Top-Down Network Design, Second Edition

364 Chapter 11: Selecting Technologies and Devices for Enterprise Networks

For some organizations, scalability is a key design goal. The selected WAN solution must

have enough headroom for growth. As discussed in this chapter, some WAN technologies

are more scalable than others.

Another key design goal for many organizations is to save money on the cost of WAN and

remote-access circuits. Optimization techniques that reduce costs play an important role in

most WAN and remote-access designs. Methods for merging separate voice, video, and

data networks into a combined, cost-effective WAN also play an important role. These

methods must handle the diverse QoS requirements of different applications.

Remote-Access Technologies

As organizations have become more mobile and geographically dispersed, remote-access

technologies have become an important ingredient of many enterprise network designs.

Enterprises use remote-access technologies to provide network access to telecommuters,

employees in remote offices, and mobile workers who travel.

An analysis of the location of user communities and their applications should form the basis

of your remote-access design. It is important to recognize the location and number of full-

and part-time telecommuters, the extent that mobile users access the network, and the

location and scope of remote offices. Remote offices include branch offices, sales offices,

manufacturing sites, warehouses, retail stores, regional banks in the financial industry, and

regional doctors’ offices in the health-care industry. Remote offices are also sometimes

located at a business partner’s site (for example, a vendor or supplier).

Typically, remote workers use such applications as e-mail, web browsing, sales order-entry,

and calendar applications to schedule meetings. Other, more bandwidth-intensive applica-

tions include downloading software or software updates, exchanging files with corporate

servers, providing product demonstrations, managing the network from home, videocon-

ferencing, and attending online classes.

Part-time telecommuters and mobile users who access the network less than 2 hours per

day can use an analog modem line. Users who access the network for more than 2 hours

a day need a faster and more convenient technology, however. Analog modems take a long

time to connect and tend to have high latency and low speeds. (The highest speed available

for analog modems today is 56 Kbps.) For customers who have requirements for higher

speeds, lower latency, and faster connection-establishment times, analog modems can be

replaced with small office/home office (SOHO) routers that support ISDN, a cable modem,

or a DSL modem. The following sections discuss these options and provide information on

PPP, a protocol typically used with remote-access and other WAN technologies.

Remote-Access Technologies 365

Point-to-Point Protocol

The Internet Engineering Task Force (IETF) developed PPP as a standard data link layer

protocol for transporting various network layer protocols across serial, point-to-point links.

PPP can be used to connect a single remote user to a central office, or to connect a remote

office with many users to a central office. PPP is used with ISDN, analog lines, digital

leased lines, and other WAN technologies. PPP provides the following services:

• Network layer protocol multiplexing

• Link configuration

• Link-quality testing

• Link-option negotiation

• Authentication

• Header compression

• Error detection

PPP has four functional layers:

• The physical layer is based on various international standards for serial communica-

tion, including EIA/TIA-232-C (formerly RS-232-C), EIA/TIA-422 (formerly

RS-422), V.24, and V.35.

• The encapsulation of network layer datagrams is based on the standard High-Level

Data-Link Control (HDLC) protocol.

• The Link Control Protocol (LCP) is used for establishing, configuring, authenticating,

and testing a data-link connection.

• A family of Network Control Protocols (NCPs) is used for establishing and config-

uring various network layer protocols, such as IP, IPX, AppleTalk, and DECnet.

Multilink PPP and Multichassis Multilink PPP

Multilink PPP (MPPP) adds support for channel aggregation to PPP. As mentioned in

Chapter 5, “Designing a Network Topology,” channel aggregation can be used for load

sharing and providing extra bandwidth. With channel aggregation, a device can automati-

cally bring up additional channels as bandwidth requirements increase. Channel aggrega-

tion usually is used in an ISDN environment, but it can be used on other types of serial

interfaces also. (A PC connected to two analog modems can use channel aggregation, for

example.) MPPP ensures that packets arrive in order at the receiving device. To accomplish

this, MPPP encapsulates data in PPP and assigns a sequence number to datagrams. At the

receiving device, PPP uses the sequence number to re-create the original data stream. Mul-

tiple channels appear as one logical link to upper-layer protocols.