ECE 017 Data Communications Module 3.1 Week 7 Quiz

Flow Control

• Flow control is used to manage the rate of producing and consuming frames to prevent buffer overflow.
• When the buffer is full, two options are available:
  • Excess frames are automatically dropped.
  • The data-link layer sends feedback to the sending/source data-link layer to ask it to stop or slow down.

Error Control

• Error control involves:
  • Detection
  • Correction
  • Discard and retransmission
• Electromagnetic signals are susceptible to errors due to EMI, crosstalk, etc., which affect the integrity of the frame.

Categories of Links

• There are two ways the data-link layer controls the way transmission medium is used:
  • Point-to-point link
  • Broadcast link
• Point-to-point link:
  • The data-link layer uses the whole capacity of the transmission medium.
  • The link is dedicated to the two connecting devices/hosts.
  • Examples: Bluetooth, remote control, microwave link, satellite link.
• Broadcast link:
  • The data-link layer uses only a part of the capacity of the link.
  • The link is shared among several pairs of devices.
  • Examples: Wi-Fi, cellular service, radio/tv stations.

The Link Layer

• The Link Layer is also called the Network Interface Layer or Network Access Layer.
• It is composed of the Physical and Data-link layers.
• The Link Layer is the group of methods and communications protocols confined to the link that a host is physically connected to.

The Duty and Scope of the Physical Layer

• The duty of the Physical Layer is to move or transport data in signal form from one device/node to another via a transmission medium.
• The transmission medium can be copper wire, optical fiber, or air.
• The data can be texts, numbers, images, audio, or video encapsulated into a Frame at the Data-link Layer.

Nodes and Links

• Nodes are hosts and routers.
• Links are the connecting networks in between.
• Data-link Layer logical connection is Node-to-Node.
• Data units from the source host to the destination host in the Internet pass through multiple nodes and links.

Duty Scope of the Data-Link Layer

• The Data-Link Layer provides services to the Network Layer.
• Its duty is node-to-node, hop-to-hop, link-to-link (not end-to-end).
• It is responsible for the delivery of a Datagram from one node to the next.
• The Data-Link layer of the sending node encapsulates the Datagram from the Network layer to become a Frame.
• The Data-Link layer of the receiving node decapsulates the Datagram from the Frame.

Data-Link Layer Services

• The Data-Link Layer services are:
  • Framing
  • Flow Control
  • Error Control
  • Congestion Control

Framing

• Framing is the first service of the Data-Link Layer.
• At each node, the Datagram has to be encapsulated in a Frame before sending it to the next node.
• The node also has to decapsulate the Datagram from the Frame received on the logical channel.
• A Frame has a header and a trailer.
• The framing formats are dictated by the Data-link layer standard being implemented, e.g., Ethernet.