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Byte is a group of 8 bits. It is also known as an octet.


Bursty refers to data that transfers or transmits in short, uneven spurts. LAN traffic is typically bursty.

Broadcast Domain

A broadcast domain is the group of devices that can be reached by sending a frame addressed to the broadcast MAC address.

Default Route

In a routing table, a default route tells the router which way to route a packet if it encounters one with a network number it doesn't have listed in its routing table. A default route can be statically configured or learned dynamically via a routing protocol.

Data Terminal Equipment (DTE)

Data Terminal Equipment (DTE) is the device generating the data. A PC is an example of DTE.

Data Link Layer/Layer 2

In the OSI Reference Model and the Five-Layer Model, Layer 2 is the Data Link Layer. The Data Link Layer receives packets from Layer 3 (also known as the Network Layer in the OSI and Five-Layer models, or the Internet Layer in the TCP/IP Model). It then adds another header to form a frame. The Data Link Layer can also add a trailer to the frame, such as a Cyclic Redundancy Check (CRC). A CRC is a simple mathematical calculation performed on each frame to ensure it hasn't been corrupted in transit. Finally, the Data Link Layer translates the frame into binary digits, or bits, for Layer 1, the Physical Layer.

Data Communications Equipment (DCE)

Data Communications Equipment (DCE) is the device that actually puts the data on the local loop. A modem is an example of DCE.

Customer Premises Equipment (CPE)

Customer premises equipment (CPE) is any communications equipment located on the customer's premises that is used to connect to the service provider's network. The customer might own the CPE or may lease it from the service provider.


CSMA/CA stands for Carrier Sense Multiple Access / Collision Avoidance. CSMA/CA is an access method used by wireless networks to avoid collisions. This method relies on each wireless client receiving positive acknowledgements from the access point to confirm that data was received. If no acknowledgment is received, the client will resend the data. Only a single client can transmit at any one time. CSMA/CA differs from CSMA/CD, which relies on detecting collisions when they occur. CSMA/CD is not possible in an wireless environment; a wireless client cannot detect collisions while it is transmitting.

Constant Bit Rate (CBR)

Constant Bit Rate (CBR) is the ATM service category that is used for the highest priority traffic.

Connection-Oriented Protocols

Connection-Oriented Protocols require that two devices establish a formal connection that ensures they are ready to communicate. Connection-oriented Layer 2 protocols include PPP, ATM, and Frame Relay.

Connectionless Protocols

Connectionless Protocols do not require a formal connection before two devices can communicate. Ethernet is an example of a connectionless Layer 2 protocol.

Committed Information Rate (CIR)

The Committed Information Rate (CIR) is the transmission rate for a virtual circuit guaranteed by the service provider in Frame Relay network.

Collision Domain

A collision domain is a group of network devices that compete with each other for access to one physical wire or cable. A collision domain is also known as a network segment.

Coaxial Cabling

Coaxial cabling consists of a central conductive core contained within an insulating layer, a metallic shielding layer, and a plastic coating. Consumers may be familiar with coaxial cabling used today with cable television, satellite television, and cable modems.

CIDR Block

A CIDR block is a representation of the addresses in a range indicated by the prefix length in an address specified in CIDR or slash notation.

Classless Inter-Domain Routing (CIDR) Notation

Classless Inter-Domain Routing (CIDR) notation is also known as slash notation. It uses a slash after an to indicate the length of the network prefix in terms of bits. The number after the slash is also known as the prefix length.

Customer Edge (CE) Router

A Customer Edge (CE) router is a router at the customer's site that uses a single physical connection to connect to a Provider Edge router in the provider's MPLS network using any Layer 2 protocol. The CE router is at the edge of the customer's MPLS VPN even though the CE router does not run the MPLS protocol.

Classful Routing

Classful Routing or Classful IP Addressing was a scheme that divided the IP address space into different-sized chunks or classes of addresses based on how many were used for the network portion and how many were used for the host number. With classful IP addressing, the network number always fell on an octet boundary. For example, a Class A network used the first octet for the network number and the last three octets for host numbers and a Class B network number used the first two octets for the network number and the last two octets for host numbers.


