2.5 List the characteristics, requirements, and appropriate situations for WAN connection services.
X.25: ITU setup the X.25 standard for packet switching networks in 1976. Originally, X.25 was used to connect remote terminals to mainframe computes over telephone lines. At that time, the communication speed was somewhat lower (X.25 only provide a switched data service at 56Kbps or less). In addition, the media for communication was unreliable. Therefore, X.25 put a lot effort on error correction for transmission, which had a negative impact on performance. ant not necessary nowadays for the media is more reliable.
ISDN: Integrated Services Digital Network, an international communications standard for sending voice, video, and data over digital telephone lines. In the Networking Essentials exam, when taking about ISDN, another term will also be mentioned, BRI (Basic Rate Interface), which is made up of two 64Kbps B channels and one 16Kbps D channel (2B+D). You can also combine two channels to increase the speed to 128Kbps. Please note that D channel only transmits synchronization and link management signal. The capacity of the D channel is not used for calculating ISDN speed.
You can get very higher speed or even as high as T3 by the combination of B channels in ISDN. The calculation of the speed of ISDN line is quite simple. Just times the B channel number by 64Kbps. For example, if you have 10 B channels, the ISDN speed is 64Kbps * 10 = 640Kbps. While in the real world, most ISDN lines have 128Kbps transmission speed.
For the Networking Essentials exam, you also need to know ISDN is used to replace the legacy telephone network. The advent of V.90 56Kbps-modem technology and ADSL (which can provide 256Kbps and up speed) will make ISDN technology obsolete. In the real world, NEVER plug a standard PC modem into ISDN line, because it may burn the modem.
Frame Relay A high-speed packet switching protocol used in wide area networks (WANs). It works at data link layer. Frame relay is used mostly LAN to LAN connections across great distances. As such, it performs less rigorous error detection. Frame relay provides for a granular service up to DS3 rates of 44.736 Mbps and is suited for data and image transfer. In another term, it can be multiplexed to this higher speed. In most cases, frame relay only works less than 2Mbps. Because of its variable-length packet architecture, it is not the most efficient technology for real-time voice and video compared to ATM.
In some textbook such as Networking Essentials Self-Paced Training Kit, you will find following sentence, "Frame relay uses a private virtual circuit (PVC) to transmit variable length frames at the OSI Data Link layer." Be careful, here PVC is different than the PVC (Permanent Virtual Circuit) commonly used (such as in ATM network), which means a point-to-point connection that is established ahead of time.
Please remember frame relay provides increased bandwidth as needed. If you know FTQ 2.5.7, it will be fine for your Networking Essentials exam. It seems to be the only thing can be tested regarding the frame relay.
T1: A 1.544 Mbps point-to-point dedicated line provided by the telephone companies. T1 lines are widely used for private networks and high-speed links to and from Internet service providers. A T1 line provides 24 64-Kbps voice or data channels. Here is the list of channels of 64Kbps T-carriers and corresponding speed.
ATM: Asynchronous Transfer Mode (Dont mix it with the ATM that can cash the money) is expected to take computer networking to a next-generation. ATM accommodates the simultaneous transmission of data, voice, and videos, allowing for the unification of todays separate networks that were created for specific functions (computers for data, telephones for voice). ATM has four advantages compared to traditional Ethernet.
Many people confuse this question with FTQ 2.5.7. Please notice the slight difference between provide "increased bandwidth when needed" and provide "bandwidth on demand".
To better understand this, let's look at the following two statements:
Both statement 1 and statement 2 are correct. However, the implementations on how to provide different transmission speed are different. For the frame relay, it provides a granular service. In another word, it can be multiplexed to higher speed, which means that you can combine two lines to get higher speed. While for the ATM, after you purchased the specific ATM device, the maximum transmission speed are fixed. For example, if you purchased IBM 8285 switch with IBM 25Mbps ATM card. The maximum transmission speed is 25Mbps. You will never reach 155Mbps, the most popular transmission speed of the ATM products. Moreover, you cannot put two 25Mbps ATM cards in a computer and duplex your transmission speed to 50Mbps. However, in the ATM, it is possible to setup a channel and reserve the bandwidth for a specific application. You can also increase the bandwidth of that channel. In another word, you can provide bandwidth on demand. Although your total bandwidth will not exceed the maximum bandwidth, it can guarantee the QoS (Quality of Service) of the application. The researches on VOD (Video on demand) are very popular nowadays. In fact, there are tons of papers that focus on the how to implement VOD system using ATM technology. That's because ATM is the only technology that supports "on-demand" service. Because our current Internet uses Ethernet while not ATM, it doesn't support services on-demanded. That's the reason why you make an Internet phone call, the quality is not so good. We know our modem transmission speed is up to 56Kbps. While the transmission speed required for regular call is only 9.6Kbps. The only reason is that we cannot reserve the 9.6Kbps channel between the caller and receiver. When some guy sends a huge E-mail over the Internet, it will affect a lot on your phone calls over the Internet. I am sure that Internet 2 will use ATM technology to solve this problem. By the way, if you guys are playing with stocks now, I suggest you purchase FORE, the leader of ATM technologies. You will find $13 is really a bargain for this ATM industry stock. J