Cisco Certification: WAN/PPP/Frame Relay/ISDN Terms

Terminologies

WAN/PPP/Frame Relay/ISDN

TERM

DEFINITION

Customer Premise Equipment (CPE)

Customer premises equipment (CPE) is equipment that’s owned by the subscriber and located on the subscriber’s premises.

Demarcation Point (Demarc)

The demarcation location is the spot where the service provider’s responsibility ends and the CPE begins. It’s

generally a device in a telecommunications closet owned and installed by the telecommunications company (telco).

The customer is responsible to cable (extended demarc) from this box to the CPE, which is usually a connection to a CSU/DSU or

ISDN interface.

Local Loop

The local loop connects the demarc to the closest switching office, called a central office.

Central Office

This point connects the customers to the provider’s switching network. A central office (CO) is sometimes referred

to as a

point of presence (POP)

Toll Network

The toll network is a trunk line inside a WAN provider’s network. This network is a collection of switches and

facilities owned by the ISP.

Leased Lines

Typically, these are referred to as a point-to-point connection or dedicated connection. A leased line is a

pre-established WAN communications path from the CPE, through the DCE switch, to the CPE of the remote site, allowing DTE networks to communicate at any time with no setup procedures before transmitting data.

Circuit Switching

When you hear the term circuit switching, think phone call. The big advantage is cost—you only pay for the time you actually use. No data can transfer before an end-to-end connection is established. Circuit switching uses dial-up

modems or ISDN, and is used for low-bandwidth data transfers.

Terminologies

WAN/PPP/Frame Relay/ISDN

TERM

DEFINITION

Packet Switching

Packet switching can be thought of as a network that’s designed to look like a leased line, yet charges you (and costs)

more like circuit switching. Packet switching will only work well if your data transfers are bursty in nature. Frame

Relay and X.25 are packet-switching technologies. Speeds can range from 56Kbps to T3 (45Mbps).

Frame Relay

Frame Relay is a Data Link and Physical layer specification that provides high performance. Frame Relay is a

successor to X.25, except that much of the technology in X.25 used to compensate for physical errors (noisy lines)

has been eliminated. Frame Relay provides features for dynamic bandwidth allocation and congestion control, and

can typically run at speeds of 64Kbps up to 45Mbps (T3).

Integrated Services Digital Network (ISDN)

Integrated Services Digital Network (ISDN) is a set of digital services that transmit voice and data over existing

phone lines.

Link Access Procedure, Balanced (LAPB)

Link Access Procedure, Balanced (LAPB) was created to be a connection-oriented protocol at the Data Link layer for

use with X.25. It can also be used as a simple data link transport. LAPB causes a tremendous amount of overhead

because of its strict timeout and windowing techniques.

Link Access Procedure, D-Channel (LAPD)

Link Access Procedure, D-Channel (LAPD) is used with ISDN at the Data Link layer (layer 2) as a protocol for the

D (signaling) channel. LAPD was derived from the Link Access Procedure, Balanced (LAPB) Protocol and is

designed primarily to satisfy the signaling requirements of ISDN basic access.

High-Level Data Link Control Protocol (HDLC)

High-Level Data Link Control Protocol (HDLC) was derived from Synchronous Data Link Control (SDLC), which was

created by IBM as a Data Link connection protocol. HDLC is a protocol at the Data Link layer, and is a method for encapsulating datagrams over serial links. The HDLC header carries no identification of the type of protocol being

carried inside the HDLC encapsulation.

Terminologies

WAN/PPP/Frame Relay/ISDN

TERM

DEFINITION

Point to Point Protocol (PPP)

Point-to-Point Protocol (PPP) is an industry-standard protocol. Because all multiprotocol versions of HDLC are proprietary, PPP can be used to create point-to-point links between different vendors’ equipment. It uses a Network Control Protocol field in the Data Link header to identify the Network layer protocol. It allows authentication and multilink connections and can be run over asynchronous and synchronous links.

Asynchronous Transfer Mode (ATM)

Asynchronous Transfer Mode (ATM) was created for time-sensitive traffic, providing simultaneous transmission of

voice, video, and data. ATM uses cells instead of packets that are a fixed 53 bytes long. It also can use isochronous clocking (external clocking) to help the data move faster.

Link Control Protocol (LCP)

A method of establishing, configuring, maintaining and terminating the point-to-point connection.

Network Control Protocol (NCP)

A method of establishing and configuring different Network layer protocols. NCP is designed to allow the

simultaneous use of multiple Network layer protocols.

PPP Callback

Enabled, a calling router (client) will contact a remote router (server) and authenticate as described in the previous

section. Both routers must be configured for the callback feature. Once authentication is completed, the remote router

will terminate the connection and then re-initiate a connection to the calling router from the remote router.

