COM 360

1. COM 360

2. Chapter 2 Direct Link Networks

3. Network Technologies • Point-to-Point Links • Carrier Sense Multiple Access ( CSMA) – (for example the Ethernet) • Token Rings – (for example IEEE 802.5 and FDDI Fiber Distributed Data Interface) • Wireless– (for which 802.11 is the emerging standard)

4. Problems Connecting computers is a first step. There are additional problems to solve before they can exchange packets: • Encoding bits into the transmission medium • Framing the bits so they can be understood • Error detection • Reliable delivery, in spite of occasional errors • Media access control

5. Hardware Building Blocks • Networks are constructed from nodesand links • Nodes are general purpose computers such as workstations, multiprocessors or PCs as well as special purpose switches, routers. • Memory – finite – must be managed • Network Adapter (NIC) and its device driver • Links implemented on physical media, such as twisted pair, coaxial cable, optical fiber

6. Nodes Example workstation architecture

7. The work station’s adaptor component connects the rest of the work stations to link. • More then just a physical connection , it is an active intermediary between node and link with its own internal processor. • Its role is to transmit data from the work station on to the link and receiver data from the link, storing it for the workstation.

8. Network Adaptor • A network adaptor can be thought of as having two main components. • A BUS Interface • A Link Interface

9. Network Adaptor • A BUS Interface • A bus interface that understands how to communicate with host. • A Link Interface • A link Interface that understands how to use link.

10. Network Adaptor • The adaptor exports a Control Status Register(CSR) that is readable and writeable from CPU. • The CSR is typically located at some address in the memory, thereby making it possible for CPU to read /write just like any other memory location.

11. Network Adaptor • S/W on the host – A Device driver – writes to the CSR to instruct it to transmit and/or receive data and reads from the CSR to learn the current status from the CRS. • Notifications • Reception of a FRAME. • The adaptor interrupts the host.

12. Block Diagram of a Network Adaptor

13. Interrupts • The host only pays attention to the network device when the adaptor interrupts the host, (for example, when a frame has been transmitted or one arrives). • A procedure is invoked by the operating system, and an interrupt handler is invoked to take the appropriate action. • While servicing this interrupt, the OS disables other interrupts.

14. Network Adaptor • There are TWO basic mechanisms • Direct Memory Access(DMA) • Programming I/O (PIO)

15. Direct Memory Access vs. Programmed I/O • There are two ways to transfer the bytes from the frame between the adaptor and host memory: • Direct Memory Access (DMA)- the NIC directly reads/writes to the host’s memory without CPU involvement, using a pair of buffer descriptor lists. • Programmed I/O (PIO)- network adaptor (NIC) copies message into its own buffer, until CPU can copy it into the host memory.

16. Programmed I/O

17. Links • Physical media are used to propagate signals as electromagnetic waves, traveling at the speed of light. • Properties of EM waves: • Frequency- or oscillations, measured in hertz • Wavelength – distance between adjacent maxima and minima, measured in meters

18. Electromagnetic Waves • Wavelength = speed / frequency • Voice grade phone lines carry waves ranging from 300 Hz to 3300 Hz • Voice-grade example: 300Hz in copper wire • Wavelength = Speed in Copper/ Frequency = 2/3 x 3 x 108 /300 = 667 x 103 meters

19. Electromagnetic Spectrum

20. Links • A link is a physical medium carrying signals in the form of electromagnetic waves. • Binary data is encoded in the signal. • Lower layer is concerned with modulation, varying the frequency, amplitude or phase of the signal • Upper layer is concerned with encoding the data

21. Link Attributes • Another link attribute is how many bit streams can be encoded on it, at a given time. • One bit stream- connected nodes share access • Point-to-point – often two bit streams at once • Full duplex - two directions – simultaneously • Half duplex – one direction at a time • Simplex – one direction

22. Cables • Type of cable depends on technology • Coaxial – ( thick and thin) – within buildings • Category 5 ( CAT 5) – twisted pair, thicker gauge than telephone wire • Fiber –plastic or most often glass, more expensive, but used to connect buildings, and transmits light instead of electrical waves.

23. Figure 7.3 Twisted-pair cable

24. Figure 7.4 UTP and STP cables

25. Table 7.1 Categories of unshielded twisted-pair cables

26. Figure 7.5 UTP connector

27. Figure 7.7 Coaxial cable

28. Table 7.2 Categories of coaxial cables

29. Figure 7.8 BNC connectors

30. Figure 7.10 Bending of light ray

31. Figure 7.11 Optical fiber

32. Figure 7.12 Propagation modes

33. Figure 7.13 Modes

34. Figure 7.14 Fiber construction

35. Figure 7.15 Fiber-optic cable connectors

36. Local Link Cables

37. Leased Lines • To connect nodes on opposite sides of the country, or at great distances, you must lease a dedicated line from the telephone company. • DS1, DS3, T1, and T3 are relatively old technologies, defined for copper • STS-N links are for optical fiber (Synchronous Transport Signal), also called OC-N for Optical Carrier • Originally designed for voice, today can carry data, voice and video

38. Common Bandwidths

39. Last-Mile links • Leased lines range in price from $1000/month to “don’t ask” • Last mile links span the last mile from the network service provider to the home or office. • Conventional modem- POTS (plain old telephone service) • ISDN – (Integrated Services Digital Network) – uses CODEC ( coder/decoder) to encode analog to digital signal • xDSL (Digital Subscriber Line) • Cable modem- uses cable television (CATV) infrastructure, available to 95% of US households

40. Common Available Services

41. TELEPHONE NETWORK Telephone networks use circuit switching. The telephone network had its beginnings in the late 1800s. The entire network, which is referred to as the plain old telephone system (POTS), was originally an analog system using analog signals to transmit voice.

42. Figure 9.1 A telephone system

43. DIGITAL SUBSCRIBER LINE After traditional modems reached their peak data rate, telephone companies developed another technology, DSL, to provide higher-speed access to the Internet. Digital subscriber line (DSL) technology is one of the most promising for supporting high-speed digital communication over the existing local loops.

44. xDSL • Collection of technologies, able to transmit data at high speeds over standard twisted pair lines • ASDL ( Asymmetric Digital Subscriber Line)- different speeds in different directions (upstream and downstream) – called local loop • VDSL- (Very high rate Digital Subscriber Line)- runs over shorter distances – “fiber to neighborhood”

45. Note ADSL is an asymmetric communication technology designed for residential users; it is not suitable for businesses. The existing local loops can handle bandwidths up to 1.1 MHz.

46. Note • ADSL is an adaptive technology. • The system uses a data rate based on the condition of the local loop line. • Bit rate is the number of bits per • second. • Baud rate is the number of signal • elements per second.

47. Figure 9.11 Bandwidth division in ADSL

48. Figure 9.12 ADSL modem

49. Figure 9.13 DSLAM

50. 1.554 ─ 8.448 Mbps 16 ─ 640 Kbps Central Subscriber office premises Local loop ADSL downstream upstream ADSL connects the subscriber to the central office via the local loop.