Introduction to Networking

Unit 8

Lesson 3 - Connecting Networks

Connecting to the Internet

Internet access has become an indispensable tool for businesses of all sizes. Even if a small peer-to-peer network never needs to link to another LAN, it usually must provide some sort of connection to the Internet.

Fortunately, this process has become simpler. In the early days of the Internet, each organization was completely responsible for learning the Internet's rules and procedures, and installing the equipment necessary to connect to the worldwide network. However, today, telecommunication companies and independent Internet service providers (ISPs) simplify this process for their customers by handling many technical details of Internet access, providing valuable advice, and selling the necessary hardware.

Home Dial-Up Access

An individual user normally accesses the Internet by means of a modem with a dial-up connection over an analog local loop. That connection links the user to an online information service, such as America Online, or a local ISP, as illustrated on the Individual Internet Access Diagram.

Individual Internet Access

Home Internet connections generally have the following basic components:

	Local loop connection (telephone line).
	Modem.
	ISP account provides Internet access for a monthly fee, and basic services such as e-mail boxes.
	ISP connection software is normally provided by the ISP. The software provides an interface for Internet access and the software that uses the modem to access the Internet. Normally, the dial-up software calls a number to access the ISP, and the ISP then provides access to the backbone of the Internet.
	Web browser application displays Web pages and handles file downloads.
	Specialized software, either a freestanding application or a browser plug-in, provides support for special multimedia functions. For example, with an audio coder/decoder (codec) and microphone, a user may use the Internet to make telephone calls.

Two special protocols, Serial Line Internet Protocol (SLIP) and Point-to-Point Protocol (PPP), are used to transfer IP packets across a serial link. These two protocols are widely used by ISPs to provide dial-up Internet connections for home or small business users.

SLIP

SLIP is a simple encapsulation of an IP datagram asynchronously transmitted over serial lines using an RS-232 interface. At the user's computer, the IP packet is encapsulated between two SLIP control characters (hexadecimal C0), and the resulting SLIP frame is sent to the modem. The modem transmits the information across the telephone network to the ISP modem. The ISP's modem is attached to a router that decapsulates the SLIP frame. The router then encapsulates the original IP packet in a WAN protocol frame, and routes it across the Internet to the proper destination.

A SLIP frame is formed by adding the same control character (a hexadecimal "C0") to both the beginning and end of an IP packet. In other words, the packet is inserted between the two characters, that are called SLIP delimeters.

There are several drawbacks to SLIP:

	There is no protocol header that can contain information about the type of data being transported, thus a SLIP connection can only support one network protocol at a time.
	Each end must know the other end's IP address, because there is no way to exchange this information in the protocol.
	There is no checksum to allow for error detection on noisy telephone lines, which means the higher layers are responsible for error detection and recovery.

Despite these limitations, SLIP is a proven protocol that is very easy to implement. Its simplicity is very attractive on slow links, such as analog local loops.

PPP

PPP is a more robust and flexible serial protocol. This Internet standard protocol offers multiprotocol support, data compression, host configuration, and link setup. PPP is used by higher layer protocols, such as TCP/IP, to provide simple WAN connectivity between users. It replaces SLIP and solves some of its inefficiencies. PPP supports either asynchronous (character-oriented) or synchronous (bit-oriented) transmission links.

PPP is based on the High-Level Data Link Control (HDLC) standard, which deals with LAN and WAN links and operates at the Data Link Layer of the OSI model. PPP starts with the HDLC frame format; however, it adds a protocol field to identify the Network Layer protocol of each frame. This is what allows a PPP link to carry data for multiple network protocols.

Business Internet Access

There are two ways to connect a business customer to the Internet:

Nondedicated access is any connection that is not continuous. The connection is established at the time it is desired and released when no longer needed. The connection is usually made through a switched network, typically the telephone system. The most common method to access the Internet is to use an existing telephone line, although Integrated Services Digital Network (ISDN) provides much better performance if available.

Dedicated access, at a minimum, requires a permanent analog telephone number for a dial-up modem connection. However, the analog connection speed is primarily controlled by the modem speed which, as you have seen, is slow. Thus, businesses that need dedicated access usually opt for dedicated digital services, such as T1, fractional T1 (FT1) (DS0), or ISDN. Because of the additional expense of these services, ISPs tend to provide a higher level of service and support to their permanently connected corporate clients.

Some technologies, such as dial-up lines and ISDN, can be used for both dedicated and nondedicated installations. The difference between the two is the type of equipment used to make the connection. For example, the Dedicated vs. Nondedicated Access Diagram illustrates how an ISDN line might be used for either a dedicated or nondedicated application.

Dedicated vs. Nondedicated Access

When a business uses a dedicated line to connect to an ISP, the permanent connection makes it possible for the business to use a domain name and dedicated IP address. This is typically necessary if the corporation wants to establish a Web site.

Private line Internet connections generally have the following basic components, as shown on the Dedicated Connection to ISP Diagram:

	Common carrier connection--A connection such as FT1, T1, frame relay, and so on is required.
	Channel service unit/data service unit (CSU/DSU)--A CSU/DSU is required to terminate a multiplexed common carrier connection such as a T1 line.
	Router--The routers employed on the Internet are capable of accommodating multiple speeds, channels, protocols, and transmission paths. Some routers may have a built-in CSU/DSU.
	Internet domain name--Even though a business may have no plan to create a Web site in the near future, it is best to reserve a domain name to prevent someone else from registering the preferred domain name first.

Dedicated Connection to ISP

Activities

See the Activities and Extended Activities section in Unit 8 Lesson 3 in your textbook Introduction to Networking to test what you have learned so far.

[ Previous Section ] [ Unit Contents ] [ Next Lesson ]