAC protocol?

Token Ring

Frequency Division Multiple Access (FDMA)

Slotted ALOHA (correct)

Time Division Multiple Access (TDMA)

What happens when two or more nodes transmit in the same slot during Slotted ALOHA?

Only one node's data is sent successfully.

A collision is detected by all transmitting nodes. (correct)

The transmission is successful for all nodes.

The slot is automatically reused by all nodes.

Which statement best describes the efficiency of Slotted ALOHA?

The maximum efficiency is $1/e$ which is approximately 0.37. Signup and view all the answers

What is a disadvantage of the Slotted ALOHA protocol?

Collisions can waste slots and lead to idle times. Signup and view all the answers

What is the primary goal of channel partitioning in MAC protocols?

To divide the communication channel into smaller, manageable segments Signup and view all the answers

In Time Division Multiple Access (TDMA), what happens to unused slots?

They go idle until the next round Signup and view all the answers

What is the main characteristic of Frequency Division Multiple Access (FDMA)?

The spectrum is divided into frequency bands for each station Signup and view all the answers

Which of the following best describes random access protocols?

Nodes transmit whenever they have data, allowing potential collisions Signup and view all the answers

Which MAC protocol allows longer transmission times for nodes with more packets to send?

Taking Turns Protocol Signup and view all the answers

In a TDMA protocol with 6 stations, if stations 1, 3, and 4 have packets to send, what will happen to slots 2, 5, and 6?

They will remain idle and not be used in this round Signup and view all the answers

What is a disadvantage of channel partitioning MAC protocols?

They often result in underutilization of the channel Signup and view all the answers

What do random access protocols primarily rely on for collision management?

Protocols to recover from collisions after they occur Signup and view all the answers

What is the maximum number of bits that can be detected in a burst error by the CRC method?

r+1 bits Signup and view all the answers

Which of the following is NOT an example of multiple access methods?

Direct link Signup and view all the answers

Which protocol is used for dial-up access in point-to-point links?

PPP Signup and view all the answers

What happens in a multiple access protocol when two or more nodes transmit simultaneously?

Collision occurs Signup and view all the answers

In the context of an ideal multiple access protocol, what is the average rate at which each node can send data when M nodes want to transmit?

R/M Signup and view all the answers

What is a key characteristic of a decentralized multiple access protocol?

No special node coordinates transmissions Signup and view all the answers

Which of the following statements about Ethernet as a multiple access protocol is true?

Interference can cause collisions. Signup and view all the answers

In what scenario does a node on a shared channel NOT transmit correctly?

When the channel is busy Signup and view all the answers

What is the goal of error detection in data communication?

To identify and correct errors Signup and view all the answers

What is a characteristic of broadcast links in multiple access protocols?

They involve shared communication medium. Signup and view all the answers

What action does the NIC take when it detects another transmission while it is already transmitting?

Aborts the transmission and sends a jam signal Signup and view all the answers

What does the variable K represent in the backoff algorithm of the NIC?

The random number chosen from a specific set Signup and view all the answers

How is the efficiency of CSMA/CD calculated?

Efficiency = 1 / (1 + 5tprop / ttrans) Signup and view all the answers

Which feature distinguishes random access MAC protocols from channel partitioning MAC protocols?

Utilization of the channel by a single node Signup and view all the answers

In polling protocols, who initiates the transmission of data?

The master node Signup and view all the answers

What is a significant drawback of polling in ‘taking turns’ MAC protocols?

Latency and polling overhead Signup and view all the answers

In the context of MAC protocols, what does token passing achieve?

Share the channel fairly Signup and view all the answers

Which statement accurately describes CSMA/CD's performance as tprop approaches zero?

Efficiency approaches 1 Signup and view all the answers

What happens when the NIC successfully transmits an entire frame without detecting any transmission?

It is done with the frame Signup and view all the answers

Which of the following describes channel partitioning MAC protocols?

They allocate 1/N bandwidth even with 1 active node Signup and view all the answers

Study Notes

Chapter 5: Link Layer

PowerPoint slides are freely available to faculty, students, and readers
Slides can be modified and content changed to meet needs
If used in class, the source must be mentioned
If posted online, the adapted/identical nature and copyright must be noted

Chapter 5: Link Layer Goals

Understand principles behind link layer services
Error detection and correction
Sharing a broadcast channel (multiple access)
Link layer addressing
Local area networks (Ethernet, VLANs)
Instantiation and implementation of various link layer technologies

Link layer, LANs: outline

Introduction, services
Error detection, correction
Multiple access protocols
LANs (addressing, ARP, Ethernet, switches, VLANs)
Link virtualization: MPLS
Data center networking
A day in the life of a web request

