Data Link Layer Flashcards
Define the Data Link Layer
The data link layer of the OSI model (Layer 2) prepares network data for the physical network. The data link layer is responsible for network interface card (NIC) to network interface card communications.
What’s the function of the data link layer?
1) Enables upper layers to access the media. The upper layer protocol is completely unaware of the type of media that is used to forward the data.
2) Accepts data, usually Layer 3 packets (i.e., IPv4 or IPv6), and encapsulates them into Layer 2 frames.
3) Controls how data is placed and received on the media.
4) Exchanges frames between endpoints over the network media.
5) Receives encapsulated data, usually Layer 3 packets, and directs them to the proper upper-layer protocol.
6) Performs error detection and rejects any corrupt frame.
What would happen without the data link layer?
Network layer protocols such as IP would have to make provisions for connecting to every type of media that could exist along a delivery path. Additionally, every time a new network technology or medium was developed, IP would have to adapt.
Define a node
A device that can receive, create, store, or forward data along a communications path. It can be either an end device such as a laptop or mobile phone, or an intermediary device such as an Ethernet switch.
What are IEEE 802 LAN/MAN standards specific to?
Ethernet LANs, wireless LANs (WLAN), wireless personal area networks (WPAN), and other types of local and metropolitan area networks.
What does the IEEE 802 LAN/MAN data link layer consist of?
Logical Link Control (LLC)
Media Access Control (MAC)
Define LLC
Logical Link Control: an IEEE 802.2 sublayer communicates between the networking software at the upper layers and the device hardware at the lower layers. It places information in the frame that identifies which network layer protocol is being used for the frame. This information allows multiple Layer 3 protocols, such as IPv4 and IPv6, to use the same network interface and media.
It takes the network protocol data, which is typically an IPv4 or IPv6 packet, and adds Layer 2 control information to help deliver the packet to the destination node.
Define MAC
Media Access Control (MAC)– Implements this sublayer (IEEE 802.3, 802.11, or 802.15) in hardware. It is responsible for data encapsulation and media access control. It provides data link layer addressing and it is integrated with various physical layer technologies.
It controls the NIC and other hardware that is responsible for sending and receiving data on the wired or wireless LAN/MAN medium.
What types of data encapsulation does the MAC sub-layer provide?
Frame delimiting
Addressing
Error detection
The MAC sublayer also provides media access control, allowing multiple devices to communicate over a shared (half-duplex) medium. Full-duplex communications do not require access control.
Define Free Delimiting
The framing process provides important delimiters to identify fields within a frame. These delimiting bits provide synchronization between the transmitting and receiving nodes.
Define Addressing
Provides source and destination addressing for transporting the Layer 2 frame between devices on the same shared medium.
Define error detection
Includes a trailer used to detect transmission errors.
At each hop along the path, a router performs the following Layer 2 functions:
1) Accepts a frame from a medium
2) De-encapsulates the frame
3) Re-encapsulates the packet into a new frame
4) Forwards the new frame appropriate to the medium of that segment of the physical network
Engineering organizations that define open standards and protocols that apply to the network access layer include the following:
1) Institute of Electrical and Electronics Engineers (IEEE)
2) International Telecommunication Union (ITU)
3) International Organization for Standardization (ISO)
4) American National Standards Institute (ANSI)
What are the common physical WAN topologies?
1) Point-to-Point
2) Hub and Spoke:
3) Mesh:
(A hybrid is a variation or combination of any topologies)
Define Point-to-Point:
Directly connect two nodes
In this arrangement, two nodes do not have to share the media with other hosts. Additionally, when using a serial communications protocol such as Point-to-Point Protocol (PPP), a node does not have to make any determination about whether an incoming frame is destined for it or another node. Therefore, the logical data link protocols can be very simple, as all frames on the media can only travel to or from the two nodes. The node places the frames on the media at one end and those frames are taken from the media by the node at the other end of the point-to-point circuit. (the use of physical devices in the network does not affect the logical topology)
In what case is the device required to determine if the incoming frame is destined for this node?
point-to-point connection over Ethernet
Define Hub and Spoke
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.
The image shows five routers. One route in the middle is connected by four lines, representing four links, to the other four routers.
Define Mesh
provides high availability but requires that every end system is interconnected to every other system. Therefore, the administrative and physical costs can be significant. Each link is essentially a point-to-point link to the other node.
Define Half and Full Duplex communications
Half-duplex communications restrict the exchange of data in one direction at a time. Full-duplex allows the sending and receiving of data to happen simultaneously.
It is important that two interconnected interfaces, such as a host NIC and an interface on an Ethernet switch, operate using the same duplex mode. Otherwise, there will be a duplex mismatch creating inefficiency and latency on the link.
