CISS 120 - Module 6: Data Link Layer
there are two basic access control methods for shared media
- contention-based access - controlled access
the data link layer does the following:
- 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 - accepts data, usually Layer 3 packets (i.e., IPv4 or IPv6) and encapsulates them into Layer 2 frames - controls how data is placed and received on the media - exchanges frames between endpoints over the network media - receives encapsulated data, usually Layer 3 packets, and directs them to the proper upper-layer protocol - performs error detection and rejects any corrupt frame
the MAC sublayer provides data encapsulation
- frame delimiting - addressing - error detection
frame fields include the following
- frame start and stop indicator flags - addressing - type - control - data - error detection
each frame type has three basic parts
- header - data - trailer
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
Which of the following are data link layer protocols? (Choose three) - 802.11 - Ethernet - IP - PPP - UDP
802.11, Ethernet, PPP
the hub receives the frame
the Ethernet hub receives and send the frame. an Ethernet hub is also known as a multiport repeater. any bits received on an incoming port are regenerated and sent out all other ports. if another device, such as PC2, wants to transmit, but is currently receiving a frame, it must wait until the channel is clear.
there are two types of topologies used when describing LAN and WAN networks
physical topology, logical topology
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.
data link layer
prepares the encapsulated data (usually an IPv4 or IPv6 packet) for transport access across the local media by encapsulating it with a header and a trailer to create frame.
addressing
provides source and destination addressing for transporting the Layer 2 frame between devices on the same shared medium
logical topology
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 scheme. the data link layer "sees" the logical topology of a network when controlling data access to the media. it is the logical topology that influences the type of network framing and media access control used.
frame check sequence (FCS)
represents the contents of the frame. in the Ethernet trailer, the FCS provides a method for the receiving node to determine whether the frame experienced transmission errors
half-duplex
shared medium
IEEE 802 LAN/WAN standards
specific to Ethernet LANs, wireless LANs (WLAN), wireless personal area networks (WPAN), and other types of local and metropolitan area networks
What type of physical topology can be created by connecting all Ethernet cables to a central device? - bus - ring - mesh - star
star Topic 6.2.0 - Devices connected to the Ethernet star topology connect to either a hub or a switch.
What Layer 2 function does a router perform? (Choose three.) - Accepts a frame from a medium - De-encapsulates the frame - Refers to its Layer 3 routing table for a matching destination network - Re-encapsulates the packet into a new frame
- Accepts a frame from a medium - De-encapsulates the frame - Re-encapsulates the packet into a new frame
data link layer protocols
- Ethernet - 802.11 Wireless - Point-to-Point Protocol (PPP) - High-Level Data Link Control (HDLC) - Frame Relay
engineering organizations that define open standards and protocols that apply to the network access layer (i.e., the OSI physical and data link layers) including the following
- Institute of Electrical and Electronics Engineers (IEEE) - International Telecommunications Union (ITU) - International Organization for Standardization (ISO) - American National Standards Institute (ANSI)
examples of controlled access
- Legacy Token Ring - Legacy ARCNET
The IEEE 802 LAN/MAN data link layer consists of which two sublayers? (Choose two.) - Network Control Protocol - Logical Link Control - Media Access Control - Link Control Protocol
- Logical Link Control - Media Access Control
the IEEE 802 LAN/MAN data link layer consists of the following two sublayers:
- Logical Link Control (LLC) - 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
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 these are now being replaced in the WAN by Ethernet
examples of contention-based access networks
- Wireless LAN (uses CSMA/CA) - Legacy bus-topology Ethernet LAN (uses CSMA/CD) - Legacy Ethernet LAN using a hub (uses CSMA/CD)
examples of contention-based access methods
- carrier sense multiple access with collision detection (CSMA/CD) used on legacy bus-topology Ethernet LANs - carrier sense multiple access with collision avoidance (CSMA/CA) used on Wireless LANs
What kind of network would use point-to-point, hub and spoke, or mesh topologies? - PAN - LAN - WLAN - WAN
WAN wide area networks (WAN) come in many topologies, to include point-to-point, hub-and-spoke, and mesh
another form of CSMA used by IEEE 802.11 WLANs is ______
carrier sense multiple access/collision avoidance (CSMA/CA)
data
contains the frame payload (i.e., packet header, segment header, and the data)
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 Topic 6.3.0 - Encapsulation is a function of the data link layer. Different media types require different data link layer encapsulation.
