Introduction to Networks and the Internet CMPE 80N Announcements First quiz on Friday, 01.16. Covers material up to and including 01.14. Closed books, notes, etc. Winter 2004 Lecture 5 1 2 Data Transmission Modems Analog and digital transmission. Historically, communication infrastructure for analog transmission. Digital data needed to be converted: modems (modulator-demodulator). MODEM = Modulator/Demodulator Converts digital to analog before transmitting over analog channel (e.g., telephone networks). To transmit data: DAC (digital-to-analog converter) To receive data: ADC (analog-to-digital converter) 2-way communication: needs two modems. Each modem contains circuitry to encode outgoing data and decode incoming data. 3 4
Types of Communication Modems (cont d) Half-duplex communication: Modems contain complex circuitry to: Only one party can talk at a time. E.g., walkie-talkie. Full-duplex communication: Both parties can talk at the same time. E.g., telephone Modems use full-duplex communication. Modulate/demodulate the analog signal. Allows for the transmission of moderately high bit-rate over the telephone line. Compress the data. Reduces the amount of bits to be transmitted Detect bit errors due to transmission. Achievable bit rates. E.g., 14.4-56 Kb/s 5 6 Input/Output Connections I/O Connections How to connect peripheral devices to a computer. Serial and parallel connections. Serial: bits are sent one at a time. Sequence of bits sent in parallel using parallel wires. 7 8
I/O Connections: Standards USB and FireWire Standard: defines the details of a particular technology. RS-232 is a standard for serial communication between digital devices. It s full duplex. 20-30 Kb/s. Can only connect one device at a time. So if you want to connect a PC to many devices, you need as many cables coming out of your PC. USB (Universal Serial Bus). Can connect many devices through a USB hub. Bitrates: 12 Mb/s (USB 1.1) to 480 Mb/s (USB 2.0). Provides power to small devices (e.g., mouse). Firewire (IEEE 1394). Can connect many devices through a FireWire hub. Bitrates: up to 400 Mb/s. Very popular for video cameras and storage systems, also to connect two devices (without a PC). Can provide power to small devices (e.g., video cameras). 9 10 Some Considerations Solution: Broadband Coonections RS-232, USB, Firewire, all have constraints on the maximum length of the wire. We already know a solution: modem. Uses the telephone network. However, modems provide insufficient bitrate. Also, when using the modem, you cannot use the telephone for voice communication! 11 12
Broadband Connection Types Integrated Services Digital Network (ISDN). Asymmetric Digital Subscriber Line (ADSL). Cable Modem. Wireless. Satellite Links. ISDN ISDN provides for communication of digitized voice and data to subscribers over the conventional local loop (i.e., using the same wiring as for analog telephone). In the Basic Rate Interface (BRI), ISDN offers three separate digital channels (2B+D). All on the same wire! (Multiplexing) Primary Rate Interface (23B+D). Requires higher capacity lines than local loop! 13 14 ISDN (cont d) ISDN (cont d) The two B channels are intended to carry digital voice, data, or digital streams To connect computer to ISDN, user needs a special network termination device (NT1). Bitrate of each B-channel: 64 kb/s (overall, 128 kb/s) The D channel is used as a control channel E.g., to request services which are then supplied over the B channels, to carry caller ID information, etc. Bitrate of D channel: 16 kb/s. NT1 device a.k.a. ISDN modem. A modem converts a digital signal to an analog signal; ISDN is inherently digital, so no such conversion is necessary. Need to dial a number to start a connection 15 16
ISDN (cont d) ADSL ISDN was initiated in 1984, and was available in the USA in the early 90s. It was an attempt to replace the analog phone system with a digital voice+data system. It never really succeeded Currently, ISDN is obsolete, because it offers limited bitrate at a fairly high price. Still a possibility for Internet connection where other forms of broadband are not available. ADSL allows transmission of high bit-rates over local loop. It does not require any changes in the wiring. In addition, it does not preempt the local loop. A user can use the telephone for analog voice communication and at the same time transmit data or stream video. It requires a splitter and a ADSL modem The splitter separates voice/fax signals from data stream. PC to ADSL modem: typically USB. 17 18 ADSL scheme ADSL (cont d) To achieve high bitrate transmission, ADSL must use sophisticated technology It is adaptive : ADSL modems at the two ends probe the line between them to find its characteristics, and then agree to communicate using techniques that are optimal for that line. Depending on the characteristics of the wiring, different bit-rates can be achieved If a house is too far form the End office (switching center), ADSL is not available. 19 20