Link Layer: Introduction

Hosts and routers are nodes
Communication channels connect adjacent nodes along the communication path (links).
Links include wired links, wireless links, and LANs
Layer-2 packet: frame, encapsulates datagram
Data-link layer is responsible for transferring datagram from one node to a physically adjacent node over a link

Link Layer: Context

Different link protocols transfer datagram over different links (e.g., Ethernet on the first link and frame relay on intermediate links)
Each link protocol provides different services (e.g., may or may not provide reliable data transfer (RDT) over link)

Link Layer Services

Framing and link access: Encapsulating datagram into frame, adding header and trailer
Channel access if shared medium
MAC addresses for identifying source and destination, different from IP addresses
Reliable delivery between adjacent nodes (described in Chapter 3) is seldom used for low-error links (e.g., fiber or some twisted pair)
Wireless links experience high error rates
Why both link level and end-end reliability?

Link Layer Services (more)

Flow control: pacing between adjacent sending and receiving nodes
Error detection: errors caused by signal attenuation or noise; receiver detects errors, signals sender for retransmission or drops frames
Error correction: receiver identifies and corrects bit errors without retransmission
Half-duplex and full-duplex: with half-duplex, nodes at both ends of a link can transmit but not at the same time.

Where is the Link Layer Implemented?

Implemented in each host network adapter (NIC) or chip
Ethernet card, 802.11 card; Ethernet chipset implements link and physical layers
Attaches to host system buses
Combination of hardware, software, and firmware

Adaptors Communicating

Sending side: encapsulates datagram in frame, adds error checking bits, RDT, and flow control.
Receiving side: looks for errors, performs RDT, flow control, and extracts datagram to upper layer.

Error Detection

EDC (Error Detection and Correction) bits provide redundancy
Data is protected by error checking.
Protocols may miss some errors, but rarely, and larger EDC fields provide better detection and correction.

Parity Checking

Single bit parity: detects single bit errors
Two-dimensional bit parity: detects and corrects single bit errors

Internet Checksum (review)

Goal: detect errors (e.g., flipped bits) in transmitted packets (used at transport layer only)
Sender: treats segment contents as sequence of 16-bit integers, calculates checksum (addition's one's complement sum) of segments, puts checksum value in UDP checksum field.
Receiver: computes checksum of received segment, checks if computed checksum equals checksum field value (NO=error detected).

Cyclic Redundancy Check

More powerful error-detection coding
Uses data bits (D) and CRC bits (R)
Goal: ensure <D, R> is exactly divisible by G (modulo 2)
Widely used (Ethernet, 802.11, WiFi, ATM)
Can detect all burst errors with length less than r+1 bits
D*2^r XOR R = nG

CRC Example

Calculation of remainder (R) when dividing D^2^r by G

Multiple Access Links, Protocols

Point-to-point: PPP for dial-up, point-to-point link between Ethernet switch and host
Broadcast: old-fashioned Ethernet, upstream HFC, 802.11 wireless LAN
Shared wire (e.g., wired Ethernet)
Shared RF (e.g., 802.11 WiFi)
Shared RF (satellite)
Human(s) at a cocktail party

Multiple Access Protocols

Single shared broadcast channel, two or more simultaneous transmissions by nodes.
Interference and collisions (if a node receives two or more signals at the same time).
Distributed algorithm determines how nodes share the channel; determines when a node can transmit
Communication must use the channel itself (no out-of-band channel for coordination)

An Ideal Multiple Access Protocol

A broadcast channel with rate (R bps)
If one node wants to transmit, it can send at rate R, and when M nodes want to transmit, rate R/M.
Fully decentralized, no special node or synchronization of clocks or slots, and simple
MAC protocols: channel partitioning, random access, taking turns

MAC Protocols: Taxonomy

Three broad classes: channel partitioning, divide channel into smaller pieces and allocate each piece to node for exclusive use
Random access: channels not divided; allow collisions & "recover" from them
Taking turns: nodes take turns, but those with more to send take longer turns

Channel Partitioning MAC Protocols: TDMA

TDMA (time division multiple access)
Access to channel in "rounds"
Each station gets a fixed length slot in each round.
Unused slots are idle.
Example: 6-station LAN with 1, 3, and 4 having packets, slots 2, 5, and 6 are idle

Channel Partitioning MAC Protocols: FDMA

FDMA (frequency division multiple access)
Channel spectrum divided into frequency bands
Each station is assigned a fixed frequency.
Unused transmission time in frequency bands goes idle.