A checksum, also known as a Cyclic Redundancy Check or CRC, is a simple mathematical calculation performed on each frame to ensure it hasn't been corrupted in transit.

Channel Service Unit/Data Service Unit (CSU/DSU)

A Channel Service Unit/Data Service Unit (CSU/DSU) is a device that prepares data traffic for digital lines. The DSU converts the digital frames used in the service provider's network into a frame format that the router can understand and vice versa. The CSU provides termination for the digital signal and ensures connection integrity through error correction and line monitoring.

Data Link Connection Identifier (DLCI)

A Data Link Connection Identifier (DLCI) is used to identify a Frame Relay PVC, and is similar to a destination address. Service providers assign DLCIs and each Frame Relay frame includes them.

Centralized (Switched) WLAN Architecture

Centralized architecture, also known as a switched WLAN system, is a form of wireless LAN architecture where a central switch or access point controller manages multiple access points that are Thin or Lightweight, meaning that they do not themselves contain full LAN functionality.

Central Office (CO)

Central Office (CO) is the term used to refer to the service provider's nearest exchange. A CO is like a distribution center, sending data to other COs along the path to its final destination.

Cell Transfer Delay (CTD)

Cell Transfer Delay (CTD) is the ATM QoS parameter for the delay experienced by a cell between the time it takes for the source to transmit the first bit of the cell and the destination to receive the last bit of the cell.

Default Router/Default Gateway

A default router, also known as a default gateway, is a network device, such as a router, which serves as a door or an access point to other networks. End-user devices, such as PCs, send packets to their configured default gateway when the destination network number is not the same as their network number.

Discard Eligible (DE)

Discard Eligible (DE) bits are Frame Relay frame bits that are set by the end station to indicate lower priority traffic.

DS1 Signal

DS1 signal is the signal or logical bit pattern carried on a T1 line and is the equivalent of 24 DS0s or channels.

Digital Subscriber Line (DSL)

Digital Subscriber Line (DSL) is a communications link used to transfer data over regular phone lines and can be used to connect to the Internet. A DSL line is much faster than a regular phone connection, even though the wires it uses are copper like a typical phone line.

Digital Lines

Digital lines are the wires or cable used to carry digital signals, such as the phone system within a large company. Digital lines are copper wires or fiber optic cables that operate at Layer 1 or the Physical Layer. A digital signal is an electrical signal transmitted as a binary code of 1s and 0s. Digital lines include T1/E1 and T3/E3 lines.

Distributed WLAN Architecture

Distributed architecture, also known as a wireless mesh, is a form of wireless LAN architecture in which data is repeated over wireless to neighbor nodes, which make forwarding decisions to get the data through the network. A wireless mesh can be used to create a dynamic network in environments where cabling would be impossible.


In networking, device is the generic term used to refer to any of the variety of types of computers, peripherals, and networking equipment that may be connected to a network. Personal computers, servers, and shared printers are devices that may be referred to as end-user devices or hosts. Bridges, switches, and routers are devices that may be referred to as network equipment.


DS0 is the basic digital signaling rate. DS stands for Digital Signal, and is a system of classifying digital circuits according to the rate and format of the signal. A DS0 offers 64 Kbps of bandwidth - the amount of bandwidth usually used for one telephone voice channel.

Direct Routing

Direct Routing is communication between devices without the help of a router.

Dotted Decimal Notation

Dotted Decimal Notation is the representation of a binary IP address in decimal format, separating each octet with a period or dot.


The Domain Name System (DNS) associates various information with domain names; most importantly, it serves as the phone book for the Internet by translating human-readable computer hostnames, such as www.example.com, into IP addresses, such as Networking equipment needs to know these IP addresses to deliver information.

Dynamic Ports or Private Ports

Dynamic Ports or Private Ports are software ports in the range 49,152 to 65,535 and the Internet Assigned Numbers Authority does not manage them. Randomly chosen port numbers in this range are referred to as ephemeral ports. These ports are not permanently assigned to any publicly defined application and are commonly used as the source port number for the client side of a connection. This allocation is temporary and is valid for the duration of the connection opened by the application using the protocols.

