CCNA Exploration Chapter 7 data link layer. Study questions. Answers



What are the two basic services at the data link layer?


Takes data from the upper layers and encapsulates it in preparation for placing it on the media. Controls how data is placed on the media and received.


What is the name of the Data Link layer PDU?




What are media (in the context of OSI layers 1 and 2)?


The physical materials that carry signals. These can be copper cable, fibre optic cable or the atmosphere carrying radio waves.


What are Nodes


Network devices connected to a common medium


What is a physical network?


A set of devices (nodes) connected to a common transmission medium


The sending host encapsulates the packet into a frame before putting it on the transmission medium. Does the frame stay the same until it reaches its final destination?


If the final destination is on the same physical network as the sending host then the frame will stay the same. If the final destination is on another network then the frame header and trailer will be stripped off and replaced by each intermediate router. The packet inside stays the same, apart from minor adjustments such as updating the TTL field.


Why does IP not have to change every time there are developments in transmission media (e.g. introduction of Ethernet speeds greater than 1Gbps)?


The data link and physical layer protocols are adapted to cater for the new technologies. Their interface with IP is kept the same so that IP does not need to change. This is an advantage of using a layered model of networking.


Which part of a PC is responsible for carrying out data link functions and placing messages on the medium?


The network interface card (NIC). (Alternatively the electronic components may be integrated on the motherboard.)


Why do routers often have more than one type of physical interface (port)?


Routers connect networks together. If the networks have different types of media then the router will need different interfaces for the different media.


There is only one network layer protocol (IP) generally used on the Internet for addressing and encapsulating packets. Why are there several different data link layer protocols in common use on the Internet?


Different media may require different access methods and different data link layer protocols to provide the necessary control features.


The transport layer and the network layer add addressing and control information to the data in the form of a header. How is the data link layer different?


It adds both a header and a trailer.


What is the purpose of the first field in a frame header?


It is the start field, containing a special pattern of bits that show that a frame is starting. This alerts the receiving device to the fact that a frame is arriving on the medium.


What other fields are typically found in a frame header?


Addressing or name fields, type field, quality control field.


What fields are typically found in a frame trailer?


Error detection field and stop field to show the end of the frame.


What are the two data link sublayers, and what is their function?


The logical link control sublayer defines how the packet is encapsulated into a frame and includes information to say which network layer protocol was used. The media access control sublayer adds addressing information to the frame and adds whatever start and stop information is required by the medium.


Why can it be useful to divide the data link layer into sublayers?


Ethernet, the most common LAN technology, has a common frame structure, but can be used over a variety of media, so it is convenient to divide the features that are always the same from the features that differ. Some other LAN technologies behave in a similar way.


What is defined by IEEE 802.11


Wireless LAN


What is defined by !EEE 802.3





What are shared media, and why do they need media access control?


Media are shared if more than one device is able to put signals on them. If two or more devices put signals on the medium at the same time then the signals will collide and be corrupted. There must be a method of controlling how and when devices place signals on shared media.


What are the characteristics of controlled access or deterministic media access control?


Only one device may transmit at a time. Other devices must wait until it is their turn. There are no collisions. One control method is to use token passing – a device can transmit only if it holds the token.


What are the advantages and disadvantages of controlled access?


Advantages: Well ordered. Predictable – each host will get its turn after a predictable waiting time.

Disadvantages: hosts have to wait even if there is no traffic. High overhead.


Which LAN technologies use controlled access?


Token ring, FDDI.


What are the characteristics of contention based media access control?


Any host is able to transmit at any time. Collisions can occur. There must be a method of avoiding collisions or of recovering from collisions.


What are the advantages and disadvantages of contention based media access control?


Advantages: less overhead for the control, and hosts do not need to wait unnecessarily.
Disadvantages: collisions get more frequent as more hosts are added, and the process of recovering from collisions takes up bandwidth.


Which LAN technologies use contention based media access control?


Ethernet, wireless.


What is the advantage of using full duplex communication on a point-to-point link?


Full duplex is “two-way traffic”. Both hosts can transmit and receive at the same time (using separate wires/fibres). There are only two hosts on a point to point link so there are no collisions and no need to have rules about when hosts can place signals on the medium.


What is the physical topology of a network?


The way the devices are arranged and linked by cables. (E.g. bus, star, ring, mesh)


What is the logical topology of a network?


The way the network sends frames from one node to the next. E.g. point to point, multi-access, ring. This may not be the same as the physical topology.


A network has a logical ring topology. Does it necessarily have a physical ring topology?


No. (It might be cabled as a star.)


Which logical topology typically uses CSMA/CD or CSMA/CA as a media access control method?





What is the name given to addresses added at the data link layer?


Physical addresses


If a data link layer address is used only for delivery on the local network, how can a message be sent to a remote network?


The message is delivered to a router. The router will remove the frame header and trailer and encapsulate the packet with a new frame header and trailer. The new header will contain a new data link layer address so that the message can be sent on the next hop. This continues until the message reaches its destination network.


What is the purpose of the FCS field in the frame trailer?


Frames may be corrupted as they are transmitted. The sending device calculates a CRC (cyclic redundancy check) value based on the contents of the frame, and places it in the FCS field. The receiving host calculates the CRC value and compares it with the value in the FCS field. If it is different then the frame is corrupt and is discarded.



If an error is found in a frame, can it be corrected?


Some data link layer protocols have methods of correcting errors and others do not.


Why do LANs and WANs use different data link layer protocols?


LANs typically use high bandwidth technologies and support a large number of hosts over a small geographical area. WANs typically use lower bandwidth technologies over long distances. These different technologies require different protocols.


What remains the same for all versions of Ethernet, operating at defferent speeds?


The basic frame structure.


What is a MAC address?


The name given to an Ethernet data link layer address (physical address).


How many bits are in a MAC address and how is it written?


48 bits, written in hexadecimal.


A host on an Ethernet network is sending a message to a destination host on a remote network. It puts the IP address of the destination host in the packet header. What destination address will it put in the frame header?


The MAC address of the local router. Specifically, the address of the router’s interface on the same network as the sending host.


Why does a PPP frame not contain full layer 2 addresses?


The link is point-to-point, there is only one possible destination, so full addresses are not needed.


What physical media does PPP use?


It can use a variety: copper cable, fibre optic, satellite.


PPP sets up sessions. What options can the two nodes negotiate within a session?


Authentication, compression, multilink.


Which standard is commonly called WiFi?


IEEE 802.11


What does WiFi have in common with Ethernet?


It uses the same logical link control layer protocols and the same addressing scheme. It is also susceptible to collisions (but deals with them differently).