Which organization defines standards for the network access layer (i.e., the OSI physical and data link layers)? - Cisco - IANA - IEEE - IETF
IEEE
What are two services performed by the data link layer of the OSI model? (Choose two.) - it provides media access control and performs error detection - it fragments data packets into the MTU size - it determines the path to forward packets - it accepts Layer 3 packets and encapsulates them into frames - it monitors the Layer 2 communication by building a MAC address table
- it provides media access control and performs error detection - it accepts Layer 3 packets and encapsulates them into frames Topic 6.1.0 - The data link layer is responsible for the exchange of frames between nodes over a physical network media. Specifically the data link layer performs two basic services: It accepts Layer 3 packets and encapsulates them into frames. It provides media access control and performs error detection. Path determination is a service provided at Layer 3. A Layer 2 switch builds a MAC address table as part of its operation, but path determination is not the service that is provided by the data link layer.
What method is used to manage contention-based access on a wireless network? - CSMA/CD - CSMA/CA - token passing - priority ordering
CSMA/CA Topic 6.2.0 - Carrier sense multiple access with collision avoidance (CSMA/CA) is used with wireless networking technology to mediate media contention. Carrier sense multiple access with collision detection (CSMA/CD) is used with wired Ethernet technology to mediate media contention. Priority ordering and token passing are not used (or not a method) for media access control.
Which data link layer media access control method does Ethernet use with legacy Ethernet hubs? - turn taking - token passing - CSMA/CD - determinism
CSMA/CD Topic 6.2.0 - CSMA/CD is used by Ethernet networks using legacy Ethernet hubs. CSMA/CA is used by 802.11-based wireless networks.
LAN and WAN frames
Ethernet protocols are used by wired LANs. wireless communications fall under WLAN (IEEE 802.11) protocols. these protocols were designed for multiaccess networks.
Which two engineering organizations define open standards and protocols that apply to the data link layer? (Choose two.) - Electronics Industries Alliance (EIA) - International Telecommunications Union (ITU) - Internet Society (ISOC) - Internet Assigned Numbers Authority (IANA) - Institute of Electrical and Electronics Engineers (IEEE)
International Telecommunication Union (ITU), Institute of Electrical and Electronics Engineers (IEEE) Topic 6.1.0 - The IANA is responsible for overseeing and managing IP address allocation, domain name management, and protocol identifiers. The EIA is an international standards and trade alliance for electronics organizations, and is best known for its standards related to electrical wiring, connectors, and the 19-inch racks used to mount networking equipment. IEEE defines open standards and protocols that apply to the network access layer.
What are the two sublayers of the OSI model data link layer? (Choose two.) - network access - LLC - internet - transport - MAC - physical
LLC, MAC Topic 6.1.0 - The data link layer of the OSI model is divided into two sublayers: the Media Access Control (MAC) sublayer and the Logical Link Control (LLC) sublayer.
What is another name for the OSI data link layer? - Layer 1 - Layer 2 - Layer 3 - Layer 6
Layer 2
What identifier is used at the data link layer to uniquely identify an Ethernet device? - MAC address - UDP port number - sequence number - TCP port number - IP address
MAC address Topic 6.3.0 - Ethernet frames are identified at the data link layer by their MAC addresses, which are unique to each NIC. IP addresses are used at the network layer, and TCP and UDP port numbers are used at the transport layer. Sequence numbers are fields in TCP headers.
What attribute of a NIC would place it at the data link layer of the OSI model? - MAC address - attached Ethernet cable - TCP/IP protocol stack - IP address - RJ-45 port
MAC address Topic 6.3.0 - The data link layer describes media access and physical addressing. The encoding of a MAC address on a NIC places it at that layer. Ports and cables are placed at the physical layer of the OSI model. IP addresses are placed at the network layer. The TCP/IP protocol stack describes a different model.
