Michael S Murphy | NET 125
Chapter 7: OSI Data Link Layer
Vocabulary Exercise: Matching
Term Definition
a. Frame a. Two or more devices connected to a common medium
b. Node d. A layer of the Open Systems Interconnection (OSI) model that
frames upper-layer data and controls how data is placed on a medium
c. Media b. A device on a network
d. Network e. The physical means used to carry data signals
e. Data Link c. The protocol data unit (PDU) used in Layer 2 of the OSI model
Concept Questions
1. What are the two main jobs of the data link layer?
Allows the upper layers to access the media using techniques such as framing.
Controls how data is placed onto the media and is received from the media using
techniques such as media access control (MAC) and error detection.
2. What is the difference between a logical network and a physical network?
Logical network refers a group of devices that associated by the arrangement of
a hierarchical addressing scheme.
Physical network refers the connection of devices on a common media.
3. If the data link layer didn’t exist, what changes would be required of a network layer
protocol such as Internet Protocol (IP)?
Without the data link layer, a network layer protocol, like IP, would require provisions for
connecting to every type of media that could exist along a delivery path.
Vocabulary Exercise: Completion
1. The technique for getting a frame on and off a medium is called the MAC method.
2. To connect to a network, an end device uses an adapter, such as a Network
Interface Card (NIC) on a Local Area Network (LAN). The adapter manages the
framing and media access method.
3. An intermediary device, such as routers, has physical interfaces that can
encapsulate a packet into the appropriate frame and handle the MAC method to
access each link.
4. A router uses data link layer services to receive a frame from one medium,
decapsulate the frame to the Layer 3 PDU, encapsulate the Layer 3 PDU into a new
frame, and place the frame on the medium of the next link in the network.
Concept Questions
1. Compare and contrast controlled media access control and contention-based media
access control.
Although controlled media access is well ordered and provides predictable throughput,
these methods can be inefficient because a device has to wait for its turn before it can
use the medium. Contention-based media access allows any device to try to access the
medium whenever it has data to send.
2. List the steps in the carrier sense multiple access collision detect (CSMA/CD)
process. At this point, you can keep the list simple. You will learn more about CSMA/CD
in Chapter 9.
In CSMA/CD, the device monitors the media for the presence of data signal.
3. What is the difference between full-duplex communication and half-duplex
communication?
Half-duplex communication means that the devices can both transmit and receive on
the media but cannot do so simultaneously. In full-duplex communication, both devices
can transmit and receive on the media at the same time.
Vocabulary Exercise: Completion
1. A logical multiaccess topology enables a number of nodes to communicate by using
the same shared media.
2. Having many nodes share access to the medium requires a data link MAC layer
media access control method to regulate the transmission of data.
3. Three media access control methods used by logical multi-access topologies include
Header, Data, and Trailer.
4. Data link layer rules (also called protocols) specify the media access control method
that is used for a particular Frame Check Sequence (FCS). For example, Ethernet
uses 802.3 Standards.
Vocabulary Exercise: Matching
Field Definition
a. Start Frame g. Used to indicate overloading on the medium
b. Address f. Used to start and stop traffic when overloading occurs
c. Type d. Specifies the number of bytes in the data part of the frame
d. Length c. Indicates the upper-layer service contained in the frame
e. Priority a. Tells other devices on the network that a frame is coming along the
medium
f. Flow control b. Identifies the sender and receiver
g. Congestion e. Indicates a particular type of communication service for special
control processing
Protocol Specification
a. Ethernet c. A protocol for wireless networks that uses CSMA/CA
b. Point-to-Point a. Defined by the IEEE 802.3 standard
Protocol
c. 802.11 a. Includes specifications for operation at 10, 100, 1000, and
10,000 Mbps
b. Establishes a logical connection (session) between two nodes
c. Uses a preamble field at the beginning of the frame
c. Often found on WANs
a. Provides unacknowledged connectionless service over a shared
medium using CSMA/CD
b. Defined in a Request For Comments (RFC) document
b. Uses a flag field at the beginning of the frame
c. Uses an acknowledgment to confirm that a frame was received
successfully
Concept Questions
1. Compare and contract Layer 2 addresses with Layer 3 addresses
As compared 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,
Layer 2 uses addressing to transport the frame across the local media.
