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Exam Code: 646-985

Exam Name: Data Center Networking Solution Sales (DCNSS)

Vendor: Cisco

Certs Covered:

No of Questions: 83

Last Updated: September 14, 2014

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Point-to-Point Leased Line Implementation


WAN Concepts Review

The categories of wide area networks (WANs) designs are:

  • Private network

  • Public networks

  • Leased networks


Leased WAN networks fall into either of these categories. The categories here are often referred to as the WAN connection types:

  • Dedicated leased lines: Dedicated leased lines are also called point-to-point connections or simply, leased lines. A leased line is a pre-defined or established WAN communication line from the customer premises equipment (CPE) through the data communication equipment (DCE) switch, and to the customer premises equipment of the remote site. Leased lines use synchronous serial lines up to 45Mbps.

  • Circuit-switched networks: Here, you pay for the time that is utilized. Before data can be delivered, an end-to-end connection has to be established.

  • Packet-switched networks: With packet-switched networks, bandwidth is shared between other companies, to ultimately save money.


WAN protocols that operate on point-to-point serial links pass data across the link.


The different WAN protocols that operate on point-to-point serial links are:

  • High-Level Data Link Control (HDLC)

  • Link Access Procedure Balanced (LAPB)

  • Point-to-Point Protocol (PPP)



The differences between synchronous protocols and asynchronous protocols are summarized below:

  • WAN protocols such as High-Level Data Link Control (HDLC), Link Access Procedure Balanced (LAPB) and Point-to-Point Protocol (PPP) support synchronous communications.

  • Synchronous protocols allow more throughput over a serial link than that allowed by asynchronous protocols.

  • Synchronous protocols are usually used between routers.

  • Asynchronous protocols require less expensive hardware than that need by the synchronous protocols.

  • With asynchronous protocols, the sending rate need not be monitored. Asynchronous protocols do not need the clock rate adjusted as synchronous protocols do.



The popular Data-link protocols for point-to-point leased lines are:

  • High-Level Data Link Control (HDLC)

  • Point-to-Point Protocol (PPP)


Both HDLC and PPP provide the following services:

  • Data delivery over a single point-to-point serial link

  • Data delivery on synchronous serial links


Bear in mind though that PPP supports asynchronous serial links as well.



The different WAN Data-link Protocols

The different WAN Data-link protocols and their capability of supporting specific features are shown in Table 1



TABLE 1: The different WAN Data-link protocols

WAN Data-link Protocol

Protocol Type field Provided


Error Correction

Comments

Synchronous Data Link Control (SDLC)


No

Yes

Supports multipoint links.

Link Access Procedure Balanced (LAPB)

No

Yes

Typically used for X.25

Link Access Procedure on the D Channel (LAPD)

No

No

Typically used by ISDN lines

Link Access Procedure for Frame Mode Bearer Services (LAPF)


Yes

No

Typically used over Frame Relay links.

High-Level Data Link Control (HDLC)

No

No

HDLC is the default protocol used on serial links.

A Proprietary Type field is utilized to support multi-protocol traffic.

Point-to-Point Protocol (PPP)

Yes

Yes – not enabled by default.

Designed for multi-protocol interoperability.


PPP can support multiple protocols:

  • Internet Protocol (IP)

  • Internetwork Packet Exchange (IPX)

  • AppleTalk

  • DECnet

  • OSI/CLNS

  • Transparent bridging





Point-to-Point WAN Terminology

The commonly used point-to-point WAN terminology is listed below:

  • Synchronous; implies that time ordering exists at the sending end and receiving end of the link. Devices attempt to use the same speed as that used by the device at the other end of the serial link. Devices essentially adjust their sending rates to match a clock source.

  • Asynchronous; implies that there is no time ordering at the sending end and receiving end of the link. Here, each device at the end of the link comes to agreement on the speed to use – they do not however check the rate, nor do they adjust sending rates. A start bit signals the start of a byte – one byte is sent per transfer.

  • Clock source; refers to the device to which all other devices on the link adjust their speed. Only relevant when using synchronous links.

