Multimedia multimedia over wireless and mobile networks
- 1. Multimedia Over Wireless and Mobile Networks Submitted by Shahad
- 2. Outlines • Characteristics of Wireless Channels • Wireless Networking Technologies • Multimedia Over Wireless Channels
- 3. Characteristics of Wireless Channels • Wireless radio transmission channels are far more error-prone than wire-line communications are. • Various effects cause radio signal degradation in the receiver side. • They can be classified as short-range and long-range effects. • path loss models are available for long-range atmospheric attenuation channels. • fading models are available for short-range degradation.
- 4. Path Loss For long-range communication, the signal loss is dominated by atmospheric attenuation Depending on the frequency, radio waves can penetrate the ionosphere (>3 GHz) and establish line-of-sight (LOS) communication, or for lower frequencies reflect off the ionosphere and the ground, or travel along the ionosphere to the receiver. The free-space attenuation model for LOS transmission is in inverse proportion to the square of distance (𝒅 𝟐) and is given by the Friis radiation equation • Sr and St are the received and transmitted signal power • Gr and Gt are the antenna gain factors, • λ is the signal wavelength, • and L is the receiver loss. • It can be shown that if we assume ground reflection, attenuation increases to be proportional to d4
- 5. Hata model A popular medium-scale • The basic form of the path loss equation in dB is given by • A is a function of the frequency and antenna heights. • B is an environment function, and C is a function depending on the carrier frequency. • d is the distance from the transmitter to the receiver
- 6. Multipath Fading • Fading is a common phenomenon in wireless communications ( especially mobile) , in which the received signal power (suddenly) drops • Signal fading occurs due to reflection, refraction, scattering, and diffraction • Multipath fading occurs when a signal reaches the receiver via multiple paths • Because they arrive at different times and phases, the multiple instances of the signal can cancel each other, causing the loss of signal or connection. The problem becomes more severe when higher data rates are explored.
- 7. • Doppler spread distribution of the signal power over the frequency spectrum. • when the signals is narrowband, the Doppler spread of the signal is small enough, the signal is coherent • When the signal is wideband, different frequencies of the signal have different fading paths • models For narrowband signals are: 1- Rayleigh fading and 2- Rician fading. 1- Rayleigh fading model assumes an infinite number of signal paths with non line-of-sight (NLOS) to the receiver for modeling the probability density function Pr of received signal amplitude r: • σ is the standard deviation of the probability density function • Rayleigh model does provide a good approximation when the number of paths is over 5.
- 8. Gilbert-Elliott model • A Rayleigh fading channel can be approximated using a Markov process with a finite number of states, referred to as a Finite State Markov Channel • with only two states, representing the good and the bad channel conditions. • As illustrated in Fig. 17.2, state 0 has no error and state 1 is erroneous, and the wireless channel condition switches between them with transition probabilities P00 to P11 • widely used in simulations
- 9. 2-Rician mode • A Raleigh channel assumes that all the paths arrive with random amplitude. • A Rician channel assumes that there is a line of sight component that has much larger amplitude. The Rician probability density function is • r and σ are the signal amplitude and standard deviation, respectively, • and s is the LOS signal power. Io is a modified Bessel function of the first kind with0 order. • Note that when s = 0 (K = 0) ,there is no LOS. • wideband signal • the fading paths are more empirically driven . • One way is to model the amplitude as a summation over all the paths, each having randomized fading. • An alternative technique of modeling the channel fading is by measuring the channel impulse response.
- 10. Wireless Networking Technologies • In a wide-area cellular network, a field is covered by a number of cells. • Each mobile terminal in a cell contacts its Access Point (AP) or Base Station (BS), which serves as a gateway to the network. • The AP themselves are connected through high-speed wired lines, or wireless networks or satellites that form the backbone network. • When a mobile user moves out of the range of the current AP, a handoff (or handover, as it is called in Europe) is required to maintain the communication . • The size of a cell is typically of 1,000m in cities, but can be larger (macrocell) or smaller (microcell) depending on the location and the density of users. • A Wireless Local Area Network (WLAN) covers a much shorter range, generally within 100 m. Given the short distances, the bandwidth can be very high while the access cost and power consumption can be low, making them ideal for use within a house or an office building
- 11. 1G Cellular Analog Wireless Networks • used mostly for voice communications, such as telephone and voice mail. • used an analog technology with Frequency Division Multiple Access (FDMA) . • each user is assigned a separate frequency channel during the communication. • typical data rate was 9,600 bps. • Digital data transmission users needed modems to access the network.
