Chapter 7 Flashcards
(21 cards)
How does the data link layer prepare packets for transmission?
The data link layer prepares a packet for transport across the local media by encapsulating it with a header and a trailer to create a frame.
Describe four general data link layer media access methods. Suggest data communications environments in which these access methods can be appropriately implemented
MAC methods for shared media are as follows:
■ Controlled: Each node has its own time to use the medium, a ring topology
■ Contention-based: All nodes compete for the use of the medium, a bus topology
Media access control in point-to-point connections can be one of the following:
■ Half duplex: A node can only transmit or receive at one time.
■ Full duplex: A node can both transmit and receive at the same time.
Describe the features of a logical ring topology.
In a logical ring topology, each node in turn receives a frame. If the frame is not addressed to a node, the frame is passed to the next node. If there is no data being transmitted, a signal (known as a token) can be placed on the media. A node can place\ a frame on the media only when it has the token. This is a controlled media access control technique called token passing
Name five Layer 2 protocols.
Layer 2 protocols include
■ Ethernet
■ PPP
■ High-Level Data Link Control (HDLC)
■ Frame Relay
■ ATM
List five header field types in data link frames
Typical frame header fields include
■ Start Frame field: Indicates the beginning of the frame
■ Source and Destination address fields: Indicate the source and destination nodes on the media
■ Priority/Quality of Service field: Indicates a particular type of communication service for processing
■ Type field: Indicates the upper-layer service contained in the frame
■ Logical connection control field: Used to establish a logical connection between nodes
■ Physical link control field: Used to establish the media link
■ Flow control field: Used to start and stop traffic over the media
■ Congestion control field: Indicates congestion in the media
If a node receives a frame and the calculated CRC does not match the CRC in the FCS, what action will the node take?
A. Drop the frame
B. Reconstruct the frame from the CRC
C. Forward the frame as it is to the next host
D. Disable the interface on which the frame arrives
A. The node drops the frame. The CRC provides error detection, not error correction, so B is incorrect. C is incorrect because the frame is not forwarded. The interface is not disabled, so D is incorrect.
Which of the following protocols are typically used in WANs? (Choose two.)
A. 802.11
B. Ethernet
C. HDLC
D. PPP
C, D. PPP and HDLC are designed as WAN protocols. 802.11 and Ethernet are LAN protocols, so A and B are incorrect.
What are the contents of the data field in a frame?
A. 64 bytes
B. The network layer PDU
C. The Layer 2 source address
D. The data directly from the application that generated the data
B. The network layer PDU is encapsulated in the frame. The number of bytes in the payload is variable, so A is incorrect. C is incorrect because the Layer 2 source address is in the address field of the frame header. The data from the application undergoes encapsulation before being passed down to the data link layer, so D is incorrect.
Which of the following is a characteristic of contention-based MAC?
A. Used on nonshared media.
B. Nodes compete for the use of the medium.
C. Leaves MAC to the upper layer.
D. Each node has a specific time to use the medium.
B. Nodes compete for the media. Option A is incorrect because contention-based is used on shared media. C is incorrect because one of the primary purposes of Layer 2 is MAC. D is incorrect because taking turns is a function of controlled access.
Which of the following are common data link sublayers used in LANs? (Choose two.)
A. Protocol data unit
B. Logic Link Control
C. MAC
D. Network interface card
E. Carrier access multiaccess
B, C. LLC is the upper sublayer, and MAC is the lower sublayer
Which of the following describes a virtual circuit?
A. Is an error-detection technique
B. Provides an encapsulation technique
C. Is used only with point-to-point physical topologies
D. Establishes a logical connection between two network devices
D. Virtual circuits establish a logical connection between two devices to provide a logical point-to-point topology. Option A is incorrect because CRC is an error-detection technique. Virtual circuits do not provide an encapsulation technique, so B is incorrect. C is incorrect because virtual circuits can be used over multiple types of physical topologies.
Name three basic parts that are common to all data link layer frames
Header, data, and trailer.