PC1 sends a frame
PC1 has an Ethernet frame to send to PC3. the PC1 NIC needs to determine if any device is transmitting on the medium. if it does not detect a carrier signal, it will assume the network is available to send. the PC1 NIC send the Ethernet Frame when the medium is available
What is the responsibility of the MAC sublayer? - Adds Layer 3 addresses to the frame - Communicates with the network layer (Layer 3) - 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
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
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.
data link layer protocols are generally not defined by ______
Request for Comments (RFCs), unlike the protocols of the upper layers of the TCP/IP suite.
legacy LAN topologies
early Ethernet and legacy Token Ring LAN technologies included two other types of topologies: - Bus - Ring
The router in the figure has an Ethernet interface to connect to the LAN and a serial interface to connect to the WAN. As the router processes frames, it will use data link layer services to receive the frame from one medium, de-encapsulate it to the Layer 3 PDU, re-encapsulate the PDU into a new frame, and place the frame on the medium of the next link of the network.
This animation illustrates how a Layer 2 frame is encapsulated and de-encapsulated as it travels in a network. A user sends an Ethernet frame to the default gateway router. When the router receives the frame, it decapsulates the Ethernet frame to read its content. It then processes the Layer 3 packet and makes a routing decision to choose a serial interface as the exit interface to the next hop IP address. The router then re-encapsulate the packet into a new Layer 2 frame and sends it to the next router across the serial link.
node
a device that can receive, create, store, or forward data along a communication path. a node can be either an end device such as a laptop or mobile phone, or an intermediary device such as an Ethernet switch.
cyclic redundancy check (CRC) value
a transmitting node creates a logical summary of the contents of the frame
hybrid
a variation or combination of any topologies. for example, a partial mesh is a hybrid topology in which some, but not all, end devices are interconnected.
the hub sends the frame
all devices attached to the hub will receive the frame. however, because the frame has a destination data link address for PC3, only that device will accept and copy in the entire frame. all other device NICs will ignore the frame.
bus
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.
contention-based access
all nodes are operating in half-duplex, competing for the use of the medium. however, only one device can send at a time. therefore, there is a process if more than one device transmits at the same time.
multiaccess LANs
end devices (i.e., nodes) are interconnected using star or extended star topologies. in this type of topology, end devices are connected to a central intermediary device, in this case, an Ethernet switch.
ring
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 (FDDI) and Token Ring networks used ring topologies
Which LAN topology is a hybrid topology? - bus - extended star - ring - star
extended star the extended star topology is considered a hybrid topology because it combines multiple star topologies
Which media access control method is used in legacy Ethernet LANs? - carrier sense multiple access/collision annoyance - carrier sense multiple access/collision avoidance - carrier sense multiple access/collision destruction - carrier sense multiple access/collision detection
carrier sense multiple access/collision detection CSMA/CD is the media access control method used in legacy Ethernet LANs
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 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 to many different destinations simultaneously
data that is transmitted over the network flows in one direction at a time Topic 6.2.0 - The data that is transmitted over the network can flow using one of three modes: Simplex - Data can only flow in one direction. Half-duplex - Data flows in one direction at a time. Full-duplex - Data flows in both directions at the same time.
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 - source NIC address, destination NIC address, source IP address, destination IP address - destination NIC address, source NIC address, destination IP address, source IP address
destination NIC address, source NIC address, source IP address, destination IP address
whether it is an Ethernet LAN using hubs, or a WLAN, contention-based systems ______ scale well under heavy media use
do not
controlled access
each node has its own time to use the medium. these deterministic types of legacy networks are inefficient because a device must wait its turn to access the medium. today, Ethernet networks operate in full-duplex and do not require an access method.
extended star
extends the star topology by interconnecting multiple Ethernet switches. the star and extended topologies are easy to install, very scalable (easy to add and remove end devices), and easy to troubleshoot. early star topologies interconnected end devices using Ethernet hubs.
frame fields
framing breaks the stream into decipherable 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.
full-duplex communication
full-duplex allows the sending and receiving of data to happen simultaneously. 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. Ethernet switches operate in full-duplex mode by default, but they can operate in half-duplex if connecting to a device such as an Ethernet hub.
