Slotted ALOHA Protocol Quiz

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Questions and Answers

What is a requirement for nodes in slotted ALOHA?

Nodes require a centralized control.

Frames must be of variable size.

Time must be divided into equal size slots. (correct)

Nodes must transmit at any time.

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

Only one node succeeds in transmission.

A collision occurs, affecting all nodes transmitting. (correct)

Transmission is delayed by one slot for each node.

All nodes receive their frames successfully.

What is the maximum efficiency of slotted ALOHA as the number of nodes approaches infinity?

0.25

1/e (approximately 0.37) (correct)

0.5

What is a disadvantage of slotted ALOHA?

It requires nodes to be synchronized. Signup and view all the answers

Which of the following is NOT an example of a random access MAC protocol?

Token Ring Signup and view all the answers

What is the main strategy used in random access MAC protocols to handle collisions?

Delayed retransmissions based on probability. Signup and view all the answers

What is a key characteristic of slotted ALOHA operation?

Nodes transmit in the next available time slot after acquiring a fresh frame. Signup and view all the answers

What is the primary purpose of DHCP in the scenario described?

To provide the client with a unique IP address Signup and view all the answers

Which protocol is used to encapsulate DHCP requests?

UDP Signup and view all the answers

What action does the DHCP server take after receiving a DHCP request?

It formulates a DHCP ACK with relevant information. Signup and view all the answers

Before an HTTP request can be sent, what must be obtained?

The IP address of <a href="http://www.google.com">www.google.com</a> Signup and view all the answers

Which layer of the network stack is responsible for encapsulating the DNS query?

Transport Layer Signup and view all the answers

What is the function of a MAC address in a network?

Identifying the physical interface for forwarding frames within the same network Signup and view all the answers

What type of addressing is associated with ARP?

It resolves logical addresses to physical MAC addresses Signup and view all the answers

Which MAC protocol is used in Ethernet technology?

CSMA/CD Signup and view all the answers

What does DOCSIS stand for?

Data Over Cable Service Interface Specification Signup and view all the answers

In the context of cable access networks, what does FDM stand for?

Frequency Division Multiplexing Signup and view all the answers

Which of the following best describes the concept of Time Division in MAC protocols?

Each device gets a fixed amount of bandwidth for communication Signup and view all the answers

How are MAC address spaces typically allocated?

Purchased from the IEEE by manufacturers to ensure uniqueness Signup and view all the answers

What is the primary concern related to token overhead in network protocols?

The time taken to manage the token for access to the network Signup and view all the answers

What type of multiple access method is used for upstream channels in cable networks?

Time slots are assigned, while others compete for the available slots Signup and view all the answers

What is the main goal of Multiprotocol Label Switching (MPLS)?

To enable high-speed IP forwarding using fixed length labels Signup and view all the answers

What does the Tag Control Information in an 802.1Q frame include?

A 12-bit VLAN ID field and a 3-bit priority field Signup and view all the answers

Which of the following is NOT a characteristic of MPLS?

IP address is removed from the IP datagram Signup and view all the answers

What element allows for quick lookups in MPLS?

Fixed length labels Signup and view all the answers

What does the data payload in an 802.1 frame primarily include?

User data intended for transmission Signup and view all the answers

Which component is NOT typically part of a link-layer frame?

IP header Signup and view all the answers

What field does the fixed length label in MPLS contain?

A 20-bit label Signup and view all the answers

In data communication, what do VLANs help to achieve?

Segregate network traffic Signup and view all the answers

How does MPLS differ from traditional IP routing?

It utilizes fixed length labels for routing decisions Signup and view all the answers

What is indicated by the 802.1 frame's CRC?

Integrity of the frame data Signup and view all the answers

What is the primary function of a load balancer in a data center network?

To direct workload and handle client requests Signup and view all the answers

Which of the following is NOT listed as a challenge in managing data center networks?

Integrating cloud services Signup and view all the answers

What type of routing does a load balancer perform?

