CNT140: Chapter 6: Data Link Layer
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 the packet into a new frame 4. Forwards the new frame appropriate to the medium of that segment of the physical network.
Which of the following are data link layer protocols? (choose three) 802.11 Wireless Ethernet IP PPP UDP
802.11 Ethernet PPP
Refers to the way a network transfers frames from one node to the next. This topology identifies virtual connections using device interfaces and layer 3 IP addressing schemes.
Logical Topology
When using the contention-based access method CSMA/CD, what happens if two devices transmit at the same time?
A collision will occur and both devices will detect the collision on the network. This is the collision detection (CD) portion of CSMA/CD. The NIC compares data transmitted with data received, or by recognizing that the signal amplitude is higher than normal on the media. The data sent by both devices will be corrupted and will need to be resent.
A combination of any of the WAN topologies in known as what?
A hybrid topology
What is a partial mesh?
A hybrid topology in which some, but not all, end devices are interconnected.
What is a multiaccess network?
A network that can have two ore more end devices attempting to access the network simultaneously.
What can be considered a node?
A node can be either an end device such as a laptop or mobile phone, or an intermediary device such as an Ethernet switch
Why does the data link layer add error detection to a frame?
Because the signals on the media could be subject to interference, distortion, or loss that would substantially change the bit value that those signals represent.
Early Ethernet and legacy Token Ring LAN technologies included two other types of topologies:
Bus Ring
All end systems are chained to each other and terminated in some form on each end. Infrastructure devices such as switches are not required to interconnect the end devices. Legacy Ethernet networks were often bus topologies using coax cables because it was inexpensive and easy to set up.
Bus topology
The data link layer "sees" the ------- topology of a network when controlling data access to the media. It is the -------topology that influences the type of network framing and media access control used.
Logical Topology
Which typology displays networking device layer IP address? Aerial topology IP address topology Logical Topology Physical topology
Logical Topology
What layer of the OSI model accepts data, usually Layer 3 packets (i.e., IPv4r or IPv6), and encapsulates them into layer 2 frames?
Data Link Layer or layer 2
What layer of the OSI model 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.
Data Link layer or Layer 2
What layer of the OSI model performs error detection and rejects any corrupt frame?
Data Link layer or layer 2
What Layer in the OSI model is responsible for NIC to network interface card communications?
Data Link layer or layer 2 of the OSI model
What part of the frame contains the IPv4 or IPv6 packet?
Data field of the frame
Which statement describes the half-duplex mode of data transmission? Data that is transmitted over the network flows in one direction at a time Data that is transmitted over the network flows in one direction to many different destinations simultaneously Data that is transmitted over the network flows in both directions at the same time Data that is transmitted over the network can only flow in one direction
Data that is transmitted over the network flows in one direction at a time
What determines a frames structure?
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.
What does the data link layer addressing contain?
Destination MAC address Source MAC address
Which lists the Layer 2 and Layer 3 address fields in the correct order?
Destination NIC address, source NIC address Source IP address, Destination IP address
CMSA/CA does not detect collisions but attempts to avoid them by doing what?
Each device that transmits includes the time duration that it needs for the transmission. All other wireless devices receive this information and know how long the medium will be unavailable.
When a frame is moving from node to node. How does the data link layer addressing change?
Each link frame contains the source data link address of the NIC sending the frame, and the destination data link address of the NIC receiving the frame.
What are the benefits in using the Star and Extended Star LAN topologies?
Easy to install Very scalable (easy to add and remove end devices) Easy to troubleshoot Note: Early Star topologies interconnected end devices using Ethernet Hubs
The data link layer prepares the encapsulated data (usually an IPv4 or IPv6 packet) for transport across the local media by doing what?
Encapsulating it with a header and a trailer to create a frame
Included after the data to form the trailer
Error detection
Includes a trailer used to detect transmission errors.
Error detection
What frame fields are located in the trailer of a frame?
Error detection Frame Stop
Data link layer protocols include:
Ethernet 802.11 Wireless Point-to-Point Protocol (PPP) High-Level Data Link Control (HDLC) Frame Relay
What two examples of a multiaccess network?
Ethernet LANs WLANs
IEEE 802 LAN/MAN standards are specific to what?
Ethernet LANs, wireless LANs (WLAN), wireless personal area networks (WPAN) and other types of local and metropolitan area networks.
Which LAN topology uses multiple Ethernet switches to connect to two or more star topologies?
