In the New Routing Protocol window, select RIP version 2 for Internet Protocol and

150 | Lesson 7 RIP can now take care of what we did with static routes in previous lessons. Keep in mind that for much bigger networks, other protocols are more desirable. ■ Defining Common WAN Technologies and Connections Wide area networks connect multiple local area networks together. If an organization wishes to have a wide area connection to another office, it needs to decide on a networking service and the speed at which it wishes to connect. Budgeting plays a significant role in these types of decisions. THE BOTTOM LINE CERTIFICATION READY How would you define X.25 and Frame Relay? 1.3 Defining Packet Switching Packet switching is how data packets are moved over switched wide area networks. Types of packet switching services include X.25 and Frame Relay. This section defines those two services. Most WANs utilize some type of packet switching technology. Let’s discuss the technology world prior to packet switching and talk about why packet switching is a superior solution. Packet switching services include X.25 and Frame Relay. Before packet switching, there were direct dial-up connections and other archaic forms of communication. Some of the problems associated with these included the following: • Until the early 1970s, data transfer was analog with much static and noise. It was also primarily asynchronous and conducted by dial-up modems. • Data transfer could be as much as 40 overhead and only 60 actual information. Overhead included the allowance for noise, error checking, flagging, stopstart bits, parity, and so on. • Longer data transfers could be disconnected for many reasons, including: ❍ Poor connection ❍ Network degradation ❍ Loss of circuits • After a disconnect, the entire message file would have to be resent, usually after the person dialed out again. DEFINING X.25 Then packet switching arrived. The X.25 communications protocol was one of the first implementations of packet switching, and it is still in use today. Packet switching was originally created to break down large messages into smaller, more manageable segments for transmission over a WAN. Basically, the sending computer sends its message over the LAN to the hardwaresoftware component known as the router. The router then breaks the file into more manageable pieces known as packets. Every packet gets a portion of the original message. Every packet also gets a segmentation number and address info. Each packet is then transmitted over the physical link to the switching system telco, which picks a wire for transmission from the header information of the packet. This establishes a virtual connection or virtual circuit. Next, packets are re-assembled at the receiving router. Understanding Wide Area Networks | 151 Here are the X.25 packet switching steps: 1. A computer sends data to the router as normal through the OSI model over the LAN. 2. Data is gathered by the router as the message, but the router then disassembles the entire lot into jumbled packets. Thus, the router is known as a PAD packet assembler disassembler.

3. The PAD sends the packets to a CSUDSU high-speed digital data interchange device

as serial information. The CSUDSU is the equivalent of the modem for the entire LAN. It is known as a DCE, or data communications equipment. In this scenario, the PAD or router is known as the DTE, or data terminating equipment.

4. The CSUDSU sends the packets to the demarcation point demarc in the office or

company. Quite often, the CSUDSU is the demarc, otherwise known as the point where your responsibility as an administrator ends and the telecommunications or data communications provider’s responsibility begins. The demarc could also be a network interface device or simple networking jack. Figure 7-3 illustrates the process up to this point. Figure 7-3 X.25 packet switching process Computer Switch Router PAD CSUDSU

5. This then leads to the central office of the phone company that is supporting the

X.25 service.

6. The central office C.O. picks a wire and transmits to the switching office, which then

continues to the power lines, and so on. When the central office does this, it is known as a virtual circuit.

7. The information ends up at the receiving central office, which sends the data over

another virtual circuit to the correct line that leads to the other office.

8. This then leads to their demarcation point demarc, to a CSUDSU, and then to their

receiving router PAD.

9. The receiving PAD then buffers the info, checks it, recounts, and puts the packets in

sequence.

10. It then sends over the LAN in regular OSI model fashion to the correct receiving

computer. The “cloud” is the area of the telephone company’s infrastructure that is in between the demarcation point of your office and the receiving office. All central offices, switching offices, telephone poles, and lines are part of the cloud. The cloud is represented in Figure 7-4. 152 | Lesson 7 Characteristics of X.25 include the following: • It is usually digital. • It is usually synchronous. This means that the connection is controlled by a clocking circuit so that both X.25 devices know when to transmit data without having collisions. • It involves a 56K or 64K max line. • It is also known as variable length packet switching. • A PAD decides which circuit the information is going to take as part of the virtual circuit concept. • Packets usually have 128 bytes of actual data, but some configs go up to 512 bytes. Now, let’s cover the X.25 components. Basically, an X.25 packet is made of overhead and data. Overhead is the packet’s header and trailer information combined. Therefore, if some- one asks what the two parts of a packet are, you would answer the overhead and the data. However, if someone asks about the three parts of a packet, you would say the header, data, and the trailer. Overhead is not real data. It is information sent as additional electrical impulses, but it is not part of the original message. The header information includes items such as the packet flag, HDLC high-level data link control, the from address, information with error detection, and so on. The trailer includes items such as the cyclic redundancy check CRC, which checks the size of the packet for accuracy at the destination computer. An entire X.25 packet can be seen in Figure 7-5. Figure 7-4 X.25 “cloud” Los Angeles Packet Switching Exchange Central Office Atlanta Seattle New York Packet Switching Exchange Central Office Packet Switching Exchange Central Office

X.25 Cloud PSE Mesh

Packet Switching Exchange Central Office