2. Why are Layer 2 addresses not needed in point-to-point topologies?
Point-to-Point topologies, with just two interconnected nodes, do not require addressing.
3. What is the purpose of the Frame Check Sequence (FCS) in a frame trailer?
FCS is used to determine whether errors occurred in the transmission and reception of
the frame.
Vocabulary Exercise: Completion
1. The user starts by clicking a link on a web page. A TCP segment handshake sets up
a connection with the server.
2. The user’s web browser initiates a Hypertext Transfer Protocol (HTTP) request. The
application layer adds a Layer 7 header.
3. The transport layer identifies the upper-layer service that user wants to reach as the
HTTP (or World Wide Web [WWW]) service. The transport layer places a destination
acknowledgment number of WWW in the TCP segment to indicate the WWW service.
The transport layer also places the source port number for this session in the segment.
4. TCP adds an acknowledgment number that tells the web server the sequence
number that TCP expects in the next segment it receives.
5. At the network layer, an IP packet is constructed to identify the source and
destination hosts. For the destination address, the client uses the IP address
associated with the World Wide Web (WWW). For the destination address, the client
uses the IP address associated with the WWW server’s hostname. It uses its own IPv4
address as the source address.
6. The data link layer refers to the Address Resolution Protocol (ARP) cache to
determine the Media Access Control (MAC) address that is associated with the Ethernet
interface on the client’s router. The Client builds an Ethernet II frame to transport the
IPv4 packet across the local medium.
7. The data link layer frame indicates that the upper-layer data is IPv4 by placing
0x0800 in the Type field of the Ethernet II header. The frame begins with a Preamble
and Start of Frame (SOF) field and ends with an cyclic redundancy check (CRC)
field for error detection. The client uses the CSMA/CD media access control method to
verify that the medium is not already in use.
8. The physical layer transmits the frame onto the medium bit by bit.
9. The router that receives the frame checks the FCS at the end of the frame to
determine if the frame was received intact without any errors. The router removes the
encapsulation header and pushes the packet up to the network layer.
10. At the network layer, the destination IPv4 address in the packet is compared to the
routes in the routing table. A match is found, and the router determines that the next
hop for the packet is a router at the other end of a PPP WAN link.
11. The router creates a Point-to-Point Protocol (PPP) frame to transport the packet
across the WAN.
12. The router includes a Protocol field in the PPP header with a value of 0x0021 to
indicate that an IPv4 packet is encapsulated.
13. The PPP session has already been established, so the physical layer begins
transmitting the frame onto the WAN medium bit by bit.
14. The recipient router checks the bits to determine if the frame was received intact
without any errors. The router removes the encapsulation header and pushes the
packet up to the network layer.
15. At the network layer, the packet in the destination IPv4 address is compared to
routers in the routing table. The router determines that the packet should be sent out
an Ethernet network to the web server.
16. The router consults its ARP cache to determine the MAC address of the web server.
It then builds an Ethernet II frame to transport the IPv4 packet to the server. It uses a
router to verify that the medium is not already in use.
17. The physical layer transmits the frame onto the medium bit by bit.
18. The server examines the frame. It checks the frame to determine if the frame was
received intact without any errors.
19. The server compares the destination MAC address in the frame to the MAC
address of the NIC in the server. Because it matches, the server removes the
encapsulation header and pushes the packet up to the network layer.
20. The server compares the destination IPv4 address in the packet to its own IPv4
address. Because it matches, the server removes the encapsulation header and
pushes the data to the network layer. Because the IP network layer header identified
the upper-layer protocol as 0x06, the server pushes the data to network.
21. The server examines the TCP segment to determine the session to which the data
belongs. This is done by examining the source and destination ports. The TCP
sequence number is used to place this segment in the proper order to be sent upward
to the application layer.
22. At the application layer, the HTTP GET is delivered to the WWW service. This
service can now form a response. The 22 steps reverse themselves, and a packet flows
back to the client. Eventually, as soon as all the necessary packets have been sent and
received, the user sees a web page.