Extended Star Topology
Which LAN topology is a hybrid topology? Bus extended star ring star
Extended star
Each network environment that packets encounter as they travel from a local host to a remote host can have different characteristics. Give and example of this.
For example, an Ethernet LAN usually consists of many hosts contending for access on the network medium. The MAC sublayer resolves this. With serial links the access method may only consist of a direct connection between only two devices, usually two routers. Therefore, they do not require the techniques employed by the IEEE 802 MAC sublayer.
What frame fields are located in the header of a frame?
Frame Start Addressing Type Control
The framing process provides important delimiters to identify fields within a frame. These delimiting bits provide synchronization between the transmitting and receiving nodes.
Frame delimiting
What information is added to a frame by the MAC sublayer during data encapsulation?
Frame delimiting Addressing Error detection
Used to identify the beginning and end limits of the frame
Frame start and stop indicator flags
Both devices can simultaneously transmit and receive on the shared media. This is an example of what duplex mode?
Full duplex mode
Which mode of communication when used does the data link layer assume that the media is available for transmission for both nodes at any time?
Full-Duplex mode
Ethernet switches operate in what duplex mode by default?
Full-duplex mode
What are the two common modes of duplex?
Half-Duplex Full-Duplex
Both devices can transmit and receive on the media but cannot do so simultaneously. This is an example of what duplex mode?
Half-duplex
Which duplex communication method is used in WLANs?
Half-duplex
If a device is set to half duplex-mode, how does it affect communication between two devices?
Half-duplex allows only one device to send ore receive the time on the shred medium.
In contention-based multiaccess networks, all nodes are operating in what duplex mode, competing for the use of the medium?
Half-duplex mode
WLANs and legacy bus topologies with Ethernet hubs use what duplex mode?
Half-duplex mode
Wireless LANs, Legacy bus-topology, and Legacy Ethernet LANs using a hub all operate in what duplex-mode?
Half-duplex mode
If an Ethernet switch is connected to a Ethernet hub what duplex mode will the Ethernet switch run in?
Half-duplex this is because the Ethernet hub can only operate in half duplex. The Ethernet switch can run in both half and full duplex modes.
Data link addressing is contained in the which part of the frame?
Header
Although there are many different data link layer protocols that describe data link layer frames, each frame type has three basic pars:
Header Data (Layer 3 packet) Trailer
What does the data link layer add to a layer 3 packet to create a frame (Choose two)? Flags Sequence Number Header Trailer
Header Trailer
If more control methods are used to ensure delivery how will it effect the frame?
Header and trailer fields are larger as more control information is needed. Greater effort is needed to endure delivery. This means higher overhead and slower transmission rates.
This is a WAN version of a 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.
Hub and Spoke
Which two engineering organizations define open standards and protocols that apply to the data link layer? (Choose 2) Electronic Industries Alliance (EIA) Internet Society (ISOC) International Telecommunication Union (ITU) Institute of Electrical and Electronics Engineers (IEE) Internet Assigned Numbers Authority (IANA)
IEE and ITU
Which organization defines standards for the network access layer (ie. the OSI physical and data link layer)?
IEEE
In a network using legacy Ethernet LANs that use a hub CSMA/CD is used. How do devices decide whether they can send a frame on the network?
If it does not detect a carrier signal (in other words, it is not receiving transmissions from another device), it will assume the network is available to send.
How does the data link layer protocols determine if the frame arrived without errors?
In a process called error detraction, 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.
What is a node in a computer network?
In computer networks a node is a device that can receive, create, store, or forward data along a communications path.
Why does the router de-encapsulate the frame?
In order to examine the hierarchical address, which is the IP address.
Engineering organizations that define open standards and protocols that apply to the network access layer (i.e., the OSI physical and data link layers) include the following:
Institute of Electrical and Electronics Engineers ( IEEE) International Telecommunication Union (ITU) International Organization for Standardization (ISO) American National Standards Institute (ANSI)
If data must pass onto another network, what type of device is needed?
Intermediary device, such as a router
When a network using legacy Ethernet LANs that use a Ethernet hub, if another device, such as PC2, wants to transmit, but is currently receiving a frame it must do what?
It must wait until the channel is clear
What are two services performed by the data link layer of the OSI model? It provides media access control and performs error detection It fragments data packets into MTU size It accepts layer 3 packets and encapsulates them into frames It monitors the layer 2 communication by building a MAC address table It determines the path to forward packets
It provides media access control and performs error detection It accepts layer 3 packets and encapsulates them into frames
Understanding duplex communication is important when discussing LAN topologies because what?