ADSL (cont d) ADSL is asymmetric: it provides a higher bitrate downstream than upstream. Downstream: 32 kb/s to 6.4 Mb/s (more typically, 1.5 Mb/s) Upstream: 32 to 640 kb/s (more typically, 256 kb/s) Asymmetry is OK when high bitrate data is transmitted to the user. E.g.: Video-On-Demand, Internet radio In some cases, symmetric communication is preferable. E.g.: Videoconferences Cable Modems CATV (Community Antenna TV, or cable TV) uses coax cable (less susceptible to interference) 1-Km coax cable can accommodate bitrates of 1-2 Gb/s! Only one cable is used for a neighborhood Different TV channels are multiplexed on it. Cable systems are designed to carry many more television signals than currently available. There is unused capacity that can be used for data communication! >80% of US homes are already reached by CATV 21 22 Cable Modems (cont d) Cable Modems (cont d) User can connect using a cable modem A splitter separates the TV and the data signals. Problem: all users in the neighborhood share the same available capacity in the same cable! If all users in the neighborhood transmit data at the same time, the available bitrate is reduced. E.g., if there are 50 Mb/s available, and 100 users in the neighborhood use it simultaneously, each user has only 0.5 Mb/s Coax cables from several neighborhoods connect to a concentrator The concentrator uses high capacity fiber optics cables to connect to the head end, which is connected to the Internet. Communication is asymmetric Originally, CATV was designed only for downstream communication! Available bitrates: 23 Downstream: 1.5 to 2 Mb/s Upstream: 128 kb/s 24
Satellite Systems Satellite Systems (cont d) Digital communication satellites were deployed by telecommunication companies as an alternative to terrestrial lines. They can now be used as local loop technology (e.g., DirectPC). Advantages: Can reach arbitrary geographic locations. Does not require wiring. Has high bandwidth. Perfect for broadcasting (can reach many users at once). 25 Disadvantages: It s a shared medium (the bitrate depends on the number of simultaneous users). Delay (latency) can be relatively high (<1s) Not ideal for playing interactive app s (e.g., games). You have to put a dish on your roof! Initially, uplink was not provided. Needed to use a separate phone line to uplink information. Nowadays it is a two-way system. 26 Physical Layer: Summary Different types of signal: Analog and digital. Analog communication infrastructure: Need to convert digital to analog before transmitting: ADC. DAC before entering computer. Digitization: Sampling. Sampling period and frequency (samples/sec or Hertz). Sample representation (quantization). Bit rate. Physical Layer: Summary (Cont d) Modems. Input/output connections. RS 232. USB. Firewire. Broadband. ISDN. ADSL. Cable modem. Satellite. 27 28
Other Wireless Networks Cellular Networks Cellular phones:voice. Cellular networks: shift from voice to data. New wireless devices: pagers, PDAs. New services: Web access, e-mail, instant messaging, etc. 29 30 Cellular Concept: Motivation Some Cellular Terminology Early mobile radio systems: Mobile. Large coverage with single, high-powered transmitter. But, no frequency re-use due to interference. Since finite spectrum allocation, need: high capacity (number of users) with limited spectrum and wide coverage. Base station. Mobile Switching Center (MSC). Handoff. Cell. 31 32
Cellular Architecture Cellular Fundamentals mobile System-level idea, no major technological changes. Many low-power transmitters instead of single, high power on (large cell). Service area divided into small cells covered by each low power transmitter. Each transmitter (or base station) allocated a portion of the spectrum. Nearby BSs assigned different channel group to minimize interference. Scalability: as more users subscribe, more BSs can be added using lower transmission power). 33 34 Frequency Reuse Handoff/Handover Mobile hosts can change cells while communicating. Hand-off occurs when a mobile host starts communicating via a new base station. Handoff decision made based on signal strength. 35 36
Cellular Networks: Evolution Second Generation (2G) Evidence of the wireless success! Since 1996, number of new mobile phone subscribers exceeded number of new fixed phone subscribers! 1 st. Generation (1G): analog technology. FDMA. Analog FM. Most of today s cellular networks use 2G standards. Early 90s. Digital technology. Lighter, smaller devices with longer battery life. Better reception and channel utilization. 37 38 3G Wireless Networks Multi-megabit Internet access, VoIP, ubiquitous always-on access. Wireless Local Area Networks Local area network connectivity using wireless communication. Single mobile device for everything (integrated service approach). New, world-wide standard. International Mobile Telephone 2000 (IMT 2000) IEEE 802.11 WLAN standard. Example: WaveLan, Aironet Wireless LAN may be used for Last hop to a wireless host. Wireless connectivity between hosts on the LAN. 39 40
Other WLAN Standards MANETs HomeRF Mobile, (wireless), multi-hop ad-hoc networks. Proponents of 802.11 frequency hopingspread spectrum (FH-SS). HomeRF 2.0 10 Mbps FH-SS. Formed by wireless hosts which may be mobile. Without (necessarily) using a pre-existing infrastructure. HIPERLAN Europe, mid 1990s. Similar capability to IEEE 802.11b. Routes between nodes may potentially contain multiple hops. Mobilitty cause routes to change. 41 42 Multi-hop Why MANETs? May need to traverse multiple hops to reach destination. Ease of deployment. Speed of deployment. Decreased dependence on infrastructure. 43 44
Many Applications Personal area networking. Cell phone, laptop, ear phone, wrist watch. Military environments. Soldiers, tanks, planes. Civilian environments. Smart environments. Emergency operations Search-and-rescue Policing and fire fighting Monitoring and surveillance. 45