Data Link Layer, OSI Model

 In the OSI (Open Systems Interconnection) model, the data link layer sits just above the physical layer. Therefore, it defines the protocols that directly interact with the physical  components of the link, such as the network adapters and cable. It frames data and controls   the flow of information across the link. Originally, data link protocols were designed for point-to-point links, and this is still the primary way that communication is handled, although shared LANs such as Ethernet require additional medium access control protocols to arbitrate access to the shared medium.

Data Link Protocols 

Often called layer 2 protocols, data link protocols exist in the protocol layer just above the physical layer relative to the OSI protocol model. Data link protocols provide communication between two devices. Because there are many different ways to connect devices, there are many different data link protocols. The defining factors are:                                                  

 *  Dedicated point-to-point links between two devices, such as modem, bridges, or routers           

 *  Shared media links in which multiple devices share the same cable (i.e., Ethernet LAN)   

DCE (Data Circuit–Terminating Equipment)

DCE equipment is typically a modem or other type of communication device. The DCE  sits between the DTE (data terminal equipment) and a transmission circuit such as a phone  line. Originally, the DTE was a dumb terminal or printer, but today it is a computer, or a bridge or router that interconnects local area networks. In an IBM mainframe environment, a communication controller and a link-attached cluster controller are examples of DTEs.       

DDR (Dial-on-Demand Routing)

DDR provides a way to link two sites over a public network and provide needed bandwidth by setting up additional lines as required. DDR is a feature of routers from Cisco and other vendors It allows a device to establish a circuit-switched asynchronous modem connection, an ISDN                

digital connection, or Switched 56 connections to a remote location.

Delay, Latency, and Jitter

Delay and latency are similar terms that refer to the amount of time it takes a bit to be transmitted from source to destination. Jitter is delay that varies over time. One way to view latency is how long a system holds on to a packet. That system may be a single device like a router, or a complete communication system including routers and links.

Diff Serv

Diff Serv or differentiated services is a method of ensuring quality of

service on large networks such as the Internet.

Diff Serv achieves Quality of Service by dealing with bulk data flow rather than single flows. This means that a single policy will be made for all of the packets from, for example, a single ISP. The contracts specifying the packet policies are called "service level agreements". These service level agreements will specify what classes of traffic will be provided, what guarantees are needed for each class, and how much data will be sent for each class.

DLC (Digital Loop Carrier)   

 A DLC system is a part of the copper-wire, local-loop telephone system. A DLC system aggregates the subscriber calls within a neighborhood, office building, or industrial park and multiplexes the calls over a single line (T1/E1 or fiber), back to the telephone company central office where all the switching equipment is located. DLC is an impediment to the expansion of  DSL (Digital Subscriber Line) services.

DLC (Data Link Control)

See Data Link Layer, OSI Model; Data Link Protocols.   

DSCP

 An IP packet contains all the information about where it should go and what class it belongs, each part of information in a packet is called a field. Differentiated Services Code Point (DSCP) is a field in an IP packet that enables different levels of service to be assigned to network traffic. This is achieved by marking each packet on the network with a DSCP code, the switch then reads the field and provides it with the corresponding level of service. The different levels  of service ensures Quality of Service (QoS) for the network. In QoS, six bits are reallocated for use as the DSCP field, where each DSCP specifies a particular per-hop behavior that is applied to a packet. Support for DSCP still is lacking in some network equipment, however (see RFC 2474). DSCP is not compatible with IP Precedence 

DSL (Digital Subscriber Line)     

DSL is a technology that provides high-speed data transmissions over the so-called “last-mile” of “local-loop” of the telephone network, i.e., the twisted copper wire that connects home and small office usrs to the telephone company central offices(COs).Dem and for high-speed access methods is increasing with growing Internet access, electronic commerce, IP telephony, and  videoconferencing. A number of methods for providing this bandwidth are available, including   DSL technologies, cable (CATV) networks, and wireless and satellite technologies. All of these fit into the category of “residential broadband services.”   

DVMRP

Short for Distance Vector Multicast Routing Protocol, DVMRP is an interior gateway protocol based on RIP that supports connection less multicast data transmission, a connectionless network service is a service that transfers information between end users without establishing a logical connection or virtual circuit between those specific users. DVMRP sends multicast transmission within unicast packets that are reassembled into multicast data when they arrive at their destination.

DTE(Data Terminal Equipment)

DTEs are the source or destination of data in a communication connection. DTEs are connected to DCE (data circuit–terminating equipment), which in turn is connected to the communication        channel. Dumb terminals were originally classified as DTEs, but computers also fall into this category. A DCE is a modem if the circuit is an analog voice line, or a DSU/CSU if the line is digital.

DWDM(Dense Wavelength Division Multiplexing)

See WDM (Wavelength Division Multiplexing).

Dynamic Routing 

Dynamic routing is a process in which routers automatically adjust to changes in network topology or traffic. The opposite is static routing, in which the router manager enters the routes manually. Dynamic routing is used in all modern routers, but some amount of programming is still available for customizing routes if necessary.