  • Data Services Unit and Channel Services Unit (DSU/CSU); refers to the interface used on digital data links to the telephone company. Routers use the attached DSU/CSU as the clock source.

  • Telco; refers to the telecommunications company.

  • 4-wire circuit; a line from the telephone company that has four wires. Two is a twisted-pair wire used to send in only one direction at any given point in time. This means that a 4-wire circuit allows for full-duplex communication.

  • T/1; a line from the telephone company which allows for sending of data at 1.544Mbps. The T/1 line can be used with a T/1 multiplexor (mux).

  • E/1; a line from the telephone company allows for sending of data at 2.048Mbps. The E/1 can be separated into 32 different 64kbps channels.



Comparing HDLC and PPP

The features used to compare the HDLC and PPP WAN protocols are:

  • Whether the protocol supports synchronous communications.

  • Whether the protocol supports asynchronous communications.

  • Whether the protocol supports both synchronous communications and asynchronous communications.

  • Whether the protocol provides error recovery.

  • Whether an architected Protocol Type field exists.



HDLC supports synchronous links only. PPP supports both synchronous communications and asynchronous communications.


PPP can run on different DTE/DCE physical interfaces:

  • Synchronous serial

  • Asynchronous serial

  • High-Speed Serial Interface (HSSI)

  • Integrated Services Digital Network (ISDN)



HDLC does not provide for error recovery. PPP on the other hand provides support for error recovery. The feature is however disabled by default.


HDLC does not provide an architected Protocol Type field. PPP does however provide an architected Protocol Type field.



PPP Specific Features

Because PPP was defined later than the initial HDLC specifications, PPP includes a number of additional features which were not previously included in the other WAN protocols. This makes PPP the main WAN data link layer protocol.


PPP specific features are categorized as follows:

  • Features specific to each particular Layer 3 protocol.

  • Features required, irrespective of the Layer 3 protocol



The PPP Link Control Protocol (LCP) provides the main PPP features. LCP performs the following functions:

  • Establishes the point-to-point connection.

  • Tests and configures the point-to-point connection.

  • Maintains the point-to-point connection.

  • Terminates the point-to-point connection.



The main PPP LCP features are listed here:

  • Error detection through Link Quality Monitoring (LQM).

  • Looped link detection by using different magic numbers.

  • Authentication through PAP and CHAP.

  • Multilink support.

  • Software compression.



LCP uses a number of phases to perform its function of setting up, configuring and managing point-to-point links:

  • Establishing the link and negotiating configuration parameters. Acknowledgment frames are sent and received during this phase.

  • Determining link quality: This consists of testing of the point-to-point connection to ascertain whether the quality of the line will support the Network layer protocols.

  • Negotiation of Network layer protocols: Here, Network control programs (NCPs) configure PPP to support Network layer protocols. The PPP devices send NCP packets to choose and configure the Network layer protocol(s). Data is transmitted over the link once a Network layer protocol has been selected and configured.

  • Termination of the point-to-point links: LCP can terminate a point-to-point link at any point in time. Link termination usually occurs when the expiration value of a timeout parameter is reached, through a user requesting it, or when there carrier loss.



Where features are specific to Layer 3 protocol, PPP uses PPP control protocols. PPP control protocols include protocols such as the IP Control Protocol (IPCP).



Understanding Authentication over WAN Links

The authentication protocols that can authenticate the endpoints on each end of point-to-point serial links are:

  • Password Authentication Protocol (PAP):

  • Challenge Handshake Authentication Protocol (CHAP):



With PAP, a two-way handshake process is used by hosts to authenticate to each other:

  • The request is initiated by the remote host. The remote host transmits the username and password information to the local host. The remote host continues to pass this information until the local host either accepts the connection request, or rejects the connection request.

  • The local host that receives the connection call can either proceed to accept the username and password information or reject it. If the request is rejected, the connection is immediately terminated.



With CHAP, three-way handshake process is used by hosts to authenticate to each other:

  • When a router receives a call, the routers transmit a challenge packet to the remote host that started the call.