Random Access Protocols

When a node has a packet to send, it transmits at full channel data rate.
Two or more transmitting nodes generate collisions
Random access MAC protocol specifies how to detect and recover from collisions (e.g., via delayed retransmission).
Examples: slotted ALOHA, ALOHA, CSMA, CSMA/CD, CSMA/CA

Slotted ALOHA

Assumptions: all frames same size, time divided into equal size slots. Nodes start transmitting only at beginning of slot, nodes synchronized.
Operation: when a node obtains a fresh frame, it transmits in the next slot. If no collision, node can send a new frame in the next slot. If a collision occurs, the node retransmits the frame in each subsequent slot with a probability (p) until success.

Slotted ALOHA: Efficiency

Long-run fraction of successful slots, given many nodes each with many frames to send.
Probability of success = Np(1-p)^N-1 (N nodes, probability p for transmission in a slot)

Pure (Unslotted) ALOHA:

Simpler than slotted ALOHA, no synchronization
Transmits immediately when a frame arrives
Collision probability increases when multiple frames sent in same time slot

Pure ALOHA Efficiency

Probability of success, given other nodes don't transmit in the frame's time slot.
Using optimum (p) and larger (n) generates even worse efficiency than slotted ALOHA

CSMA (Carrier Sense Multiple Access)

Listen before transmit
If channel sensed idle, transmit entire frame.
If channel sensed busy, defer transmission
Human analogy: don't interrupt others.

CSMA Collisions

Collisions can occur due to propagation delay.
Two nodes may not hear each other's transmissions.
Collision means entire packet transmission time is wasted.
Distance and propagation delay determine the probability of collision

CSMA/CD (Collision Detection)

Carrier sensing, deferral, and collision detection within a short time.
Colliding transmissions aborted, reducing channel wastage
Easy in wired LANs (measure signal strengths).
Difficult in wireless LANs (received signal strength overwhelmed by local transmission strength)
Human analogy: being a polite conversationalist

Ethernet CSMA/CD Algorithm

NIC receives datagram from network layer; creates frame.
If NIC senses channel idle, starts frame transmission. If busy, waits until idle.
If NIC transmits entire frame without detecting another transmission, it completes.
If any collision occurs during transmission, the NIC sends a jam signal and enters into binary exponential back-off.

CSMA /CD Efficiency

Efficiency = 1 / (1 + 5 * Tprop/ Ttrans), where Tprop = maximum propagation delay between two nodes and Ttrans = time to transmit.
Efficiency tends toward 1 as Ttrans goes to infinity, and toward 0 as Tprop approaches 0.

“Taking Turns” MAC Protocols

Channel partitioning MAC: efficient at high load, but inefficient at low load. Delay in channel access when only one or few active nodes.
Random access MAC: efficient at low load, but has collision overhead at high load.
Taking turns protocols look for a balance of both world

Polling

Master node invites slave nodes to transmit in turn.
Typically used with "dumb" slave devices, but problems include polling overhead, latency, and single point of failure.

Token Passing

Control token passed from one node to the next sequentially.
Concerns: token overhead, latency, and potential single point of failure

Cable Access Network

Internet frames, TV channels, and control transmitted downstream.
Multiple downstream (broadcast) channels using a single CMTS (Cable Modem Termination System).
Multiple upstream channels for users to contend for certain upstream channel time slots.
DOCSIS (data over cable service interface specification) uses FDM over upstream and downstream channels. TDM upstream: assigned and contention; MAP frame assigns upstream slots, and request for up/down stream slots are made and transmitted in random access (binary backoff) in selected slots.

Ethernet Switch

Link-layer device, takes an active role
Stores and forwards Ethernet frames
Examines incoming frame MAC addresses and selectively forwards to one or more outgoing links.
Uses CSMA/CD to access the segment
Transparent: hosts unaware of switches.
Plug-and-play, self-learning; switches don't need configuration.

Switch: Multiple Simultaneous Transmissions

Hosts have dedicated direct connections to switches
Switches buffer packets
Ethernet protocol used on each incoming link; no collisions
Each link is its own collision domain
Switching: A-to-A' and B-to-B' can transmit simultaneously without collisions

Switch Forwarding Table

How switch knows A' reachable via interface 4, B' via 5.
Each switch has a table. Each entry includes MAC address of host, interface to reach host, and time stamp, which makes it look like a routing table.

Switch: Self-Learning

Switch learns which hosts can be reached through which interfaces.
When a frame is received, the switch learns the sender's location. (incoming LAN segment)
Records sender/location pair in the switch table.