Application-layer routing Signup and view all the answers

What benefit does increased interconnection among switches provide in a data center?

Increased throughput and reliability Signup and view all the answers

Which component typically directs client requests to specific servers within a data center?

Load balancer Signup and view all the answers

What does the term 'TOR switches' refer to in a data center network?

Top of Rack switches Signup and view all the answers

Which of the following protocols is related to error detection and correction in link layer networking?

Ethernet Signup and view all the answers

What does the acronym ARP stand for in LAN addressing?

Address Resolution Protocol Signup and view all the answers

Which of these describes the journey of a web request within the protocol stack?

It completes a cycle through all layers before reaching the server. Signup and view all the answers

What is meant by 'link virtualization' in the context of LANs?

Creating multiple virtual links over physical connections Signup and view all the answers

Study Notes

Chapter 5: Link Layer

The link layer is responsible for transferring datagrams from one node to another physically adjacent node over a link.
It encapsulates datagrams in frames.
Hosts and routers are nodes.
Communication channels that connect adjacent nodes along communication paths are links (wired links, wireless links).
Link layer services cover framing, link access for encapsulating datagrams into frames, adding headers, and trailers, channel access if needed, and MAC addresses in frame headers to identify source and destination.
Reliable delivery is between adjacent nodes.
Flow control paces adjacent sending and receiving nodes.
Link layer services include error detection, error correction, and half-duplex and full-duplex.
Error detection and correction are crucial for handling signal attenuation, noise.
Receivers detect and signal for retransmission or drop frames.
Receivers identify and correct bit errors (without resorting to retransmission) with flow control.

Link Layer Implementation

Link layer is implemented in network interface cards (NICs) or on a chip in each host.
Ethernet cards, 802.11 cards, and Ethernet chipsets implement link and physical layers.
These attach to the host's system buses.
They combine hardware, software, and firmware.

Adaptors Communication

Sending side encapsulates datagrams in frames, adds error checking bits, rdt, flow control, etc.
Receiving side looks for errors, rdt, flow control, etc. Extracts the datagram, and passes it up to the receiver’s upper layer.

MAC Protocols: Taxonomy

Three classes of MAC protocols include channel partitioning, random access, and taking turns.
Channel partitioning divides the channel into smaller pieces (time slots, frequency, code).
Random access allows collisions.
Taking turns involves nodes taking turns; nodes with more to send might take longer turns.

Channel Partitioning MAC Protocols: TDMA and FDMA

TDMA (Time Division Multiple Access) access channel in rounds. Each station receives a fixed length slot in each round, and unused slots are idle.
FDMA (Frequency Division Multiple Access) divides the channel spectrum into frequency bands with each station assigned to a fixed frequency band. Unused time goes idle.

Random Access Protocols

When a node wants to send a packet, it transmits at full channel data rate R; no prior coordination.
Two or more nodes transmitting → collision. Random access MAC protocol specifies how to detect and recover from collisions.

Slotted ALOHA

All frames have the same size, and time is divided into equal size slots (time for transmitting one frame).
Nodes start to transmit at the beginning of a slot.
Nodes are synchronized.
If two or more nodes transmit in the same slot, all nodes detect collision.

Slotted ALOHA Efficiency

Maximizes efficiency by finding the probability that maximizes the long run fraction of successful slots given many nodes with many frames to send.
The max efficiency is 1/e = .37

Pure (Unslotted) ALOHA

Simpler than slotted ALOHA with no synchronization.
When a frame arrives, the node transmits immediately.
Collision probability increases when the frame sent at t₀ collides with other frames sent in [to-1,t₀+1].

Pure ALOHA Efficiency

Probability of success by a given node is the probability of no other node transmitting in [t₀-1,t₀].
The efficiency is worse than Slotted ALOHA with a value of 1/(2e) = .18.

CSMA (Carrier Sense Multiple Access)

Listen before transmission.
If the channel is sensed idle, transmit the entire frame, otherwise, defer the transmission to prevent collisions from occurring.
Human analogy: don’t interrupt others.

