Data Link Layer 2019/2/19

Two basic services of Data Link 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 and error detection 2019/2/19

Data Link Terms 2019/2/19

Note … Medium or media refer to the material that actually carries the signals representing the transmitted data. Not audio, video, and the likes … 2019/2/19

Physical vs logical network Logical networks are defined at the Network layer by the arrangement of the hierarchical addressing scheme. Physical networks represent the interconnection of devices on a common media. Sometimes, also referred to as a network segment. 2019/2/19

Recall … A network model allows each layer to function with minimal concern for the roles of the other layers. The Data Link layer relieves the upper layers from the responsibility of putting data on the network and receiving data from the network. 2019/2/19

Data Link: supporting its upper layer In any given exchange of Network layer packets, there may be numerous Data Link layer and media transitions. At each hop along the path, an intermediary device - usually a router - accepts frames from a medium, de-capsulates the frame, and then forwards the packet in a new frame appropriate to the medium of that segment of the physical network. 2019/2/19

A layer 3 packet may have to go through various data link networks Satellite Satellite Satellite Wifi Wifi Ethernet Ethernet Ethernet 2019/2/19

Data Link: supporting its upper layer The Data Link layer effectively insulates the communication processes at the higher layers from the media transitions that may occur end-to-end. A packet is received from and directed to an upper layer protocol that does not need to be aware of which media the communication will use. 2019/2/19

Data Link: supporting its upper layer Due to the higher number of communication services provided by the Data Link layer … it is difficult to generalize their role and provide examples of a generic set of services. any given upper layer protocol may or may not support all these Data Link layer services. 2019/2/19

Transfer of Frames 2019/2/19

Media Access Control Each network environment that packets encounter as they travel from a local host to a remote host can have different characteristics. one network environment may consist of many hosts contending to access the network medium on an ad hoc basis. Another environment may consist of a direct connection between only two devices over which data flows sequentially as bits in an orderly way. 2019/2/19

Media Access Control (MAC) Define the processes by which network devices can access the network media and transmit frames in diverse network environments. 2019/2/19

MAC A node that is an end device uses an adapter to make the connection to the network. For example, to connect to a LAN, the device would use the appropriate Network Interface Card (NIC) to connect to the LAN media. The adapter manages the framing and media access control. 2019/2/19

Network Interface Card (NIC) Different physical interfaces on the router are used to encapsulate the packet into the appropriate frame, and a suitable media access control method is used to access each link. E.g., a router may have an Ethernet interface to connect to the LAN and a serial interface to connect to the WAN. 2019/2/19

Framing : Creating a frame 2019/2/19

Framing 2019/2/19

Connecting upper layer to the Media 2019/2/19

Data Link Sub-layers defines the software processes that provide services to the Network layer protocols defines the media access processes performed by the hardware 2019/2/19

Data Link Standards 2019/2/19

MAC - again Regulating the placement of data frames onto the media is known as media access control. 2019/2/19

MAC Regulating the placement of data frames onto the media is known as media access control The absence of any media access control would be the equivalent of vehicles ignoring all other traffic and entering the road without regard to the other vehicles. However, not all roads and entrances are the same. Traffic can enter the road by merging, by waiting for its turn at a stop sign, or by obeying signal lights. A driver follows a different set of rules for each type of entrance. 2019/2/19

MAC The method of media access control used depends on: Media sharing If and how the nodes share the media Topology How the connection between the nodes appears to the Data Link layer 2019/2/19

MAC 2019/2/19

MAC for shared media Two basic media access control methods for shared media: Controlled - Each node has its own time to use the medium Token Ring (deterministic) Contention-based - All nodes compete for the use of the medium CSMA/CD (non-deterministic) 2019/2/19

CSMA/CD Multiple Access (MA) Describing the fact that media is shared The device monitors the media for the presence of a data signal  Carrier Sense (CS) If a data signal is absent, indicating that the media is free, the device transmits the data. If signals are then detected that show another device was transmitting at the same time, all devices stop sending and try again later  Collision Detection (CD) 2019/2/19

CSMA/CA The device examines 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. The device then sends the data. Used by 802.11 wireless networking technologies. 2019/2/19

MAC for non-shared media Require little or no control before placing frames onto the media. These protocols have simpler rules and procedures for media access control. Such is the case for point-to-point topologies. 2019/2/19

In Point-to-point connection … The Data Link layer has to consider whether the communication is half-duplex devices can both transmit and receive on the media but cannot do so simultaneously full-duplex both devices can transmit and receive on the media at the same time 2019/2/19

Logical Topology What about physical topology? 2019/2/19

Logical Point to Point 2019/2/19

Adding intermediate physical connections to Logical Point to Point 2019/2/19

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. Virtual circuits are important logical communication constructs used by some Layer 2 technologies.  like frame relay 2019/2/19

Logical multiple-access topology 2019/2/19

Ring topology 2019/2/19

Framing - again Data Link layer frames has three basic parts Header Trailer 2019/2/19

In a fragile environment … 2019/2/19

In a protected environment … 2019/2/19

Role of header & trailer 2019/2/19

Addressing in multiple access topology 2019/2/19

Addressing in point-to-point topology the frame has only one place it can go 2019/2/19

Examples of Data Link Frames HDLC frame PPP frame Frame Relay frame Ethernet frame 2019/2/19

Ethernet Frame 2019/2/19

PPP Frame 2019/2/19

802.11 Frame 2019/2/19

A simple data transfer between 2 hosts 1. Assume all routing tables are converged 2. ARP tables are complete 3. A TCP session is already established between the client and server. 4. DNS lookup for the WWW server is already cached at the client. 2019/2/19

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