Example of a Cellular Wireless Network Picture: http://www.scom.hud.ac.uk
1G Cellular Networks • 1st generation cellular networks are purely analog cellular systems. • The transmission of data is sent via a continuously variable signal
2G Cellular Networks • 2nd generation cellular networks refer to digital cellular and PC wireless systems. • voice and low speed data services. • They consist of digital traffic channels, perform encryption, error detection & correction • Users share channels dynamically
3G Cellular Networks • 3rd generation refers to the next generation of wireless systems. • This is digital with high speed data transfer • It is voice quality comparable with a switched telephone network. • Data transmission rates can be asymmetric or symmetrical • It provides support for circuit switched and packet switched data services
Cellular Operation • Three basic devices • A mobile station • A base transceiver • A Mobile Telecommunications Switching Office (MTSO)
Cellular Operation Picture: www.Xanthippi.ceid.upatras.gr
Cellular Network Organization • Base Station (BS) • includes an antenna, a controller, and a number of receivers • Mobile Telecommunications Switching Office (MTSO) • connects calls between mobile units Two types of channels available between mobile unit and BS • Control channels • used to exchange information having to do with setting up and maintaining calls (out-band or in-band through stealing bits) • Traffic channels • carry voice or data connection between users
Cellular Operation Public Land Mobile Network (PLMN) refers to a cellular network that has land and radio based sections. This network consists of: • Mobile station (MS): A device used for communication over the network. • Base station transceiver (BST): A transmitter/receiver used to transmit/receive signals over the network.
Cellular Operation Mobile switching center (MSC): Sets up and maintains calls made over the network. Base station controller (BSC): Communication between a group of BSTs and a single MSC is controlled by the BSC Public switched telephone network (PSTN): Section of the network that is land based
Cellular Operation • Outgoing from mobile • input phone number and press send • mobile links to base transceiver via control channel • base to MTSO to PSTN • MTSO routes connection back to mobile via voice channel • mobile shifts from control to voice
Cellular Operation • Incoming to mobile • call goes from PSTN to MTSO • on control channel, MTSO searches for mobile by PAGING every active mobile • If found, MTSO rings it and establishes voice channel connection • uses transceiver with strongest signal from mobile
Cellular Network Organization (Cells) • Cells use low powered transmitters. • Each cell is allocated a band of frequencies, and is served by its own antenna as well as a base station consisting of a transmitter, receiver and control unit.
Frequency Reuse • Each colour/letter uses the same frequency band Picture: netlab.cis.temple.edu/~jmulik/teaching/8550s03-slides/ 8550-Cellular-14.sxi.pdf
Factors limiting frequency reuse • Co-channel interference • Adjacent channel interference
Adjacent Channel Interference Adjacent channel interference can be controlled with transmit and receive filters Picture: xanthippi.ceid.upatras.gr/courses/ mobi_net/Lecture1.ppt
Coping with increasing capacity • Adding new channels • Frequency borrowing • frequencies are taken from adjacent cells by congested cells Picture: www.its.bth.se/courses/etc019/handouts/ ch10_Cellular_wireless_netw.pdf
Coping with increasing capacity • Cell splitting • cells in areas of high usage can be split into smaller cells • Cell sectoring • cells are divided into a number of wedge-shaped sectors, each with their own set of channels • Microcells • antennas move to buildings, hills, and lamp posts
Handoffs • Network protocols must refresh and renew paths as a mobile station host moves between cells. • Handoffs are the function of one cell handing over the communication link between itself and a mobile station as the mobile station moves out of the boundary of its region into the boundary of an adjacent cell.
Handoffs • This practice must preserve end-to-end connectivity in a dynamically reconfigured network topology.
Encoding: Modulation(1) Amplitude Modulation Frequency Modulation Phase Modulation are the three different methods of encoding binary information on a regular wave.
Encoding: Modulation(2) When using digital signals the methods are known as Amplitude Shift Keying (ASK), Frequency Shift Keying (FSK), and Phase Shift Keying (PSK).
Encoding: Multiplexing(1) Multiplexing allows many mobile users to use cellular radio transmission schemes at the same time. The different schemes are: • Frequency Division Multiplexing • Time Division Multiplexing • Code Division Multiplexing
Encoding: Multiplexing(2) Frequency Division Multiplexing involves a different frequency channel given to each user
Encoding: Multiplexing(3) Time Division Multiplexing involves a channel with a given number of time slots (per millisecond) where each user is assigned certain time interval. Code Division Multiplexing gives each user a “code” for differentiation purposes. The receiver picks out each channel from the “noise” using the code. Wide frequency band is used. Does not contain single frequencies or time slots.
Advantages of Code Division Multiplexing • better protection against interference • good security • signal difficult to jam
Disadvantages of Code Division Multiplexing • pseudo-random code sequences generated by the transmitters and receivers are not always random • fast power control system needed so that strong signals don’t overpower weaker signals.
Analogy:Multiplexing Lectures at a learning institute: • Frequency Division: takes place in different rooms • Time Division: taking turns in a single room • Code Division: lectures on different subjects.
Wired vs. Wireless (1) A problem with wireless networks is that anyone with a wireless network card is able to access this network and is potentially harmful since they are able to corrupt and steal important files. These networks transmit data over an area such that the network signals may penetrate physical areas such as walls.
Wired vs. Wireless (1) Although this problem is relevant to a wired network also, it exists to a greater degree in a wireless network. With regard to wired networks, the electromagnetic waves that are given off from the current traveling through the network cables.
Advantages of wireless networks • Flexible • Cost is less (long term) • Mobile user choice • Accesses areas that wired networks cannot reach
Disadvantages of wireless networks • Compared to wired networks the data rates are slower • User location determines performance • Devices such as microwaves, cordless phones, etc may cause interference • Can be accessed by hackers from the outside
Wired Equivalent Privacy Algorithm (WEP) (1) Wireless networks may include additional security elements, which are not supported in wired networks. For eg. The use of security algorithms such as WEP (Wired Equivalent Privacy Algorithm), that uses an encryption algorithm which deals with unauthorized access to the network (eavesdropping).
WEP (2) WEP is a implemented such that a block of plaintext (input text) is bitwise XORed with an equal length random key sequence. A random number generator is used on the initialization vector and the secret key and outputs a key sequence of random of octets. An integrity check value is produced to protect against data modification. The key sequence combined with plaintext combined with the integrity check value gives the enciphered message. The integrity check value and the ciphertext is the combination of the output.
Block Ciphers Another security algorithm is Block Ciphers which is the most common of the encryption techniques. The Block Cipher consists of: Data Encryption Standard (DES) Triple Data Encryption (TDEA) Advanced Encryption Standard (AES).