It refers to the direction of data transmission between two devices.
Unlike a serial connection in a point to point topology, A point to point connection over Ethernet requires a node to do what?
It requires the device/node to determine if the incoming frame is destined for this node.
What are the two sublayers of the OSI model data link layer? (Choose two) LLC physical MAC network access transport internet
LLC and MAC
Which data link sublayer 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?
LLC sublayer or Logical Link Control
The IEEE 802. LAN/MAN data link layer consists of which two sublayers?
LLC-Logical Link Control Layer MAC-Media Access Control Layer
What is another name for the OSI data link layer?
Layer 2
What layer of the OSI model receives encapsulated data, usually layer 3 packets, and directs them to the proper upper-layer protocol?
Layer 2 or Data Link layer
What is contained in the data portion of a frame?
Layer 3 packet ( IPv4 or IPv6)
In a TCP/IP network, all OSI Layer 2 protocols work with IP at what OSI Layer?
Layer 3 the Network Layer, however, the layer 2 protocol used depends on the logical topology and the physical media.
Examples of multiaccess networks that use controlled access include the following:
Legacy Token Ring Legacy ARCNET
The data link layer address is only used for what type of delivery?
Local delivery
This 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 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.
Logical Link Control (LLC)
The IEEE LAN/MAN data link layer consists of what two sublayers?
Logical Link Control (LLC) Media Access Control (MAC)
Do contention-based systems scale well under heavy media use?
No, contention-based systems do not scale well under heavy media use
End systems are connected to their respective neighbor forming a ring. The ring does not need to be terminated, unlike in the bus topology. Legacy Fiber Distributed Data Interface (FDII) and Token Ring networks used ring topologies.
Ring topology
Why is the layer 2 addressing 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
What type of physical topology can be created by connecting all Ethernet cables to a central device? mesh star ring bus
Star
Which LAN topology requires end devices to be connected to a central intermediary device such as as Ethernet witch?
Star Topology
In multiaccess LANs, end devices (ie nodes) are interconnected using what type of topologies?
Star and Extended Star topologies
The layer 2 protocol that is used for a particular network topology is determined by what?
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 happens to the LAN header when a local router receives the frame sent by a local host and sends that frame outside of the network?
The LAN header is removes and replaced by a WAN header. This is due to routers using serial links and there only being two devices interacting with each other
The difference in bandwidth normally results in the use of different _______ for LANs and WANs.
protocols
Which is a function of the Logical Link Control (LLC) sublayer? to define the media access processes that are performed by the hardware to accept segments and package them into data units that are called packets to identify which network layer protocol is being used to provide data link layer addressing
to identify which network layer protocol is being used
Unlike other encapsulation protocols, the data link layer appends information in the form of what at the end of the frame?
trailer
Contains the frame payload (ie packet header, segment header, and the data)
Data
What layer of the OSI model controls how data is placed and received on the media?
Data Link Layer
What layer of the OSI model exchanges frames between endpoints over the network media?
Data Link Layer
What Layer 2 function does a router perform?
Accepts a frame from a medium De-encapsulates the frame Re-encapsulates the packet into a new frame Forwards the new frame appropriate to the medium of that segment of the physical network
Indicated the destination and source nodes on the media
Addressing
Provides source and destination addressing for transporting the Layer 2 frame between devices on the same shared medium.
Addressing
When a frame is broadcasted through the local network, what do other devices do with the frame?
All devices attached to the hub will receive the frame. However, because the frame has a destination data link address for a certain device, only that device will accept and copy in the entire frame. All other device NICs will ignore the frame
The MAC sublayer also provides media access control, what is does this do?
Allows multiple devices to communicate over a shared (half-duplex) medium. Full-duplex communications do not require access control.
After a wireless device sends an 802.11 frame, the receiver returns what?
An acknowledgment so that the sender knows the frame arrived.
When network using legacy Ethernet LANs that use a Ethernet hub , what does that Ethernet hub do with the frame?
Any bits/frames received on an incoming port are regenerated and sent out all other ports of the Ethernet hub.
Which data link layer media access method does Ethernet use with legacy Ethernet HUBS? determinism CSMA/CD turn taking token passing
CSMA/CD or Carrier Sense Multiple Access with Collison Detection
What method is used to manage contention-based access on a wireless network?
Carrier Sense Multiple Access with Collision Avoidance or CSMA/CA
Which media access control method is used in legacy Ethernet LANs?
Carrier sense multiple access with Collison detection
Which contention-based access method is used on Wireless LANs?