Switch: Frame Filtering/Forwarding

Records incoming link and MAC address of sending host.
Indexes the switch table using the destination MAC address, and checks if an entry is found.
If the destination is on the segment, the frame is dropped; otherwise, the frame is forwarded. The algorithm floods the frame on all interfaces except the one it arrived on.

Self-learning, Forwarding: Example

Frame destination unknown; flood.
Destination A location known; selectively send the frame on just one link.

Interconnecting Switches

Switches can be interconnected.
Self-learning mechanism works exactly the same as in the single-switch case.

Virtual LANs (VLANs)

Motivation: CS user moves office to EE but wants access to CS switch.
Single broadcast domain: all layer-2 broadcast traffic (ARP, DHCP, unknown destination MAC addresses) must cross entire LAN. Security/ privacy issues, and efficiency issues.

Port-Based VLAN

Switch ports grouped (by management software).
Single physical switch operates as multiple virtual switches.

Port-based VLAN (cont.)

Traffic isolation: frames to/from ports 1-8 reach only ports 1-8.
Dynamic membership: ports can be dynamically assigned to different VLANs.

VLANs Spanning Multiple Switches

Trunk ports: carry frames between defined VLANs over multiple switches.
Frames forwarded within VLANs between switches can use a vanilla 802.1 protocol. It adds/removes additional header fields for frames that are forwarded between trunk ports.

802.1Q VLAN Frame Format

2-byte Tag Protocol Identifier (value: 81-00)
Tag Control Information (12-bit VLAN ID, 3-bit priority field).

Data Center Networks

10's to 100's of thousands of closely coupled hosts.
Includes e-business, content servers, search engines, and data mining.
Challenges: managing/balancing load, avoiding processing and networking bottlenecks

Data Center Networks (load balancer)

Load balancer receives external client requests, directs workload within the data center, and returns results to external clients (hiding data center internals).

Data Center Networks (Rich Interconnection)

Rich interconnection among switches and racks result in increased throughput and reliability via redundancy.

Synthesis: A Day in the Life of a Web Request

Journey down the protocol stack (application, transport, network, link layer).
Putting-it-all-together: understanding protocols involved in seemingly simple scenarios.
Scenarios like a student requesting www.google.com from their laptop connected to a campus network.

A Day in the Life… Scenario

A diagram visualizing the process, including parts of the school network, Comcast network, the DNS server, and Google's network.

A Day in the Life… Connecting to the Internet

DHCP assigns IP address, router address, and DNS server address.
DHCP request encapsulated in UDP; encapsulated in IP, and encapsulated in 802.3 (Ethernet)
Ethernet frame broadcast on LAN, received by DHCP server.

A Day in the Life… ARP (before DNS, before HTTP)

Before sending HTTP request that requires IP address of www.google.com, a DNS query must be made.
DNS query is encapsulated in UDP, encapsulated in IP, encapsulation into Eth. To send a frame to a router, the MAC address of the router interface is necessary (ARP).
ARP query is broadcasted to the network.
Router replies with an ARP reply containing the MAC address of the router interface.

A Day in the Life… Using DNS

IP datagram that contains a DNS query is forwarded via LAN switch to the first-hop router.
IP datagram forwarded from campus network into Comcast network
Routed via RIP, OSPF, IS-IS or BGP routing protocols to DNS server
Demuxed and responded to with IP address of www.google.com.

A Day in the Life... TCP Connection Carrying HTTP

Client initially opens TCP socket to web server to send HTTP request.
TCP SYN segment is sent to web server, inter-domain routed.
Web server responds with SYNACK segment in the 3-way handshake
TCP connection established once the handshake is complete

A Day in the Life... HTTP Request/Reply

HTTP request sent to TCP socket, IP datagram containing HTTP request routed to web server at www.google.com
Web server responds with HTTP reply. The reply, including the web page, is contained within the IP datagram and routed back to client.

Chapter 5 Summary

Principles behind data link layers and their services, error detection, correction, sharing broadcast channels, and link-layer addressing.
Installation and implementation of various link layer technologies (Ethernet, switched LANs, VLANs)
Virtualized networks (MPLS)
Synthesis: a day in the life of a web request.
Summary of additional topics about the concepts in the lesson.

Chapter 5: Let's take a breath!

Journey down protocol stack is complete (except PHY).
Solid understanding of networking principles and practice.
Potentially stopping here, but introducing topics like wireless, multimedia, security, network management.

Studying That Suits You

Use AI to generate personalized quizzes and flashcards to suit your learning preferences.

Description

Test your knowledge on the Slotted ALOHA protocol with this quiz. You'll answer questions about its characteristics, efficiency, disadvantages, and how it operates in random access scenarios. Perfect for those studying network protocols or computer communications.