Ethernet LANs using switches do not use a contention-based system because the switch and the host NIC operate in ______
full-duplex mode
in a fragile environment, more controls are needed to ensure delivery. the header and trailer fields are larger as more control information is needed.
greater effort is needed to ensure delivery. this means higher overhead and slower transmission rates.
Which duplex communication method is used in WLANs? - full-duplex - half-duplex - simplex
half-duplex wireless LANs (WLANs) only support half-duplex because only one device can access the media at a time
there are two common modes of duplex
half-duplex communication full-duplex communication
half-duplex communication
half-duplex communications restrict the exchange of data to one direction at a time. both devices can transmit and receive on the media but cannot do so simultaneously. WLANs and legacy bus topologies with Ethernet hubs use the half-duplex mode. half-duplex allows only one device to send or receive at a time on the shared medium.
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
control
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.
type
identifies the Layer 3 protocol in the data field
physical topology
identifies the physical connections and how end devices and intermediary devices (i.e., routers, switches, and wireless access points) are interconnect. the topology may also include specific device location such as room number and location on the equipment rack. physical topologies are usually point-to-point or star
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.
error detection
included after the data to form the trailer
error detection
includes a trailer used to detect transmission errors
addressing
indicates the source and destination nodes on the media
how the data link layer adds Layer 2 Ethernet destination and source NIC information to a Layer 3 packet
it would then convert this information to a format supported by the physical layer (i.e., Layer 1)
What is true concerning physical and logical topologies? - 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
logical topologies refer to how a network transfers data between devices Topic 6.2.0 - Physical topologies show the physical interconnection of devices. Logical topologies show the way the network will transfer data between connected nodes.
Which topology displays networking device layer IP addresses? - aerial topology - IP address topology - logical topology - physical topology
logical topology the logical topology shows the IP addresses assigned to device interfaces
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? - bus - star - ring - mesh - hierarchical
mesh Topic 6.2.0 - The mesh topology provides high availability because every node is connected to all other nodes. Mesh topologies can be found in WANs. A partial mesh topology can also be used where some, but not all, end points connect to one another.
the two sublayers - LLC and MAC - of the data link layer
the LLC 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 mode. the MAC sublayer controls the NIC and other hardware that is responsible for sending and receiving data on the wired or wireless LAN/MAN medium
topology
the arrangement, or the relationship, of the network devices and the interconnections between them
Layer 2 addresses
the data link layer provides the addressing used in transporting a frame across a shared local media. device addresses at this layer are referred to as physical addresses.
frame delimiting
the framing process provides important delimiters to identify fields within a frame. these delimiting bits provide synchronization between the transmitting and receiving nodes.
The Internet Engineering Task Force (IETF) maintains ______
the functional protocols and services for the TCP/IP protocol suite in the upper layers, but they do not define the functions and operation of the TCP/IP network access layer
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.
Although CSMA/CD is still a feature of Ethernet, why is it no longer necessary? - the use of full-duplex capable Layer 2 switches - the development of half-duplex switch operation - the use of Gigabit Ethernet speeds - the virtually unlimited availability of IPv6 addresses - the use of CSMA/CA
the use of full-duplex capable Layer 2 switches Topic 6.2.0 - The use of Layer 2 switches operating in full-duplex mode eliminates collisions, thereby eliminating the need for CSMA/CD.
logical link control (LLC)
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 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
hub and spoke WAN topology
this is a 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.
point-to-point WAN topology
this is the simplest and most common WAN topology. it consists of a permanent link between two endpoints. physical point-to-point topologies directly connect two nodes.
mesh WAN topology
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-point link to the other node.
What is the function of the last field in a data link layer frame? - to determine whether the frame experienced transmission errors - to identify special flow control services such as 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
Which is a function of the Logical Link Control (LLC) sublayer? - to identify which network layer protocol is being used - to provide data link layer addressing - 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 Topic 6.1.0 - Defining the media access processes that are performed by the hardware and providing data link layer addressing are functions of the MAC sublayer. The data link layer accepts Layer 3 packets and packages them into data units that are called frames.
frame start and stop indicator flags
used to identify the beginning and end limits of the frame