Carrier sense multiple access with collision avoidance (CSMA/CA)
In a wireless LAN what kind of contention based access is used?
Carrier sense multiple access with collision avoidance or CSMA/CA
Which contention-based access method is used on legacy bus-topology Ethernet LANs?
Carrier sense multiple access with collision detection (CSMA/CD)
What are the two examples of contention based access control methods?
Carrier sense multiple access with collision detection (CSMA/CD) Carrier sense multiple access with collision avoidance (CSMA/CA)
In a legacy bus-topology what type of contention based access is used?
Carrier sense multiple access with collision detection or CSMA/CD
When using a legacy Ethernet LAN using a hub what type of contention based access is used?
Carrier sense multiple access with collision detection or CSMA/CD
Another form of CSMA used by IEEE 802.11 WLANs is ?
Carrier sense multiple access/collision avoidance (CSMA/CA)
Which access control method is used if only one device can send at a time?
Contention-based access
Some multiaccess networks require rules to govern how devices share the physical media. There are two basic access control methods for shared media:
Contention-based access Controlled access
Identifies special flow control services such as quality of service (QoS). QoS gives forwarding priority to certain types of messages. For example, voice over IP (VoIP) frames normally receive priority because they are sensitive to delay.
Control
Which multiaccess network control method allows each node to have its own time to use the medium?
Controlled access Note: These deterministic types of legacy networks are inefficient because a device must wait its turn to access the medium.
What is a CRC value and who crates it?
Cyclic redundancy check (CRC) value is a logical summary of the contents of the frame created by a transmitting node.
What is true concerning physical and logical topologies? Logical topologies refer to how a network transfers data between devices. Logical topology is always the same as the physical topology. The logical topology is always the same as the physical topology. Physical topologies display the IP addressing scheme of each network Physical topologies are concerned with how a network transfers frames.
Logical topologies refer to how a network transfers data between devices.
What attribute of a NIC would place it at the data link layer of the OSI model? attached Ethernet cable RJ-45 port TCP/IP protocol stack IP address MAC address
MAC address
What identifier is used at the data link layer to uniquely identify an Ethernet device? TCP port UDP port MAC address IP address Sequence number
MAC address
Which data link sublayer controls the NIC and other hardware that is responsible for sending and receiving data on the wired or wireless LAN/MAN medium?
MAC sublayer Media Access Control
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.
Media Access Control (MAC)
The media access control method used depends on which two criteria? Layer 3 IP protocol Media Sharing Topology Transport Layer protocol Type of data
Media Sharing Topology
A technician has been asked to develop a physical topology for a network that provides a high level of redundancy. Which physical topology requires that every node is attached to every other node on the network? star hierarchical mesh ring bus
Mesh
This topology 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-pint link to the other noe.
Mesh
An Ethernet hub is also known as what?
Multiport Repeater
The data link protocol is responsible for what type of communications within the same network?
NIC-to-NIC communications
The data link layer corresponds to what layer of the TCP/IP model?
Network Access Layer
As the IP packet travels from host to router, router to router, and finally router to host, at each point along the way the IP packet is encapsulated in a _______ data link frame.
New
In a logical point-to-point topology does having intermediary devices between the two nodes change the topology?
No, the use of physical devices in the network does not affect the logical topology. Adding intermediary physical connection may not change the logical topology. The logical point-to-point connection is the same.
Do Ethernet LANs using switches use contention-based systems?
No, they do not used a contention-based system because the switch and the host NIC operate in full-diplex mode.
When using point-point protocol on a serial connection does the node have to determine the frame is destined for it or another node?
No, 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.
If you moved a device to another network. What would happened the the devices layer 2 address?
Nothing would happened to the devices layer 2 address. A device will still function with the same layer 2 physical address even if the device moves to another network or subnet.
What is half-duplex transmission?
Only one device can send or receive at a time. This requires a process to govern when a device can send and what happens when multiple devices send at the same time.
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. Why is this the case?
Otherwise, there will be a duplex mismatch creating inefficiency and latency on the link.
What is an example of a hybrid topology?
Partial mesh is a hybrid topology
Identifies the physical connections and how end devices and intermediary devices ( routers, switches, and wireless access points) are interconnected. The topology may include specific device location such as room number and location on the equipment rack.
Physical Topology
What are the two types of topologies used when describing LAN and WAN networks?
Physical Topology Logical Topology
The data link layer provides the addressing used in transporting a frame across a shared local media. Device address at this layer are referred to as what?