Password Authentication Protocol (PAP)

The Password Authentication Protocol (PAP) is the less secure of the two methods. Passwords are sent in clear text,

and PAP is only performed upon the initial link establishment. When the PPP link is first established, the remote node

sends back to the originating router the username and password until authentication is acknowledged.

Challenge Handshake Authentication Protocol (CHAP)

The Challenge Handshake Authentication Protocol (CHAP) is used at the initial startup of a link and at periodic

checkups on the link to make sure the router is still communicating with the same host.

Terminologies

WAN/PPP/Frame Relay/ISDN

TERM

DEFINITION

Link-Establishment Phase

LCP packets are sent by each PPP device to configure and test the link. These packets contain a field called the configuration Option that allows each device to see the size of the data, compression, and authentication. If no configuration Option field is present, then the default configurations are used.

Authentication Phase

If required, either CHAP or PAP can be used to authenticate a link. Authentication takes place before Network layer protocol information is read. It is possible that link-quality determination may occur at this same time.

Network Layer Protocol Phase

PPP uses the Network Control Protocol (NCP) to allow multiple Network layer protocols to be encapsulated and sent

over a PPP data link. Each Network layer protocol (e.g., IP, IPX, AppleTalk, which are routed protocols) establishes

a service with NCP.

Committed Information Rate (CIR)

The maximum bandwidth of data guaranteed to be delivered. However, in reality, this is the average amount that the service provider will allow you to transmit.

Access Rate

The maximum speed at which the Frame Relay interface can transmit.

Permanent Virtual Circuits

Permanent Virtual Circuits (PVCs) are by far the most common type in use today. What permanent means is that the

telco creates the mappings inside their gear, and as long as you pay the bill, they will remain in place.

Switched Virtual Circuits

Switched Virtual Circuits (SVCs) are more like a phone call. The virtual circuit is established when data needs to be transmitted, then is taken down when data transfer is complete.

Terminologies

WAN/PPP/Frame Relay/ISDN

TERM

DEFINITION

Data Link Connection Identifiers (DLCI)

Used to identify virtual circuits in a Frame Relay network.

Inverse Address Resolution Protocol (IARP)

Used to map a known DLCI to an IP address.

Local Management Interface (LMI)

Local Management Interface (LMI) is a signaling standard used between your router and the first Frame Relay switch

it’s connected to. It allows for passing information about the operation and status of the virtual circuit between the provider’s network and the DTE (your router). It communicates information about Keepalives, Multicasting, Global Addressing and the status of virtual circuits

Discard Eligibility (DE)

Used in Frame Relay networks to tell a switch that a frame can be preferentially discarded if the switch is too busy.

The DE is a field in the frame that is turned on by transmitting routers if the committed information rate (CIR) is oversubscribed or set to 0.

Forward Explicit Congestion Notification (FECN)

A bit set by a Frame Relay network that informs the DTE receptor that congestion was encountered along the path

from source to destination. A device receiving frames with the FECN bit set can ask higher-priority protocols to

take flow-control action as needed.

Backward Explicit Congestion Notification (BECN)

the bit set by a Frame Relay network in frames moving away from frames headed into a congested path. A DTE that receives frames with the BECN may ask higher-level protocols to take necessary flow control measures.

Terminologies

WAN/PPP/Frame Relay/ISDN

TERM

DEFINITION

Terminal Equipment Type 1 (TE1)

A terminal equipment type 1 (TE1) device refers to those terminals that understand ISDN standards and can plug

right into an ISDN network.

Terminal Equipment Type 2 (TE2)

A terminal equipment type 2 (TE2) device refers to those that predate ISDN standards (are not natively

ISDN-compliant). To use a TE2, you have to use a terminal adapter (TA) to be able to plug into an ISDN network.

Network Termination 1 (NT1)

The network termination 1 (NT1) device implements the ISDN Physical layer specifications and connects the user

devices to the ISDN network by converting the network from a four wire to the two-wire network used by ISDN.

Network Termination 2 (NT2)

The network termination 2 (NT2) device is typically a provider’s equipment, such as a switch or PBX. It also provides

Data Link and Network layer implementation. It’s very rare to find these on a customer’s premises.

Terminal Adapter (TA)

A terminal adapter (TA) converts TE2 non-ISDN signaling to signaling that’s used by the ISDN switch. It connects

into an NT1 device for conversion into a two-wire ISDN network.

Basic Rate Interface (BRI)

ISDN Basic Rate Interface (BRI) service, also known as 2B+D, provides two B channels and one D channel. The

BRI B-channel service operates at 64Kbps and carries data, while the BRI D-channel service operates at 16Kbps and usually carries control and signaling information. The total bandwidth for ISDN BRI is then 144Kbps

(64 + 64 + 16 = 144)

Primary Rate Interface (PRI)

ISDN Primary Rate Interface (PRI) service—also known as 23B+D—delivers 23 64Kbps B channels and one 64Kbps

D channel, for a total bit rate of 1.544Mbps.

Dial on Demand Routing (DDR)