802.11 Group of Standards

The 802.11 Group of Standards are specifications for wireless local area networking. The original standard was released in 1997 and many extensions have been added over the years.


802.3 is a networking standard established by the Institute of Electrical and Electronics Engineers (IEEE). Among other things, 802.3 standardized the operation of CSMA/CD. While Ethernet and 802.3 differ slightly in their terminology and their frame format, their media access control (MAC) protocols are virtually identical. Today, the term Ethernet refers generically to both the sets of Ethernet standards.

E3 Line

E3 is a dedicated copper telephone line that supports data rates of 34.37 Mbps and like an E1, is also built on the base DS0 signal. An E3 is 16 E1s-or 512 DS0s-bundled together.

E1 Line

E1 is a dedicated copper telephone line that supports data rates of 2.05 Mbps. An E1 line operates at Layer 1 or the Physical layer and is commonly available world-wide with the exception of North America and Japan where T1 lines are used. An E1 line consists of 32 individual channels or DS0s, each of which supports 64 Kbps. Each channel or DS0 can transport voice or data. A customer can lease an entire E1 line or only a few channels, which is known as fractional E1 service.

Forwarding Storm

A forwarding storm is network congestion caused by a looped network. When a bridge or switch receives a frame for which it doesn't have the destination MAC address in its table, or when it receives broadcast or multicast traffic, it floods the frame out every other port. In a looped network, this can result in the same data being received on multiple ports and being forwarded again and again. A forwarding storm can block all other network traffic and bring a network to a complete stand still. To prevent loops, bridges and switches can use the Spanning Tree Protocol, or STP.

Forward Explicit Congestion Notification (FECN)

Forward Explicit Congestion Notification (FECN) bits are bits in the Frame Relay header used to notify an end station, such as a router, of network congestion. When congestion occurs, Frame Relay switches set the FECN bit to a 1 in frames going toward the destination location or going downstream. In this way, all downstream devices learn about congestion.


For a switch or bridge, flooding means forwarding data on of all ports except the port on which data was received. When one of these network devices receives data destined for an unknown address, data destined for the broadcast address, or data destined for a multicast address, it will flood the data on all of its ports except for the port on which the data was received.

Five-Layer Reference Model

The Five-Layer Reference Model is a commonly used combination of the OSI Reference Model and the TCP/IP Reference Model. The five layers defined in this model are as follows: Application Layer,Transport Layer/Layer 4, Network Layer/Layer 3, Data Link Layer/Layer 2, Physical Layer/Layer 1

Fiber Distributed Data Interface (FDDI)

Fiber Distributed Data Interface (FDDI) is a standard for data transmission on fiber optic lines that can extend in range up to 200 km (124 miles).

Extension Headers

In IPv6, extension headers are used to extend the functionality of the IPv6 header. IPv6 has a fixed length header. Optional extension headers are then used to extend functionality when needed. The IPv6 header and each extension header contain a Next Header field, which specifies the type of the next header that follows. This allows a variable number of extension headers to be added.

Ethernet Cabling

Ethernet cabling is twisted-pair cabling that meets standards for use on an Ethernet-based network. Category 5 (Cat 5) and Category 5e (Cat 5e) cabling are commonly in use today. Straight through Ethernet cables can be used to connect an end-user device to a networking device such as a switch. Crossover Ethernet cables can be used to connect one end-user device directly to another.


Ethernet is the most common set of rules controlling network communications for local area networks. It is a set of standards that define rules such as frame format as well as how computers communicate with each other over a single wired shared by all devices on the network. These rules give any new device attached to the wire the ability to communicate with any other attached device. Ethernet can also be used as a wide area network (WAN) technology. In this application, it is often referred to as carrier Ethernet. Organizations such as the Metro Ethernet Forum are helping to accelerate adoption of carrier Ethernet networks and services.

Fiber Optic Lines

Fiber optic lines are glass or plastic threads or fibers used to transmit data. Fiber optic lines operate at Layer 1 or the Physical Layer. SONET/SDH connections use fiber optic lines.

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