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Maximum Burst Size (MBS)

Maximum Burst Size (MBS) is the ATM traffic descriptor for the maximum number of cells that can be sent back-to-back at the peak cell rate.

Maximum Receive Unit (MRU)

Maximum Receive Unit (MRU) is the maximum allowable size in bytes for the PPP link. Devices negotiate the MRU during the connection establishment process. The MRU must be agreed upon by two devices before they can exchange data.

MEF Three-Layer Model

The Metro Ethernet Forum (MEF) Three-Layer Model for Carrier Ethernet is somewhat like a collapsed version of the Five-Layer Reference Model referred to in this course. The three layers are as follows: Application Services Layer, Ethernet Services Layer, Transport Services Layer, Each layer has its own data, control, and management planes.

Minimum Cell Rate (MCR)

Minimum Cell Rate (MCR) is the ATM traffic descriptor for the minimum rate at which cells can be transported.


A modem is an example of DCE that converts or modulates the digital signal into an analog signal for an analog phone line and then demodulates the analog signal back into a digital signal for a computer.

Maximum Transmission Unit (MTU)

Maximum Transmission Unit (MTU) is the fixed upper limit on the size of packets can be sent in a single frame.

Multicast MAC Address

A multicast MAC address is used to identify a group of devices on the same network. Multicast addresses always have the first bit of the first byte set to 1. Other types of MAC addresses include unicast addresses and the broadcast address.

Multicast IP Address

Multicast IP addresses are similar to multicast MAC addresses and are the addresses a device uses to send the same data to a specific group of devices.

Multiprotocol Label Switching (MPLS)

Multiprotocol Label Switching (MPLS) is a data packet forwarding technology used to improve the forwarding speed of routers by using labels to make forwarding decisions. When a packet enters the MPLS network, the first MPLS router adds a simple label based on fields in the IP header. Routers within the MPLS network forward the packet based on the contents of the label. MPLS is often referred to as a Layer 2.5 protocol because it operates between Layer 2 and Layer 3, is Layer 2-independent, provides the privacy and security of a Frame Relay or ATM network, and yet allows for the inherentany-to-any connectivity and flexibility typical of an IP-based network.

MPLS Domain

An MPLS domain is an MPLS network.


Nak is an abbreviation for Negative Acknowledgement. It is the negative response of one device to another in the connection establishment process.


Negotiation is the process of resolving differences in parameters that two devices go through to establish a connection.

Network Layer/Layer 3

In the OSI Reference Model and the Five-Layer Reference Model, Layer 3 is the Network Layer. The Network Layer receives a segment from Layer 4, the Transport Layer, adds a header to it to create a packet, and sends the packet to Layer 2, the Data Link Layer. The Network Layer is responsible for delivering the packet to the destination computer. If there is more than one route to the destination computer, the Network Layer chooses the best path for the packet to take. The Network Layer treats each packet independently, so it is possible that two packets from the same transmission might take different paths to arrive at the destination computer. This is the same as the TCP/IP Reference Model's Internet Layer.

Network Access Layer

In the TCP/IP Reference Model, the Network Access Model focuses on how data is transmitted over any type of physical network, regardless of whether it's a LAN or a WAN. Don't confuse this with the OSI Reference Model's Network Layer.

Network Control Protocol (NCP)

Network Control Protocol (NCP) is a Layer 3-specific protocol used during the PPP connection establishment process to establish and configure the different Layer 3 protocols-such as IP, IPX, or Appletalk-running over the connection.

Network Interface Controller (NIC) or Card (NIC)

Network Interface Controllers or Cards (NICs) or hardware ports are physical circuit cards installed in a computer or printer so that it can connect to a network. The terms NIC, network adapter, network card, and LAN adapter are used interchangeably. In the 1980s, a NIC was an expansion card that plugged into a computer bus. Today, most computers have a NIC built into the motherboard and a separate card is not necessary unless you need connections to multiple networks.

Network Layer Protocol Identifier (NLPID)

Network Layer Protocol Identifier (NLPID) is the Frame Relay frame field used to specify the Layer 3 protocol.

Network Masks

Network Masks are used to identify the network portion of the address. Also known as masks, address masks, subnet masks, and netmasks.

Network Number

The network number is the first part of a Layer 3 address. (The host number is the second part.) The network number portion of the Layer 3 address identifies the network the device is on. Both the network number and host number are assigned by a network administrator.

Network Prefix

Network Prefix is another term for Network Number.

Network Segment

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

Next Hop Address

The next hop is the next device in the path to the destination device. The next hop address is the address of that device, and either indicates that the destination network is directly connected to the router or provides the Layer 3 address of another router on a directly connected network.


A nibble is 4 bits, which is the number represented by a hexadecimal digit or symbol.

Non-Real Time Variable Bit Rate (nrt-VBR)

Non-Real Time Variable Bit Rate (nrt-VBR) is an ATM service category that is similar to rt-VBR but is used for applications with bursty transmission characteristics that tolerate high cell delay, but require low cell loss.


An octet is a group of 8 bits. It is also known as a byte.

Optical Carrier (OC)

Optical Carrier (OC) refers to a SONET signal being carried over a fiber optic network. The OC level expresses the speed of an OC-n line, where the speed is equal to n times 51.84 Mbps. For example, an OC-3 signal has a bit rate of 3 * 51.84 Mbps or 155.52 Mbps.

Optical Transport Network (OTN)

Optical Transport Network (OTN) uses wavelength division multiplexing to transport data over optical networks. OTN systems in use today can operate on 40 or even 80 channels simultaneously. Incoming signals can be SONET, SDH, Ethernet, or native OTN traffic.

