25 terms

Chap.4 The data link layer


Terms in this set (...)

What layers are equivalent to network access layer in TCP/IP model?
Data link (Layer 2)
Physical (Layer 1)
What is the 2 main function of the data link layer?
*It accepts Layer 3 packets and packages them into data units called frames.
*It controls media access control and performs error detection.
What is the 2 sublayers of the data link layer?
Logical Link Control (LLC): This upper sublayer defines the software processes that provide services to the network layer protocols. It places information in the frame that identifies which network layer protocol is being used for the frame.
Media Access Control (MAC): This lower sublayer defines the media access processes performed by the hardware. It provides data link layer addressing and delimiting(delning) of data.
Benefit of separating the data link layer?
It allows for one type of frame defined by the upper layer to access different types of media defined by the lower layer.
Media access control
The packets do not have a way to directly access these different media. It is the role of the OSI data link layer to prepare network layer packets for transmission and to control access to the physical media.
What does a data link frame include?
Header: Contains control information, such as addressing, and is located at the beginning of the PDU(protocol data unit).
Data: Contains the IP header, transport layer header, and application data.
Trailer: Contains control information for error detection added to the end of the PDU.
What fields does a frame include?
*Frame start and stop indicator flags: Used by the MAC sublayer
*Addressing: Used by the MAC sublayer to identify the source and destination nodes.
*Type: Used by the LLC to identify the Layer 3 protocol.
*Control: Identifies special flow control services.
*Data: Contains the frame payload (i.e., packet header, segment header, and the data).
*Error Detection: Included after the data to form the trailer, these frame fields are used for error detection.
What id the main roll of MAC-sublayer?
Regulating the placement of data frames onto the media
Physical and logical topologies?
Physical topology: Refers to the physical connections and identifies how end devices and infrastructure devices are interconnected.
Logical topology: Refers to the way a network transfers frames from one node to the next. logical signal paths are defined by data link layer protocols.
Common physical WAN topologies?
Point-to-Point: This is the simplest topology which consists of a permanent link between two endpoints. this is a very popular WAN topology.
Hub and Spoke: A WAN version of the star topology in which a central site interconnects branch sites using point-to-point links.
Mesh: This topology provides high availability, but requires that every end system to be interconnected to every other system. Therefore the administrative and physical costs can be high.
Physical point-to-point topology
it directly connects two nodes.
Logical point-to-point topology
the source and destination node may be indirectly connected to each other over some geographical distance.
In point-to-point networks, how can data flow? (2 ways)
Half-duplex communication: Both devices can both transmit and receive on the media but cannot do so simultaneously.
Full-duplex communication: Both devices can transmit and receive on the media at the same time.
4 types of Physical LAN topologies?
Star: End devices are connected to a central intermediate device(switch). The star topology is the most common physical LAN topology.
Extended star or hybrid: In an extended star topology, central intermediate devices interconnect other star topologies.
Bus: All end systems are chained to each other and terminated(avslutas) in some form on each end.
Ring: End systems are connected to their respective neighbor forming a ring.
2 Media access control methods for shared media?
Contention-based access: All nodes compete for the use of the medium but have a plan if there are collisions.
Controlled access: Each node has its own time to use the medium.
Contention-based access
*Stations can transmit at any time
*collisions exist
*there are mechanisms to resolve contention for the media
*Carrier Sense Multiple Access (CSMA) process is used to detect if the media is carrying a signal
What is the 2 common methods for resolving contention in contention-based access?
*Carrier sense multiple access with collision detection (CSMA/CD): The end device checks the media if there's a data signal. If a the media is free, the device transmits the data. If signals are detected, all devices stop sending and try again later. Traditional forms of Ethernet use this method.
*Carrier sense multiple access with collision avoidance (CSMA/CA): The end device checks the media for the presence of a data signal. If the media is free, the device sends a notification across the media of its intent to use it. Once it receives a clearance to transmit, the device then sends the data. This method is used by 802.11 wireless networking technologies.
what is a logical multi-access topology?(LAN)
enables a number of nodes to communicate by using the same shared media.
Data from only one node can be placed on the medium at any one time. Every node sees all the frames that are on the medium, but only the node to which the frame is addressed processes the contents of the frame.
How does a ring topology work?
each node in turn receives a frame. If the frame is not addressed to the node, the node passes the frame to the next node.
How does a data link frame built?
Each frame type has three basic parts:


All data link layer protocols encapsulate the Layer 3 PDU within the data field of the frame. However, the structure of the frame and the fields contained in the header and trailer vary according to the protocol.
What describes data link layer protocols?
Difference between sending data in a fragile environment and secure environment?
They describe the features required for the transport of packets across different media.

In a fragile environment, more controls are needed. Header and trailer fields are larger as more control inf are needed.
Fields in an Ethernet frame header?
*Start Frame field: Indicates the beginning of the frame.
*Source and Destination Address fields: Indicates the source and destination nodes on the media.
*Type field: Indicates the upper layer service contained in the frame.
The data link layer(layer 2) address
Device addresses at this layer are referred to as physical addresses. the physical address is a unique device specific address. Data link layer addressing is contained within the frame header and specifies the frame destination node on the local network.
Functions and fields in frame trailers?
FCS: used for error checking. The sourse calculates and place a number in the FCS field. The destination calculates and check if the FCS matches. If it doesn't match, the frame is deleted.
Stop Frame: an optional field that's used when the length of the frame is not specified in the length field. indicates the end of the frame.
Main cable types in UTP Cabling?
*Ethernet Straight-through: The most common type of networking cable. It is commonly used to interconnect a host to a switch and a switch to a router.
*Ethernet Crossover: An uncommon cable used to interconnect similar devices together. For example to connect a switch to a switch, a host to a host, or a router to a router.
*Rollover: A Cisco proprietary cable used to connect to a router or switch console port.