What are the three basic types of a frame?
Header
Data
Trailer
Explain this sentence: “There is no one frame structure that meets the needs of all data transportation across all types of media”
Depending on the environment, the amount of control information needed in the frame varies to match the access control requirements of the media and logical topology.
Define framing
Framing breaks the stream into understandable groupings, with control information inserted in the header and trailer as values in different fields. This format gives the physical signals a structure that are recognized by nodes and decoded into packets at the destination.
What do frame fields include?
1) Frame start and stop indicator flags: to identify the beginning and end of a frame
2) Addressing: indicates the source and destination nodes on media
3) Type: identifies the Layer 3 protocol in the data field
4) Control: identifies special flow control services such as QoS
5) Data: contains the frame payload
5) Error Detection: Included after the data to form the trailer
What does the data link add to the end of each frame and what is its function?
A trailer
In a process called error detection, the trailer determines if the frame arrived without error. It places a logical or mathematical summary of the bits that comprise the frame in the trailer.
Why does the data link layer add error detection?
because the signals on the media could be subject to interference, distortion, or loss that would substantially change the bit values that those signals represent.
Define CRC
Cyclic redundancy check
a logical summary of the contents of the frame created by a transmitting node
This value is placed in the frame check sequence (FCS) field to represent the contents of the frame
Note: In the Ethernet trailer, the FCS provides a method for the receiving node to determine whether the frame experienced transmission errors.
Where is the data link addressing contained?
within the frame header. It is typically at the beginning of the frame, so the NIC can quickly determine if it matches its own Layer 2 address before accepting the rest of the frame. The frame header may also contain the source address of the frame.
Explain this sentence “Unlike Layer 3 logical addresses, which are hierarchical, physical addresses do not indicate on what network the device is located”
The physical address is unique to the specific device. A device will still function with the same Layer 2 physical address even if the device moves to another network or sub-net. Therefore, Layer 2 addresses are only used to connect devices within the same shared media, on the same IP network.
Talk about Layer 2 addresses
1) Host-To-Router
2) Router-To-Router
3)Router-To-Host
Talk about Host-To-Router
The source host encapsulates the Layer 3 IP packet in a Layer 2 frame. In the frame header, the host adds its Layer 2 address as the source and the Layer 2 address for R1 as the destination.
Talk about Router-To-Router
R1 encapsulates the Layer 3 IP packet in a new Layer 2 frame. In the frame header, R1 adds its Layer 2 address as the source and the Layer 2 address for R2 as the destination.
Talk about Router-To-Host
R2 encapsulates the Layer 3 IP packet in a new Layer 2 frame. In the frame header, R2 adds its Layer 2 address as the source and the Layer 2 address for the server as the destination.
What are the data link addresses only used for?
The data link layer address is only used for local delivery. Addresses at this layer have no meaning beyond the local network unlike Layer 3 addresses
What happens if the data must pass onto another network segment?
The router must accept the frame based on the physical address and de-encapsulate the frame in order to examine the hierarchical address, which is the IP address. Using the IP address, the router can determine the network location of the destination device and the best path to reach it. When it knows where to forward the packet, the router then creates a new frame for the packet, and the new frame is sent on to the next network segment toward its final destination.
Explain this sentence: “Each protocol performs media access control for specified Layer 2 logical topologies”
This means that a number of different network devices can act as nodes that operate at the data link layer when implementing these protocols. These devices include the NICs on computers as well as the interfaces on routers and Layer 2 switches.
What does Layer 2 protocols depend on?
In a TCP/IP network, all OSI Layer 2 protocols work with IP at OSI Layer 3. However, the Layer 2 protocol used depends on the logical topology and the physical media.
The Layer 2 protocol that is used for a particular network topology is determined by what?
the technology used to implement that topology. The technology used is determined by the size of the network, in terms of the number of hosts and the geographic scope, and the services to be provided over the network.
What does a LAN typically use?
a high bandwidth technology capable of supporting large numbers of hosts. The relatively small geographic area of a LAN and its high density of users make this technology cost-effective.
Using a high bandwidth technology for WANs that cover large geographical areas is what?
using a high bandwidth technology is usually not cost-effective for WANs that cover large geographic areas. The cost of the long-distance physical links and the technology used to carry the signals over those distances typically results in lower bandwidth capacity.
What does the difference in bandwidth result in?
the use of different protocols for LANs and WANs.
Data link layer protocols include:
1) Ethernet
802.11 Wireless
2) Point-to-Point Protocol (PPP)
3) High-Level Data Link Control (HDLC)
3) Frame Relay