OSI Reference Model

The Open Systems Interconnection (OSI) Reference Model is a seven-layer model that identifies the steps and functions that must be completed when computers communicate over a network. The OSI model provides only guidelines on how computers communicate over a network. It does not provide detailed procedures on how to actually make this communication happen. The seven layers of the OSI Reference Model are as follows: Application Layer/Layer 7, Presentation Layer - Layer 6, Session Layer - Layer 5, Transport Layer - Layer 4, Network Layer - Layer 3 Data Link Layer - Layer 2 Physical Layer - Layer 1

Open Shortest Path First (OSPF)

Open Shortest Path First (OSPF) is a routing protocol by which routers in one network learn how to communicate with routers in another network.

Organizationally Unique Identifier

The Organizationally Unique Identifier (OUI) is a 3-byte identifier that each Ethernet equipment manufacturer is assigned by the IEEE. The first three bytes of a MAC address identify a device's manufacturer using the manufacturer's OUI.

Plain Old Telephone Service (POTS) lines

Plain Old Telephone Service (POTS) lines are analog copper phone lines capable of data transfer speeds of up to 30 kbps.


A packet is one unit of data encapsulated at Layer 3 (also known as the Network Layer in the OSI and Five-Layer models, or the Internet Layer in the TCP/IP Model). Each packet contains a header followed by the data. The packet's header specifies the data's source and destination IP addresses. Each packet header also specifies the IP protocol number, which indicates whether the data should be processed with the UDP or TCP protocol on the receiving computer.

Packet Switched Network

A Packet Switched Network is a type of connection-less network where data is divided into small packets or frames and routed through the network based on the destination address contained within each packet. Because each packet contains the destination address, multiple users can share a network-such as a Frame Relay network.

Passive Scanning

In a wireless environment, passive scanning is when a client listens passively for access point beacons that indicate available networks that it may attempt to associate with.

Provider Edge (PE) Router

A Provider Edge (PE) router is a type of edge router used in MPLS VPNs, and must provide completely private and secure connectivity within a VPN. PE routers also exchange routing information with customer edge routers and other PE routers within the VPN. The PE routers store the customer's routing information in a Virtual Routing and Forwarding table or instance.

Peak-to-Peak Cell Delay Variation (CDV)

Peak-to-Peak Cell Delay Variation (CDV) is the ATM QoS parameter for the difference between the maximum and minimum cell transfer delay experienced during the connection.

Permanent Virtual Circuits (PVCs)

Permanent virtual circuits (PVCs) are connections that are like leased lines in that the service provider defines a permanent path to each customer location. PVCs are always on and ready to use. Frame Relay and ATM networks use PVCs.

Peak Cell Rate (PCR)

Peak Cell Rate (PCR) is the ATM traffic descriptor for the maximum rate at which cells can be transported.

Physical Cable Segment

A physical cable segment is the cable or wire used to connect network devices.

Physical Layer/Layer 1

In the OSI Reference Model and the Five-Layer Reference Model, Layer 1 is the Physical Layer. Layer 1 is where binary digits, or bits, move across a physical medium. Layer 1 defines the electrical or optical signal that equals a one, and the signal that equals a zero. Physical Layer standards include cabling specifications, electrical or optical signaling, and lower-level framing of ones and zeros.

Point-to-Point Connections

Point-to-Point connections are dedicated communication lines between two locations, such as T1/T3 or E1/E3 lines using PPP as the Layer 2 protocol.

Point-to-Point Protocol (PPP)

Point-to-Point Protocol (PPP) is a WAN Layer 2 protocol that runs over a dedicated leased line between two locations.

PPP Authentication Protocols

PPP Authentication Protocols are used to verify the identity of a user. Common PPP authentication protocols include Password Authentication Protocol (PAP), Challenge Handshake Authentication Protocol (CHAP), Microsoft Challenge Handshake Authentication Protocol (MS CHAP), and Extensible Authentication Protocol (EAP).


In an Ethernet LAN, the preamble is a specific short sequence of bytes used by a network interface controller to allow receiving devices to recognize the start of a new frame. The preamble is not considered part of the Ethernet frame itself.

Presentation Layer/Layer 6

In the OSI Reference Model, Layer 6 is the Presentation Layer. The Presentation Layer accepts the data from the Application Layer and converts or encodes it into a standard format that the Application Layer on the other computer can understand. For example, text can be encoded as ASCII or HTML, while graphics can be encoded using standard formats like JPEG or TIFF. The Presentation Layer can also involve standard data compression or data encryption schemes, ensuring that the receiving computer can properly uncompress or decipher the data.


PROM stands for Programmable Read-Only Memory. A PROM chip is a special chip that initially has no programming encoded. Device manufacturers can inexpensively encode, or burn, a PROM chip, but this can only be done once and the programming cannot be changed later. Manufacturers use a PROM chip to store the unique MAC address on each NIC.


A protocol is a formal set of written rules or procedures that computers must understand, accept, and use to be able to talk to each other over a network. Different protocols are used at different layers of the reference model. Two computers must use the same protocol at the same layer for the data to be communicated correctly.

Provider (P) Routers

Provider (P) routers are transit LSRs that do not connect to customer devices and perform only label switching.

Quality of Service (QoS)

Quality of Service (QoS) is the level of service guaranteed by a WAN service provider.

Real-Time Variable Bit Rate (rt-VBR or real-time VBR)

Real-Time Variable Bit Rate (rt-VBR or real-time VBR) is the ATM service category that is used for compressed video applications that can tolerate bandwidth variations but that are sensitive to both cell loss and delay variation.

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