CSMA Collisions

Collisions can occur because propagation delays occur.
Two nodes may not hear each other's transmission, and thus, the entire packet transmission time is wasted.

CSMA/CD (Collision Detection)

Collisions are detected in a short time.
Colliding transmissions are aborted, reducing channel wastage.
Easily done with wired LANs.
Difficult with wireless LANs.

Ethernet CSMA/CD Algorithm

Step 1: NIC receives, creates frame.
Step 2: If NIC senses idle, transmits; if busy, waits until idle and transmits.
Step 3: If NIC transmits the entire frame and no more transmissions occur, NIC is done with frame.
Step 4: If NIC detects transmission while transmitting, it aborts the transmission.
Step 5: After aborting the transmission, the NIC enters binary (exponential) backoff.

CSMA/CD Efficiency

The efficiency is 1/(1+5 * (propagation delay / transmission time))
Efficiency increases as propagation delay decreases and transmission time increases.
Better performance than ALOHA.

"Taking Turns" MAC Protocols

Channel partitioning MAC protocols share the channel efficiently in a high load.
Inefficient at low load due to delays in channel access.
Random access MAC protocols are efficient at low load because a single node fully utilizes the channel.
High load with collision overhead.

Polling

The master node invites slave nodes to transmit in turn.
Typically used with dumb slave devices.
Concerns include polling overhead, latency, and single point of failure.

Token Passing

Control token is passed sequentially.
Considers token overhead and latency, and single point of failure.

Cable Access Network

Internet frames, TV channels, and controls are transmitted downstream at different frequencies.
Multiple 40 Mbps downstream channels and single CMTS transmissions are present.
Multiple 30 Mbps upstream channels allow multiple access.

DOCSIS

Data over Cable Service Interface Specification uses FDM over the upstream and downstream channels.
TDM over upstream channels allows some slots for assignment with some contention.
Downstream MAP frame assigns upstream slots, and requests for upstream slots (and data) are transmitted through random access (binary backoff).

Virtual LANs

A switch (or switches) supports VLAN capabilities to define multiple virtual LANs over a single physical LAN infrastructure to group the ports.
Port-based VLAN groups switch ports, so that single physical switches operate as multiple virtual switches. Using a combination of ports and MAC addresses of endpoints enables dynamic membership among VLANs.

Ethernet Switch

A link-layer device takes on an active role, storing and forwarding Ethernet frames.
It examines incoming frame MAC addresses.
Selectively forwards frames based on outgoing links.
It uses CSMA/CD to gain access to the segment.
It is transparent, hosts are not aware of its presence, and plug-and-play.

Switch Forwarding Table

A switch maintains a table to determine if a host is reachable through a specific interface.
The table contains the MAC address of the host, the interface to reach the host, and a timestamp.

Switch (Self-Learning)

Switches learn which hosts are reachable and through which interfaces.
The switch learns this when frames arrive; the location of the sender is recorded.
The sender/location pair is then stored in the switch table.

Self-Learning, Forwarding, Example

When a frame’s destination location is unknown, the switch floods the frame to all ports.
If the destination’s location is known, the switch selectively sends the frame on a single link.

Interconnecting Switches

Switches can be connected together.
The protocols enable self-learning to work exactly as in the single-switch case.

Data Center Networks

Data center networks involve thousands of closely coupled hosts.
Challenges involve managing/balancing the load among the applications.
Load balancers are crucial for directing external requests to the appropriate workloads.

Synthesis: a Day in the Life of a Web Request

Journey down the protocol stack is complete.
A solid understanding of networking principles along with the practice can be gained by revisiting the protocols involved in a seemingly simple scenario, such as the request to receive www.google.com.
A student connects to the campus network and requests a webpage from www.google.com

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Description

Test your knowledge on the slotted ALOHA protocol and its characteristics. This quiz covers the requirements for nodes, collision handling, efficiency, and disadvantages associated with slotted ALOHA. Ideal for students and professionals exploring random access MAC protocols.