Cloud PSE Mesh It then sends over the LAN in regular OSI model fashion to the correct receiving

Understanding Wide Area Networks | 153 Packet Flag 8 bits From Address 16 bits HDLC Type 8 bits GFI 4 bits DATA up to 128 Bytes

X.25 Packet Header

LGN 4 bits CRC 16 bits End Flag 8 bits LCN 8 bits PTI 8 bits Trailer Figure 7-5 X.25 packet Generally, an X.25 packet will be a maximum of 128 bytes, but remember that a packet’s data can be up to 512 bytes and is always of variable length. Some packets have no data at all; they are informational only to the X.25 system. Now, let’s move on to PSEs and switching to virtual circuits. A PSE is a packet switching exchange. These are located in the central offices just inside the cloud, and they are really mega switching computers that handle huge numbers of packets and decide which cir- cuit out of tens of thousands each packet will take. Quite often, these PSEs are UNIX powered. Immense amounts of processing power are required for the task of sending X.25 packets. The PSE reads the address and framing information of the packet and then routes it in the correct direction. This is another example of the fact that computers can be routers as well; in fact, they are the original routers. These computers act as routers because they can decide multiple paths for the packet. The PSE chooses a circuit out of thousands that is used least, is most direct, or is most available. The PSE then orders up a leased line from the local exchange carrier LEC. It uses this line as the circuit for the packets. In the old days, this was an analog line 2400 bps. Today, it is a digital line, usually at the speed of 64K. It is also synchronous, which means that there is a clocking circuit that controls the timing of commu- nications between the different routers. Remember, the PSE has thousands of circuits from which to choose. These are known as a circuit set. The chances of the entire message of packets taking one circuit are slim, because so many different users and companies are utilizing the bandwidth. Therefore, a typical message of ten packets could be spread over five circuits. Because multiple circuits are being used and not just one, the entire circuit set is known as the virtual circuit. There could be several PSE stops along the way. These PSEs are also PADs, and they disas- semble and reassemble the packets. These stops are also known as hops. For every hop along the way, the PSE buffers the packets into RAM and holds them there until the next PSE gets the packet and acknowledges it. This way, if a packet is lost between two PSEs, then the 154 | Lesson 7 first can send it again. At the receiving office, the PAD router reassembles the packets and the overhead header and trailer is discarded. The router then sends the information in the regular OSI format to the receiving computer on the LAN. X.25 has several advantages compared to dial-up analog lines, including the following: • If any data fails, X.25 automatically recovers and sends it again. This is assuming that there are circuits available in the virtual circuit. If this is not the case and all circuits are being used by others, then other arrangements are made. There is a TTL time to live for the packets to be buffered in the PSE, but if a virtual circuit is not available past the TTL, then the PSE notifies the previous PSE or sending router. • X.25 allows shared access among multiple users on the LAN. They share access through the LAN via the router and the CSUDSU out to a 64K line. This is as opposed to each user having a separate dial-up line. • X.25 has full error and flow control. • There is also protection from intermediate link failure. X.25 is not completely fault tolerant, but it is 70 effective. This is because of the virtual circuit, whereas, on a dial- up line, you are using the same circuit to move a file through the whole transfer. If that circuit is lost, then the whole message must be sent again. • Pricing is per shared packet sent, not per minute. • X.25 is a synchronous, digital transmission. Digital is inherently better and faster because there is less noise, and also because the information does not have to be converted from analog to digital and back. So, this is less overhead in the form of conversion. • There is less overhead per file. For dial up, there can be as much as 40 overhead per file, but with X.25, it can be as little as 8. DEFINING FRAME RELAY Frame Relay is the advancement of X.25 packet switching. It is a newer form of packet switching designed for faster connections. With this system, the packets are now referred to as frames. Like X.25, Frame Relay uses transmission links only when needed. It also uses a virtual circuit, but one that is more advanced. Frame Relay created the “virtual network” that resides in the cloud. Many customers use the same groups of wires or circuits known as shared circuits. Like private connections T1 and so on, Frame Relay transmits very quickly. It might use a T1 connection, but not in a private manner. The T1 is a trunk carrier, a physi- cal connection that has a data transfer rate of 1.544 Mbps. Unlike X.25, much less processing is needed in Frame Relay. Inside the switches or PSEs, most overhead is eliminated. The net- work only looks at the address in the frame. Unlike dedicated T1 private connections, Frame Relay uses a public leased line. Frame Relay was created to take advantage of the low-error, high-performance digital infra- structure now in place and to better service synchronous transmissions only. It is a much sim- pler network compared to a private line network. Figure 7-6 offers an example of a T1 mesh network. Connections are between each city. This is similar conceptually to the mesh topology.