- 12. • Figure 17.4 illustrates a sample geometric layout for an FDMA cellular system. A cluster of seven hexagon cells can be defined for the covered cellular area. As long as each cell in a cluster is assigned a unique set of frequency channels, interference from neighboring cells will be negligible. • The same set of frequency channels (denoted f1 to f7 in Fig. 17.4) will be reused once in each cluster,
- 13. 2G Cellular Networks : GSM and Narrowband CDMA • mainly designed for voice communications with circuit switching and had very limited support for Internet data access. • transmitted for applications such as text messaging, streaming audio, and electronic publishing. • Used digital technologies . • The digital cellular networks adopted two competing technologies Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA). 1- TDMA and GSM • The Global System for Mobile communications (GSM) which was based on TDMA, is the most widely used worldwide. • TDMA creates multiple channels in multiple time slots while allowing them to share the same carrier frequency, TDMA is generally combined with FDMA. • GSM was established with the objective of creating a standard for a mobile communication network capable of handling millions of subscribers and providing roaming services throughout
- 14. • GSM provides a variety of data services, through sending and receiving data to users. and packet-switched or circuit-switched public data networks • GSM supports Short Message Service (SMS). • GSM adoption of the subscriber identity module (SIM), a smart card that carries the mobile user’s personal number and enables ubiquitous access to cellular services • GSM network is circuit switched, data rate is limited to9.6 kbps which is hardly useful for general data services as multimedia data. • General Packet Radio Service (GPRS), developed supports packet- switched data over GSM wireless connections, so users are “always connected. • Preliminary multimedia content exchange was supported by GPRS through the Multimedia Messaging Service (MMS) • To send a multimedia content, the sending device first encodes it with an MMS Message Encapsulation Specification. The encoded message is forwarded to the carrier’s MMS store and to the forward server, known as the Multimedia Messaging Service Centre (MMSC).
- 15. 2-Code Division Multiple Access • CDMA is a major breakthrough in wireless communications. • It is a spread spectrum technology, in which the bandwidth of a signal is spread before transmission. • the spread signal might be indistinguishable from background noise, and so it has distinct advantages of being secure and robust against intentional interference (known as jamming). • foundation of CDMA is Direct Sequence (DS) spread spectrum. • frequency band can also be allocated to multiple users in all cells This has the potential to greatly increase the maximum number of users
- 16. • Fig. 17.6 shows, for each CDMA transmitter a unique spreading code is assigned to a DS spreader. • The spreading code is multiplied with the input data by the DS spreader .When the data bit is 1, the output DS code is identical to the spreading code, and when the data bit is 0 (represented by −1), the output DS code is the inverted spreading code. • The despreading process involves taking the product of the DS code and the spreading sequence. • to support more users and achieve better spectrum utilization, non orthogonal Pseudo-random Noise (PN) sequences can be used as codes. This is based on the observation that in general not all users are active in a cell.
- 17. 3G Cellular Networks : Wideband CDMA • multimedia services have become the core issues for the cellular network development. Applications include continuous media on demand, mobile interactive video call, remote medical service. • GPRS is considered as the first major step in the evolution of GSM networks toward 3G, which started to support the Multimedia Messaging Service (MMS). • GPRS networks evolved to the Enhanced Data rates for GSM Evolution . • The 3G standardization process started in 1998, when the ITU called for Radio Transmission Technology (RTT) proposals for International Mobile Telecommunication-2000 (IMT-2000). Since then, the project has been known as 3G or Universal Mobile Telecommunications System (UMTS).
- 18. • 3G wireless networks use Wideband CDMA (WCDMA) • The key differences in WCDMA air interface from a narrow band CDMA air interface are 1- To support bitrates up to 2 Mbps. 2- To effectively use the 5MHz bandwidth, longer spreading codes at higher chip rates are used. 3- WCDMA supports variable bitrates, from 8 kbps up to 2 Mbps. • The bandwidth made available by 3G networks gives rise to applications not previously available to mobile phone users. • Examples include online maps, online gaming, mobile TV, and instant picture/video content sharing. • The multimedia nature of these 3G wireless services also calls for a rapid development of new generations of handsets, where support for high-quality video, better software and user interface, and longer battery life are key factors.
- 19. 4G Cellular Networks and Beyond • Several new radio techniques are employed to achieve higher rates and lower latencies than 3G. • They include Space Division Multiplexing via Multiple Input/Multiple Output (MIMO), Space Time Coding (STC) using higher order of modulation and encoding schemes, sophisticated beam forming and beam directionality control, and intercell interference mitigation. • MIMO and beam forming are advanced antenna technologies. • Using multiple sending and receiving antennas, MIMO creates multiple channels to carry user information, leading to higher capacity and less impact from interference.
- 20. • The beam-forming techniques temporarily improve gain and offer higher capacity. • Space Time Coding STC improves the number of bits transmitted per Hz over the available bandwidth. • Orthogonal Frequency Division Multiplexing (OFDM) is a techniques that reduce interference are also used to further boost the capacity . • OFDM can be a better transport mechanism for multimedia data.
- 21. Wireless Local Area Networks • Wireless Local Area Networks (WLANs) provide much higher throughput with much lower costs than the wide area cellular wireless networks. • IEEE 802.11 • working group. specify Medium Access Control (MAC) and Physical (PHY) layers for wireless connectivity in a local area. • addressing the following important issues: 1- Security. Enhanced authentication and encryption. 2- Power management. Saves power during no transmission. 3- Roaming. Permits acceptance of the basic message format by different AP. • The initial 802.11 standard uses the 2.4GHz radio band.