Physical addresses
What type of topology is usually a star or point to point?
Physical topology
This is the simplest and most common WAN topology. It consists of a permanent link between two endpoints.
Poin-to-Point
What does PPP stand for?
Point-to-Point Protocol
What protocol is used for serial communications when you have a point-to-point connection between two routers?
Point-to-Point Protocol
WANs traditionally used other types of protocols for various types of point-to-point, hub-spoke, and full-mesh topologies. Some of the common WAN protocols over the years have included:
Point-to-Point Protocol (PPP) High-Level Data Link Control (HDLC) Frame Relay Asynchronous Transfer Mode (ATM) X.25 Note: These layer 2 protocols are now being replaced in the WAN by Ethernet
Which protocol used in serial communications does not have to make any determination about whether an incoming frame is destined for it or another node?
Point-to-Point protocol Note: 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.
What does the FCS/frame check sequence provide?
Provides a method for the receiving node to determine whether the frame experienced transmission errors.
What is the responsibility of the MAC sublayer? Adds Layer 3 addresses to the frame Communicates with the network layer provides the method to get the frame on and off the media Transmits the bits on the media
Provides the method to get the frame on and off the media
How does layer 2 support layer 3?
Receives encapsulated data, usually Layer 3 packets, and directs them to the proper upper-layer protocol. Accepts data, usually Layer 3 packets (i.e., IPv4 or IPv6), and encapsulates them into Layer 2 frames.
The data link layer is what layer in the OSI Model?
The data link layer is layer 2
If a frame needs to go outside of the network what needs to be true in order for the router to accept the frame?
The frame must have the routers Layer 2 MAC address as its destination MAC address in the header of the frame
When layer 2 encapsulates a layer 3 packet. What is inside a layer 2 header?
The header contains the destination NIC and Source NIC or the destination MAC address and source MAC address.
A LAN typically uses a high bandwidth technology capable of supporting large number of host. Give and example of this?
The relatively small geographic area of a LAN ( a single building or a multi-building campus) and its high density if users make this technology cost-effective.
What is a topology of a network?
The topology of a network is the arrangement, or the relationship, of the network devices and the interconnections between them.
Although CSMA/CD is still a feature of Ethernet, why is it no longer necessary? the use of CSMA/CA the development of half-duplex switch operation the use of full-duplex capable Layer 2 switches the virtually unlimited availability of IPv6 addresses the use of Gigabit Ethernet speeds
The use of full-duplex capable layer 2 Switches
Each protocol performs media access control for specified layer 2 logical topologies. What does this mean?
This means that a number of different network devices can act as nodes that operate at the data link layer when implementing these protocols. Note: These devices include the NICs on computers as well as the interfaces on routers and Layer 2 switches
Where is the CRC value located?
This value is placed in the frame check sequence (FCS) field to represent the contents of the frame.
What is the function of the last field in data link layer frame? To determine whether the frame experienced transmission errors To identify special flow control services such a quality of service (QoS) To identify the beginning and end limits of the frame To identify the Layer 3 protocol in the data field
To determine whether the frame experienced transmission errors
Where is the FCS or frame check sequence located in the frame?
Trailer
True or False: Layer 2 address are only used to connect devices within the same shared media, on the same IP network.
True
True or False: Today, Ethernet networks operate in full duplex and do not require an access method.
True
Ture or False: In wireless environments it may not be possible for a device to detect a collision.
True
Identifies the layer 3 protocol in the data field
Type
How are layer 2 address different form layer 3 address?
Unlike layer 3 logical addresses, which are hierarchical, physical addresses do not indicate on what network the device is located. Rather, the physical address is unique to the specific device.
When the router obtains the IP packet from the frame what does it use it for?
Using the IP address obtained from the IP packet, 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 finale destination.
What kind of network would use point-to-point, hub and spoke, or mesh topologies? PAN LAN WLAN WAN
WAN
In a WAN why is it not cost effective to use high bandwidth technology?
WANs cover large geographic areas (cities or multiple cities). 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.
Examples of contention based access includes the following three:
Wireless LAN (uses CSMA/CS) Legacy bus-topology Ethernet LAN (USES CSMA/CD) Legacy Ethernet LAN using a hub (uses CSMA/CD)
Do all data link layer protocols encapsulate the data within the data field of the frame?
Yes, they do however, the structure of the frame and the fields contained in the header and trailer vary according to the protocol.
Which layer of the OSI model is responsible for specifying the encapsulation method used for specific types of media? data link application physical transport
data link
