Long-Distance Digital Technologies

1. Long-Distance Digital Technologies Lecture 7

2. Why Digital? • Digital communication allows us to handle more amounts of data, as well as allows us to send that data farther distances. • Digital signals are encoded on some medium, so they propagate farther than non-encoded analog signals.

3. Digitization of a Signal • In general, a waveform is sampled, and a digital approximation at those sampled points becomes the new signal.

4. An Example • When the signal gets transformed into a digital signal, a sequence of discrete values are created • 0,2,4,4,7,1,1,1,1,1,1,1,2,4,4,4,4.

5. Sampling Frequency • Since our modern data networks were modeled on the older analog networks, we have adopted some of their technology. • Nyquist’s Theory: To recreate a signal of frequency x, you must take samples of the original signal at a minimum frequency of 2*x.

6. Voice Sampling in A/D Systems • Voice reproduction requires a system of 4000Hz, so sampling must occur at 2*4000Hz = 8000Hz. • A/D conversion in a phone system samples the signal every 1s/8000Hz = 0.125s. • This 0.125s sampling constant is ever-present in telephony.

7. Sampling Drawbacks • Very accurate sampling requires more bits. • Pick some range of analog values (say 1-1000V). If you wanted to represent these voltages digitally, you would want a high resolution, but that requires more bits. • Pulse Code Modulation (PCM) uses 0-255 to represent a range of analog values. • Using PCM for 1-1000V would have a 4V resolution. Your digital value would be (at worst)  2V.

8. Synchronous Communication • Synchronous communications occur over long distances with some form of clocking (synchronizing). • Synchronous links can move data at a constant rate, so even as traffic increases, the rate stays the same. • When synchronous links are idle, they send null information. Synchronous links must send something at all times!

9. Why is Synchronous Good? • Consider making an long-distance telephone call: You’re calling your grandmother in Los Angeles. (I really like this example, for some reason.) • Asynchronous: You start to talk at normal speed, no delay. Suddenly a huge delay is heard in LA. Then it becomes normal. Annoying!

10. Synchronous (cont.) • Synchronous: You call your grandmother in LA. You start talking normally. You may experience some SMALL delay, but it’s always constant. Bearable. • The synchronous link has clocking built in to guarantee a constant stream of data from one point to the next. Even on a multipoint network!

11. I Want It – The CSU/DSU • Synchronization (clocking) is provided by Channel Service Unit/Data Service Unit (CSU/DSU). • Much of today’s modern equipment has CSU/DSU capabilities built-in, but external devices are still common. • The CSU (for short) also makes sure the connection conforms to existing telephone standards.

12. The CSU/DSU – DNS1500

13. DNS1500 Specifications • T1 - 1.544Mbps with B8ZS or AMI encoding • DDS - 56/64Kbps with AMI encoding • DSX-1 – 1.544Mbps AMI or B8ZS/ESF encoding • V.35 Interface

14. How CSU/DSUs Fit In… • A CSU/DSU is required at each end of any digital point-to-point connection. • The CSU/DSU converts between the digital standards of the telephone system and those of the computer (router) vendors.

15. Common Diagram

16. The CSU Part • The CSU has the following functions: • Line termination – deals with the electrical signals on the leased line • Testing circuitry – loopback testing, link-level testing • Prevents excessive 1 bits. (Original thought: too many 1’s would lead to too much current on the line.)

17. Excessive 1’s • Two main methods are used to prevent too many 1’s: • Bit stuffing • 0V for a logical 0, alternating +3V, -3V for a logical 1.

18. The DSU Part • The DSU handles the data: • Translates the digital information from the telephone circuit to the form which the CPE desires. (V.35) • CPE – Customer Premise Equipment

19. Telephone Standards • The “T” in T1 refers to the “Telephone Standards • The standards are not internationally standard- Japan, Europe use their own slightly different schemes.

20. North American Standards

21. European Standards

22. Digital Signal Level Circuits • DS0 – 64kbps • DS1 – 24xDS0 • DS2 – 4xDS1 = 96xDS0 • DS3 – 28xDS1 = 672xDS0 • DS4 – 168xDS1 = 4032xDS0

23. Fractional Circuits • The “fractional” circuit is getting a smaller bandwidth on a given medium. • When you have a fractional T3 installed, you have a pair of 75 ohm coaxial cables. You get a fraction of the T3 by using TDM.

24. Too Big, Too Small… Just Right? • You can get multiple smaller circuits and combine them in such a fashion that allows you to use the combined bandwidth. • This is termed inverse multiplexing. Special hardware is required in addition to CSU/DSUs at both ends of the circuits. • Multi-homing is a form of inverse multiplexing.

25. An Inverse Multiplexing Example

26. Fat Pipes • Bigger circuits have their own standards. • STSx – Synchronous Transport Signal • OCx – Optical Carrier • Note: The x refers to a different level.

27. Fat Pipes (cont.)

28. Fat Pipes (cont.)

29. Fat Pipes (cont.) • OC3  OC3C. • An OC3 is really 3 separate OC1 circuits running on the same physical medium. • The “C” stands for concatenated (or clear-channel). • Remember: A T3 is NOT 3 T1s!

30. Framing Schemes • Three major types of framing schemes exist: • AMI • B8ZS • ESF

31. AMI • Alternate Mark Inversion • AT&T definition: A line code that employs a ternary signal to convey binary digits, in which successive ones are represented by digital elements that are normally of alternating, positive and negative polarity but equal in amplitude, and in which binary zeros are represented by signal elements that have zero amplitude.

32. B8ZS • Bipolar 8 Zero Substitution. • Telco dictionary: Rather than inserting a one for every seven consecutive zeros, B8ZS inserts two violations of the bipolar line encoding technique used for digital transmission links. • Bipolar Violation: Two consecutive marks of the same polarity on the T line.

33. ESF • Extended (Extended) Super Frame • T1 frames occur 8000 times a second, each frame preceded by a framing bit. ESF requires 2000 framing bits (which are used for synchronization purposes). The remaining 6000 bits are used for error detection, data link monitoring, cyclic redundancy checks, etc.

34. SONET • Synchronous Optical Network • Specifies the following: • Framing on optical circuits • Multiplexing smaller circuits on to larger ones • How clocking information is sent alongside data

35. SONET Framing

36. Why the Weird Size? • Take STS-1 for example: 51.840Mbps/125s = 6480 bits = 810 octets. • STS-3 is exactly 3 times that size, such that 3 STS-1 frames can fit in a STS-3 frame! • To talk about in the future: SONET Ring