- 22. • The basic channel access method of 802.11 is Carrier Sense Multiple Access (CSMA). However, Collision Detection (CD) in Ethernet is not employed. This is because of the unique Hidden Terminal problem in wireless communications • To address the hidden terminal problem, 802.11 uses Collision Avoidance (CA); that is, during carrier sensing, if another node’s transmission is heard, the current node should wait for a period of time for transmission to finish before listening again for a free communications channel. • CSMA/CA can optionally be supplemented by the exchange of a Request to Send RTS packet sent by the sender, and a Clear to Send CTS packet sent by the intended receiver
- 24. Bluetooth and Short-Range Technologies • Bluetooth: is a protocol intended for such short-range (called piconet) wireless communications. • Bluetooth uses Frequency Hopping (FH), a spread spectrum technology for data transmission in the 2.4GHz ISM short-range radio frequency band • Bluetooth also employs a master-slave structure. One master may communicate with up to seven slaves in a piconet • Packet exchange is based on a basic clock • Bluetooth provides a secure and low-cost way to connect and exchange information between devices such as faxes, mobile phones • it permits moving or still pictures to be sent from a digital camera or mobile phone, at a speed of over 700 kbps, within a distance of 10m. • Many other short-range wireless communication protocols have also been developed like Near Field Communication (NFC) and Wi-Fi Direct
- 25. Multimedia Over Wireless Channels • The main driving force toward the new generation of higher speed wireless networks are from multimedia communications over wireless. Suggested multimedia applications range from streaming video, videoconferencing, online gaming, collaborative work, and slide show presentations. • The characteristics of wireless handheld devices First, both the handheld size and battery life limit the processing power and memory of the device. Thus encoding and decoding must have relatively low complexity. the smaller screen sizes well accept relatively lower resolution videos, which helps reduce the processing time. Second, due to memory constraints and reasons for the use of wireless devices, as well as billing procedures, real-time communication is likely to be required.
- 26. • wireless channels have much more interference than wired channels • bitrate for wireless channels is also much more limited. • The 3GPP/3GPP2 group has defined the following QoS parameters for wireless videoconferencing services 1- Synchronization. Video and audio should be synchronized to within 20 ms. 2- Throughput. The minimum video bitrate to be supported is 32 kbps. 3- Delay. The maximum end-to-end transmission delay is defined to be 400 ms. 4- Jitter. The maximum delay jitter (maximum difference between the average delay and the 95th percentile of the delay distribution) is 200 ms. 5- Error rate. A frame error rate of 10−2 or a bit error rate of 10−3 should be tolerated.
- 27. We will introduce solutions for error detection, error correction, error-resilient entropy coding, and error concealment in the wireless network context. 1- Error Detection • Error detection is to identify errors caused by noise or other impairments during transmission from the sender to the receiver. • error detection tools include: 1-parity checking 2- checksum 3- Cyclic Redundancy Check (CRC) A-Parity Checking • With binary data, errors appear as bit flips. • Parity checking adds a parity bit to a source bit string to ensure that the number of set bits (i.e., bits with value 1) in the outcome is even (called even parity) or odd (called odd parity). • For example, • With even parity checking, a bit 1 should be appended to bit string 10101000, and a bit 0should be appended to 10101100.
- 28. B-Checksum • A checksum of an input message is a modular arithmetic sum of all the code words in the message. The sender can append the checksum to the message, and the receiver can perform the same sum operation to check whether there is any error. • The Internet checksum algorithm in these protocols works as follows 1. First pair the bytes of the input data to form 16-bit integers. If there is an odd number of bytes, then append a byte of zero in the end. 2. Calculate the 1’s complement sum of these 16-bit integers. Any overflow encountered during the sum will be wrapped around to the lowest bit. 3. The result serves as the checksum field, which is then appended to the 16-bit integers. 4. On the receiver’s end, the 1’s complement sum is computed over the received 16-bit integers, including the checksum field. Only if all the bits are 1 will the received data be correct.
- 29. example • suppose we have the following input data of 4 bytes: 10111011, 10110101, 10001111, and 00001100. They will be grouped as 1011101110110101 • and 1000111100001100. • The sum of these two 16 bit integers is 1011101110110101 +1000111100001100 0100101011000010 • 1s complement of the above sum becomes 1011010100111101, which becomes the checksum. • The receiver will perform the same grouping and summation for the received bytes, and then add the received checksum too • if there is no error, then the outcome should be 1111111111111111. Otherwise, if any bit becomes 0, then errors happen during transmission.
- 30. C-Cyclic Redundancy Check • The basic idea behind Cyclic Redundancy Check (CRC) is to divide a binary input by a keyword K that is known to both the sender and the receiver. • The remainder R after the division constitutes the check word for the input. • The sender sends both the input data and the check word, and the receiver can then check the data by repeating the calculation and verifying whether the remainder is still R.