The challenge packet contains the following information:

  • ID

  • Random number

  • Name of the local host or a username on the remote host.

  • When the remote host receives the challenge packet, it has to respond with the following information:

  • Encrypted unique ID

  • A one-way encrypted password

  • Either the remote hostname or a username

  • Random number

  • When the router receives the above information from the remote host, the router performs a number of calculations to determine whether the values it has, matches to those received from the remote host.



PPP and HDLC Configuration Commands

The PPP and HDLC configuration commands are:

  • encapsulation {hdlc | ppp | lapb}; the interface configuration mode command used to specify encapsulation.

  • compress [predictor | stac | mppc [ignore-pfc ]]; the interface configuration mode command used to specify compression.



The commands used to verify PPP and HDLC configuration are:

  • show interfaces [type number]; used to display the status and configuration information of the physical interface.

  • show processes [cpu]; displays processor utilization and task utilization. Ideal for detecting increased utilization as a result of compression.

  • show compress [predictor | stac | mppc [ignore-pfc ]]; shows compression ratios.



The OSI Physical Layer for Point-to-Point WANs

The Physical layer (layer 1) of the OSI reference model deals with the actual sending of bits and the receiving of bits. The physical layer is hardware specific and deals with the actual physical connection between the computer and the network medium.


The Physical layer identifies the interface between the data terminal equipment (DTE) and the data communication equipment (DCE). A point-to-point WAN link behaves like a trunk between two Ethernet switches.


The physical layer specifications define specific physical aspects of the transmission medium, including the following:

  • Encoding

  • Electrical currents

  • Connectors and pins, and so forth.


The various Physical layer standards are:

  • EIA/TIA-232, EIA/TIA-449,

  • V.24, V.35,

  • X.21,

  • G.703,

  • RJ45,

  • EIA-530,

  • Ethernet,

  • High-Speed Serial Interface (HSSI)

  • 802.3, 802.5,

  • NRZI, NRZ,

  • B8ZS



A few concepts pertaining to WAN circuits are listed below:

  • Data Services Unit and Channel Services Unit (DSU/CSU); refers to the interface used on digital data links to the telephone company. Routers use the attached DSU/CSU as the clock source.

  • Central office (CO); also called a point of presence (POP). The central office connects customers to the switching network of the provider.

  • Customer premises equipment (CPE); refers to equipment which is located at the premises of the subscriber, and is owned by the subscriber.

  • Demarcation point (demarc); refers to the point at which the responsibility of the CPE starts and the responsibility of the service provider stops.

  • Local loop; connects the demarcation point to the closest switching office. A switching office is also called a central office.



Cisco provides quite a variety of different WAN interface cards for Cisco routers. The different standards which specify the types of connectors and physical signaling protocols used on WAN interfaces are listed in Table 2:


TABLE 2: The types of connectors and physical signaling protocols used on WAN interfaces

Standard connectors


Governing body

Number of pins on connector

EIA/TIA-232

TIA

25

EIA/TIA-449

TIA

37

EIA/TIA-530

TIA

25

V.35

ITU

34

X.21

ITU

15




The maximum speeds for the different cables are shown in Table 3:


TABLE 3: The maximum speeds for the different cables

Data in bps


Distance EIA/TIA-232


Distance EIA/TIA-449, V.35, X.21 and EIA-530

2400

60

1250

4800

30

625

9600

15

312

19,200

15

156

38,400

15

78

115,200

3.7

N/a

T1 (1.544 Mbps)

N/a

15




The different standards for WAN speeds are shown in Table 4:


TABLE 4: The different standards for WAN speeds

Line type


Signalling type


Bit rate

56

DS0

56 kbps

64

DS0

64 kbps

T1

DS1

1.544 Mbps: 24 DS0s + 8 kbps overhead.

T3

DS3

44.736 Mbps: 28 DS1s + overhead.

E1

ZM

2.048 Mbps: 32 DS0s

E3

M3

34.064 Mbps: 16 E1s + overhead.

J1

Y1

2.048 Mbps: 32 DS0s - Japanese standard





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