Chapter 6 - Data Link Layer

What are two basic access control methods for shared media?

Contention-based access Controlled access

Describe a hybrid WAN technology.

It's a variation or combination of any topologies. For example, a partial mesh in which some, but not all, end devices are interconnected.

Which layer of the OSI model is Data Link Layer? (number)

2

What is Request for Comments (RFCs)?

A document that describes the standards, protocols, and technologies of the Internet and TCP/IP.

What does the data link layer add to the packet passed from network layer?

A header and a trailer

What is multiaccess network?

A network that can have two or more end devices attempting to access the network simultaneously.

How the logical topology changes when adding physical devices in between the nodes?

A source and destination node may be indirectly connected to each other over some geographical distance using multiple intermediary devices. However, the use of physical devices in the network does not affect the logical topology.

What is typical for Contention-based access to shared media?

All nodes are operating in half-duplex, competing for the use of the medium.

What is typical for full-duplex communication?

Both devices can simultaneously transmit and receive on the shared media. The data link layer assumes that the media is available for transmission for both nodes at any time. Switches operate in full-duplex.

What is typical for half-duplex communication?

Both devices can transmit and receive on the media but cannot do so simultaneously. Hubs operate in half-duplex.

What technology is used for error detection in a frame?

CRC (Cyclic Redundancy Check)

What protocols correspond to Contention-based access to shared media?

Carrier sense multiple access with collision detection (CSMA/CD); Carrier sense multiple access with collision avoidance (CSMA/CA).

What is a node?

Device that can receive, create, store, or forward data along a communications path.

What is typical for Controlled access to shared media?

Each node has its own time to use the medium.

What are the purposes of Data Link layer?

Enables upper layers to access the media (upper layers have no idea what media is being used). Packets <--> frames. Exchanges frames between endpoints over the network media. Performs error detection and rejects any corrupt frame.

What is IEEE 802.3?

Ethernet (Various Ethernet standards for Fast Ethernet, Gigabit Ethernet, etc.)

What are some data link layer protocols?

Ethernet, 802.11 Wireless, Point-to-Point Protocol (PPP), High-Level Data Link Control (HDLC), Frame Relay,

What is the PDU called in the data link layer?

Frame

What are the fields of a general frame?

Frame start indicator flag, addressing, type, control, data, error detection, frame end indicator flag

What are the three basic parts of frame?

Header, data, trailer

What organizations do maintain protocols for data link layer?

IEEE, ITU, ISO, ANSI

What is typical for physical network topology?

Identifies the physical connections and how end devices and intermediary devices are interconnected. It may also include specific device locations such as room number etc.

What organization does not maintain protocols for data link layer?

Internet Engineering Task Force (IETF)

What does the Media Access Control mean and what does it do? (Not the sublayer as a whole, but only the media access control)

It allows multiple devices to communicate over a shared (half-duplex) medium. Full-duplex does not require Access Control.

Describe point-to-point WAN topology.

It directly connects two nodes. They do not need to share the media with other hosts. Example: The cable between two routers is just for the two routers. When using a serial communications protocol (Point-to-Point Protocol - PPP), there is no need to verify if the incoming frame was meant for the node. There is no other device the frame could be meant for.

Error detection is one of the functions of MAC sublayer. How it is done?

It includes a frame trailer. In the trailer, you can find stuff like CRC - Cyclical Redundancy Checks.

What is the purpose of Media Access Control (MAC) sublayer?

It is responsible for data encapsulation; it provides data link layer addressing. It controls the NIC and other hardware that is responsible for sending and receiving data on the wired or wireless LAN/MAN medium. It provides the method to get the frame on and off the media.

How is error detection in a frame done?

It places a logical or mathematical summary of the bits that comprise the frame in the trailer.

What is the purpose of Logical Link Layer (LLC)?

It places information in the frame that identifies which network layer protocol (IPv4, IPv6) is being used for the frame.

Describe Mesh WAN topology.

It provides high availability but requires that every end system is interconnected to every other system.

Describe Hub and Spoke WAN topology.

It's WAN version of the star topology in which a central site interconnects branch sites through the use of point-to-point links. Branch sites cannot exchange data with other branch sites without going through the central site.

What is IEEE 802.2?

LLC

What is in the type filed in a frame?

Layer 3 protocol which is used

What protocols corresponds to Controlled access to shared media?

Legacy Token Ring, Legacy ARCNET

Where is CSMA/CD used?

Legacy bus-topology Ethernet LAN, Legacy Ethernet LAN using a hub

What is typical for Bus LAN technology?

Legacy; switches are not required; devices connected to one bus (coax most often); inexpensive, easy to set up.

What is typical for Ring LAN technology?

Legacy; systems are connected to their neighbor forming a ring. Legacy Fiber Distributed Data Interface (FDDI) and Token Ring networks used ring topologies.

Data Link layer is responsible for which type of communication?

NIC to NIC

What media access control is being used today?

None. Modern networks operate in full-duplex and do not require an access method.

Where is CSMA/CA used?

On Wireless LANs.

What are the two types of network topologies?

Physical and logical

What is Addressing in MAC sublayer?

Provides source and destination addressing for transporting the Layer 2 frame between devices on the same shared medium.

What is typical for logical network topology?

Shows the way the network transfers frames. You can see IP addresses, device interfaces.

What is in the addressing field in a frame?

Source and destination MAC address

What the Control field specifies in a frame?

Special flow control services (QoS); if the frame should have a forwarding priority or not.

What are some LAN topologies?

Star or Extended Start topologies, Bus, Ring

What is a network topology?

The arrangement, or the relationship, of the network devices and the interconnections between them.

What is Frame delimiting in MAC sublayer?

The framing process provides important delimiters to identify fields within a frame. These delimiting bits provide synchronization between the transmitting and receiving nodes.

Why is error detection for a frame needed?

The signals on the media could be subject to interference, distortion, or loss that would substantially change the bit values that those signals represent.

What are frame start and stop indicator flags used for?

To identify the start and end of the frame.

What are physical addresses used for?

Transporting frame across a shared local media.

What is typical for Star or Extended Star LAN topologies?

Used in multiaccess LANs; end devices connected to a central intermediary device. Extended Start extends the Start topology by interconnecting multiple switches. Easy to install, very scalable, easy to troubleshoot. There's a single point of failure (the central device).

What is typical for CSMA/CD?

Used on wired half-duplex networks. When two devices communicate at the same time, it will be detected by both devices. Each device will then wait for a random amount of time before trying to transmit data again.

What is IEEE 802.11?

WLAN (Various WLAN standards for different types of wireless communications.)

What is IEEE 802.15?

WPAN (Various WPAN standards for Bluetooth, RFID, etc.)

Smaller bandwidth usually have a) LANs b) WANs

b

What is true for physical address? a) It indicates the network on which the device is located. b) It is unique for the specific device. c) Physical address changes as the device connects to a different network.

b