Which of the following functions does the data link layer perform?
A. Provides user interfaces
B. Ensures end-to-end delivery of data between hosts
C. Connects the network software to the network hardware
D. Establishes and maintains sessions between applications
C. The data link layer provides the connection between hardware and software. A is incorrect; this is a role of the application layer. B is incorrect; this is a function of the network layer. D is incorrect; this is a function of the transport layer
Which of the following is true about the logical topology of a network?
A. Is always multiaccess
B. Provides the physical addressing
C. Is determined by how the nodes in the network are connected
D. Influences the type of MAC used in the network
D. The logical topology influences MAC. The logical topology can be many types of MAC, so A is incorrect. The MAC sublayer provides the physical address, so B is incorrect. The logical and physical topologies are not always the same, so C is incorrect
Explain the purpose of the Frame Check Sequence field in a data link frame trailer.
The media is a potentially unsafe environment for data. The signals on the media could be subject to interference, distortion, or loss that would substantially change the bit values that those signals represent. To ensure that the content of the received frame at the destination matches that of the frame that left the source node, a transmitting node creates a logical summary of the contents of the frame. This is known as the Frame Check Sequence (FCS) and is placed in the trailer to represent the contents of the frame. When the frame arrives at the destination node, the receiving node calculates its own logical summary, or FCS, of the frame. The receiving node compares the two FCS values. If the two values are the same, the frame is considered to have arrived as transmitted. If the FCS values differ, the frame is discarded. There is always the small possibility that a frame with a good FCS result is actually corrupt. Errors in bits can cancel each other out when the FCS is calculated. Upperlayer protocols would then be required to detect and correct this data loss
How do data link layer addresses differ from network layer addresses?
Unlike Layer 3 logical addresses that are hierarchical, physical addresses do not indicate on what network the device is located. If the device is moved to another network or subnet, it will still function with the same Layer 2 physical address. Because the frame is only used to transport data between nodes across the local media, the data link layer address is only used for local delivery. Addresses at this layer have no meaning beyond the local network. Compare this to Layer 3, where addresses in the packet header are carried from source host to destination host, regardless of the number of network hops along the route.
Compare and contrast the logical point-to-point and logical multiaccess topologies
A logical point-to-point topology connects two nodes directly. In data networks with point-to-point topologies, the MAC protocol can be very simple. All frames on the media can only travel to or from the two nodes. The frames are placed on the media by
the node at one end and taken off the media by the node at the other end. In point-topoint networks, if data can only flow in one direction at a time, it is operating as a halfduplex link. If data can successfully flow across the link from each node simultaneously, it is a full-duplex service.
Describe the issues in a router when it is interconnecting interfaces of different speeds, such as connecting an Ethernet network to a WAN on a serial interface.
If a router is interfacing media of different speeds, the router will have to buffer the frames for transmission. If not enough buffers are available, the packets can be lost.
Discuss the purpose of including a source address in the frame header. Could just one Layer 2 address be used? If so, how? Are there any data link layer protocols that use a single address?
The source addresses are used to identify the source node. In most cases, Layer 2 source addresses are not used. The most common use of source addresses is for security or by switches learning where the host exists. The source address is also used in the creation of dynamic maps such as ARP
Discuss the possible effect on throughput if a communication is operating in full duplex. Compare full duplex to multiaccess and half duplex.
The full-duplex communication between the two nodes could have twice the throughput of half-duplex and more than twice the throughput of multiaccess. If the underlying physical media can support it, the two nodes might be able to transmit and receive at full media bandwidth at the same time. This would be twice that of half duplex. Because multiaccess has overhead to control media access, the throughput is less than the bandwidth and, in many cases, much less. This would allow full duplex to have more than twice the throughput of multiaccess.
Describe how the router can use different frame formats to forward an IP packet.
As the router receives a frame on one interface, it decapsulates the frame down to a packet. It then refers to the routing table to determine out which interface the packet should be forwarded. The router then will encapsulate the packet into a frame of the appropriate size for the segment connected to the outbound interface.
