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Cellular Wireless Networks

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Presentation on theme: "Cellular Wireless Networks"— Presentation transcript:

1 Cellular Wireless Networks

2 Example of a Cellular Wireless Network

3 1G Cellular Networks 1st generation cellular networks are purely analog cellular systems. The transmission of data is sent via a continuously variable signal

4 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

5 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

6 Cellular Operation Three basic devices A mobile station
A base transceiver A Mobile Telecommunications Switching Office (MTSO)

7 Cellular Operation Picture:

8 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

9 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.

10 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

11 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

12 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


14 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.

15 Hexagon Reuse Clusters

16 Cellular Coverage Representation

17 Frequency Reuse Each colour/letter uses the same frequency band
Picture: 8550-Cellular-14.sxi.pdf

18 3-cell reuse pattern (i=1,j=1)

19 4-cell reuse pattern (i=2,j=0)

20 7-cell reuse pattern (i=2,j=1)

21 12-cell reuse pattern (i=2,j=2)

22 19-cell reuse pattern (i=3,j=2)

23 Relationship between Q and N

24 Factors limiting frequency reuse
Co-channel interference Adjacent channel interference

25 Adjacent Channel Interference
Picture: mobi_net/Lecture1.ppt Adjacent channel interference can be controlled with transmit and receive filters

26 Coping with increasing capacity
Adding new channels Frequency borrowing frequencies are taken from adjacent cells by congested cells Picture: ch10_Cellular_wireless_netw.pdf

27 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

28 Cell Splitting

29 Site Configurations

30 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.

31 Handoffs This practice must preserve end-to-end connectivity in a dynamically reconfigured network topology.

32 Handoff Types (cont’d)

33 Avoiding handoff: Umbrella cells

34 Encoding: Modulation(1)
Amplitude Modulation Frequency Modulation Phase Modulation are the three different methods of encoding binary information on a regular wave.

35 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).

36 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

37 Encoding: Multiplexing(2)
Frequency Division Multiplexing involves a different frequency channel given to each user

38 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.

39 Differences between FDMA, TDMA, and CDMA.

40 Advantages of Code Division Multiplexing
better protection against interference good security signal difficult to jam

41 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.

42 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.

43 Wired vs. Wireless

44 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.

45 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.

46 Advantages of wireless networks
Flexible Cost is less (long term) Mobile user choice Accesses areas that wired networks cannot reach

47 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

48 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).

49 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.

50 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).

51 The GSM Network The Global System for Mobile communications.-most widely used digital cellular communications system

52 The GSM Network Picture:

53 Mobile Station Mobile equipment or terminal
3 types: fixed, portable, handheld Subscriber Identity Module (SIM).

54 The Base Station Subsystem (BSS)
The Base Station Subsystem connects the Mobile Station and the Network and Switching Subsystem. It is responsible for transmission and reception and can be divided into two parts: The Base Transceiver Station (BTS) or Base Station. The Base Station Controller (BSC).

55 The Base Transceiver Station (BTS)
The BTS corresponds to the transceivers and antennas used in each cell of the network. It is usually placed in the center of a cell. The size of a cell defines the transmitting power. A BTS can have up to 16 transceivers. It all depends on the density of users in the cell.

56 The Base Station Controller (BSC)
The BSC controls a group of BTS. A BSC is primarily responsible for handovers, frequency hopping, exchange functions and control of the frequency power levels of the BTSs.

57 The Network and Switching Subsystem (NSS)
Its main role is to manage the communications between the mobile users and other users, such as mobile users, ISDN users, landline users, etc. It also has databases that store information about the subscribers and to manage their mobility. The different components of the NSS are described below.

58 The Mobile services Switching Center (MSC)
Central component of the NSS. It performs switching functions and also provides connection to other networks.

59 The Gateway Mobile services Switching Center (GMSC)
A gateway is a node that interconnects two networks. The GMSC is the interface between the mobile cellular network and the Public Switched Telephone Network (PSTN). It is responsible for routing calls from the fixed network towards a GSM user.

60 Home Location Register (HLR)
The HLR is an important database as it stores information about the subscribers belonging to the covering area of a MSC. It stores the current location of these subscribers and the services to which they have access. The location of the subscriber corresponds to the SS7 address of the Visitor Location Register (VLR) associated to the terminal.

61 Visitor Location Register (VLR)
The VLR contains information from a subscriber's HLR. It is necessary for the provision of subscribed services to visiting users. When a subscriber enters the covering area of a new MSC, the VLR associated to this MSC will request information about the new subscriber to its corresponding HLR. The VLR will then have enough information to assure the subscribed services without needing to confirm with the HLR each time a communication is established.

62 The Authentication Center (AuC)
The AuC register provides the parameters needed for authentication and encryption functions. These parameters help verify the user's identity.

63 The Equipment Identity Register (EIR)
The EIR is also used for security purposes. It is a register containing information about the mobile equipment. It particularly contains a list of all valid terminals. A terminal is identified by its International Mobile Equipment Identity (IMEI). The EIR uses this to forbid calls from stolen or unauthorized terminals.

64 The GSM Interworking Unit (GIWU)
The GIWU corresponds to an interface to various networks for data communications. During these communications, the transmission of speech and data can be alternated.

65 The Operation and Support Subsystem (OSS)
The OSS is connected to the different components of the NSS and BSC in order to control and monitor the GSM system. It also takes care of controlling the traffic load of the BSS.

66 In GSM, there are five main functions:
Transmission. Radio Resources management (RR). Mobility Management (MM). Communication Management (CM). Operation, Administration and Maintenance

67 GSM GSM uses an intriguing method to send data. By simply sending computer data as it would send voice data, GSM allows every phone to be "data enabled." Since the GSM network is already a packet network of sorts because of its frequency hopping, it requires no additional hardware to support data. GSM allows data rates in multiples of 300 bits per second, up to 64 kilobits per second.

68 Advantages of Cellular Networks
Cellular networks use "small" cells with low powered transceivers instead of one large area with a high powered transceiver. Using cellular networks increases overall call handling capacity. Avoids central point of failure. Allows dynamic distribution of capacity based on demand. Less interference with other wireless communications

69 Conclusion (1) Mobile phone contracts provide a useful and relatively cheap service. Cellular radio network infrastructures are growing at a tremendous rate. Cellular networks are becoming high speed data networks.

70 Conclusion (2) Speeding up the development of mobile communication technologies is the focus of the industry. The ideal communication system where both voice and data services can be delivered regardless of location, network, or terminal.

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