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Chapter One: Determining Remote Access NeedsAfter completing this chapter, you will be able to:
- Describe various wide area network (WAN) services
- List different kinds of circuit-switching connections
- List different kinds of packet-switching connections
- Describe various WAN encapsulation types
- List the signaling standards used with serial connections
- Explain the difference between synchronous and asynchronous serial communications
- Describe site considerations to use in selecting remote access services and equipment
WAN ServicesWAN communication is often called a service because the network provider often charges users for the services provided by the WAN. These charges are known as tariffs. WAN services are provided primarily through the following three technologies:
- Dedicated lines (sometimes referred to as leased lines)
- Circuit switching
- Packet switching
Dedicated Lines (Leased Lines)Dedicated lines are fixed connections, which do not involve establishing a new connection each time the link is used. They are permanent end-to-end connections. The telecommunications company provides a dedicated high-speed connection between the two desired locations, at speeds ranging from as low as 9600bps to as high as 45Mbps. The higher the speed, the greater the monthly fixed cost of the line. The connection is available 24 hours a day, 7 days a week, and is thus suited to companies that want permanent connections between their office branches, or perhaps to a company that wants a permanent connection to the Internet. One big advantage of dedicated lines is that you, the customer, own all the bandwidth on the connection, giving you a lot of flexibility for your WAN needs.
The basic unit of measurement for dedicated lines is a T1 connection, which supports 1.544Mbps. A T3 connection supports 45Mbps. Fractional T1 circuits are available in units of 64Kbps, with connections of 384Kbps, 512Kbps, and 768Kbps being common. The connection is implemented with two units:
- Channel Service Unit (CSU)�This unit provides the interface to the dedicated line.
- Data Service Unit (DSU)�This unit interfaces between the CSU and the customer's equipment, using RS-232 for low speeds up to 56Kbps, and V.35 (RS-422/499) for higher speeds. It is common to have the units as a single component. Several modular Cisco routers can include CSU/DSUs as add-on modules.
Synchronous connections must always be in sync even if they have to make adjust-ments. The accurate decoding of the data at the remote end is dependent on the sender and receiver maintaining synchronization during decoding. The receiver must sample the signal in phase with the sender. If the sender and receiver were both supplied by exactly the same clock source, transmission could take place forever with the assurance that signal sampling at the receiver was always in perfect syn-chronization with the transmitter. This is seldom the case, so in practice the receiver is periodically brought into sync with the transmitter. It is left to the internal clocking accuracy of the transmitter and receiver to maintain sampling integrity between synchronization pulses.
Signaling StandardsA number of different standards define signaling over a serial cable, including EIA/ TIA-232, X.21, V.35, EIA/TIA-449, EIA-530, and V.25bis. Each standard defines the signals on the cable and specifies the connector at the end of the cable.
Circuit SwitchingCircuit switching is a WAN switching method in which a dedicated physical circuit is established, maintained, and terminated through a carrier network for each communication session. Circuit switching accommodates two types of transmissions: datagram transmissions and data-stream transmissions. Used extensively in telephone company networks, circuit switching operates much like a normal telephone call. Circuit switching moves data between two points by setting up a physical link, or circuit, between them. Data flows in a stream along a circuit that lasts as long as necessary, at which time the switches and lines used to build the circuit are freed for another connection.
The usual example of a circuit-switched network is the plain old telephone system (POTS) network. When you dial a phone number, the phone company's switching equipment sets up a direct circuit between your telephone and the destination. The chief advantage of circuit switching is that the flow of data (for example, your voice) is not subject to delays introduced by the network, and the recipient receives data exactly as it was sent. The big disadvantage is that much of the connection's available bandwidth may be wasted due to the bursty nature of data traffic, which will rarely saturate the link's capacity. Figure 1.1 illustrates how a laptop can dial into an access server using POTS.
There are two commonly used circuit-switching methods:
- Asynchronous serial�Dial-up using modems
- Integrated Services Digital Network (ISDN)�Two types: Basic Rate Interface (BRI) and Primary Rate Interface (PRI)
Asychronous Serial CommunicationAsynchronous serial communication uses the existing telephone network with its associated low cost. Asynchronous communication happens when using the exist-ing dial-up telephone network and when a connection is made between the two modems by dialing the number assigned to the other modem. Generally, connections are established for limited durations. This suits remote access users who might want to dial into their network after hours, or small offices that dial into their Internet Service Provider at regular intervals during the day to exchange email.
To allow an asynchronous serial call, a Cisco router needs to have an asynchronous serial interface attached to a modem. If the router uses an external modem, the inter-face on the modem will need the EIA/TIA-232 signal standard, and the interface to the telephone company will need the standard RJ-11 adapter....
Figure 1.1 Circuit-switched network.
Types of Serial CommunicationsThere are two basic types of serial communications, synchronous and asynchronous. Asynchronous means "no synchronization," and thus does not require sending and receiving idle characters. However, the beginning and end of each byte of data must be identified by start and stop bits. The start bit indicates when the data byte is about to begin, and the stop bit signals when it ends. The requirement to send these additional two bits causes asynchronous communications to be slightly slower than synchronous; however, it has the advantage that the processor does not have to deal with the additional idle characters. The serial ports on IBM-style PCs are asynchronous devices and therefore support only asynchronous serial communications.
An asynchronous line that is idle is identified with a value of 1 (also called a mark state). By using this value to indicate that no data is currently being sent, the devices are able to distinguish between an idle state and a disconnected line. When a character is about to be transmitted, a start bit is sent. A start bit has a value of 0 (also called a space state). Thus, when the line switches from a value of 1 to a value of 0, the receiver is alerted that a data character is about to come down the line.
ISDNISDN connections, like asynchronous serial connections, are circuit-switching connections. They are intended to compete with POTS by providing voice, data, and other traffic across a telephone network. Logically, ISDN consists of two types of communications channels: bearer service B channels, which carry data and services at 64Kbps; and a single D channel, which carries signaling and control information that is used to set up and tear down calls. The transmission speed of the D channel depends on the type of ISDN service you've subscribed to. ISDN services can be divided into two categories: Basic Rate Interface (BRI) service and Primary Rate Interface (PRI). Both services will be described in the following sections. ISDN configuration will be discussed in greater detail later in Chapter 6.