Intro to Networking Flashcards
datalink sub layers
The data link layer is divided into two sublayers:
Logical Link Control (LLC) - This upper sublayer communicates with the network layer. 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 utilize the same network interface and media.
Media Access Control (MAC) - This lower sublayer defines the media access processes performed by the hardware. It provides data link layer addressing and access to various network technologies.
Wan physical topologies
WANs are commonly interconnected using the following physical topologies:
Point-to-Point - This is the simplest topology that consists of a permanent link between two endpoints. For this reason, 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 be 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.
wan topology hybrid
A hybrid is a variation or combination of any of the above topologies. For example, a partial mesh is a hybrid topology in which some, but not all, end devices are interconnected.
virtual circuit
In some cases, the logical connection between nodes forms what is called a virtual circuit. A virtual circuit is a logical connection created within a network between two network devices. The two nodes on either end of the virtual circuit exchange the frames with each other.
Physical Lan topology types
Physical topology defines how the end systems are physically interconnected. In shared media LANs, end devices can be interconnected using the following physical topologies:
Star - End devices are connected to a central intermediate device. Early star topologies interconnected end devices using Ethernet hubs. However, star topologies now use Ethernet switches. The star topology is easy to install, very scalable (easy to add and remove end devices), and easy to troubleshoot.
Extended Star - In an extended star topology, additional Ethernet switches interconnect other star topologies. An extended star is an example of a hybrid topology.
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. Bus topologies using coax cables were used in legacy Ethernet networks because it was inexpensive and easy to set up.
Ring - End systems are connected to their respective neighbor forming a ring. Unlike the bus topology, the ring does not need to be terminated. Ring topologies were used in legacy Fiber Distributed Data Interface (FDDI) and Token Ring networks.
half and full duplex communications
Half-duplex communication - Both devices can transmit and receive on the media but cannot do so simultaneously. The half-duplex mode is used in legacy bus topologies and with Ethernet hubs. WLANs also operate in half-duplex. Half-duplex allows only one device to send or receive at a time on the shared medium and is used with contention-based access methods. Figure 1 shows half-duplex communication.
Full-duplex communication - Both devices can transmit and receive on the media at the same time. 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 can operate in half-duplex if connecting to a device such as an Ethernet hub. Figure 2 shows full-duplex communication.
duplex communications
Duplex communications refer to the direction of data transmission between two devices. Half-duplex communications restrict the exchange of data to one direction at a time while full-duplex allows the sending and receiving of data to happen simultaneously.
Access control methods for sharing media (two)
There are two basic access control methods for shared media:
Contention-based access - All nodes operating in half-duplex compete for the use of the medium, but only one device can send at a time. However, there is a process if more than one device transmits at the same time. Ethernet LANs using hubs and WLANs are examples of this type of access control. Figure 1 shows contention-based access.
Controlled access - Each node has its own time to use the medium. These deterministic types of networks are inefficient because a device must wait its turn to access the medium. Legacy Token Ring LANs are an example of this type of access control. Figure 2 shows controlled access.
frames in a datalink layer typically has 3 parts
The description of a frame is a key element of each data link layer protocol. Although there are many different data link layer protocols that describe data link layer frames, each frame type has three basic parts:
Header
Data
Trailer
frame fields (not all frame protocols have these fields)
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 can be received by nodes and decoded into packets at the destination.
As shown in the figure, generic frame field types include:
Frame start and stop indicator flags - Used to identify the beginning and end limits of the frame.
Addressing - Indicates the source and destination nodes on the media.
Type - Identifies the Layer 3 protocol in the data field.
Control - Identifies special flow control services such as quality of service (QoS). QoS is used to give forwarding priority to certain types of messages. Data link frames carrying voice over IP (VoIP) packets normally receive priority because they are sensitive to delay.
Data - Contains the frame payload (i.e., packet header, segment header, and the data).
Error Detection - These frame fields are used for error detection and are included after the data to form the trailer.
A transmitting node creates a logical summary of the contents of the frame,
known as the cyclic redundancy check (CRC) value and is placed in the fcs
Fcs-Frame check field
. In the Ethernet trailer, the FCS provides a method for the receiving node to determine whether the frame experienced transmission errors.
