Mobile Communication Systems

List at least five versions of the IEEE 802.11 standards, with their properties briefly.

802.11-1997 (Legacy): 2.4 GHz ISM band, 1 or 2 Mbps bitrate. 802.11a-1999: 5 GHz ISM band, OFDM waveform, max 54 Mbps. 802.11b-1999: Popular extension of 802.11, 11 Mbps. 802.11g-2003: 2.4 GHz ISM band, 54 Mbps, similar to 802.11a. 802.11n-2009: 5 and 2.4 GHz ISM band, OFDM, MIMO, up to 600 Mbps. 802.11ac-2013: Advanced 802.11n, 5 GHz, OFDM, MIMO, 256QAM, over 6 Gbps

What is a handover? Summarize location management in GSM networks, including LAC.

A handover is the transfer of an ongoing call or data session from one cell to another without disconnection. Location management includes tracking mobile stations with a Location Area Code (LAC) and handling call routing and registration

What is a training sequence? What does timing advance means, how does it work?

A training sequence is a standard sequence used for bit-level synchronization of the receiver. Timing advance is a correction applied to ensure the mobile station's transmission is synchronized with the base station's signal, accounting for the distance and thus the signal's travel time

Describe a typical communication network hierarchy (access, aggregation and core)?

Access Network: Known as the 'last mile', this segment connects subscribers to the network. It can include various types such as DSL, ADSL, WiMAX, CATV, mobile 3G, and passive optical networks. Examples of access network locations are residential areas, office buildings, and streets. Aggregation Network: Typically uses IP/GigabitEthernet/optical technology or lower bitrate SDH. It serves areas like districts or towns. Core Network: Forms the backbone of the network, typically employing MPLS/IP, 10GigabitEthernet, or optical technologies. Core networks are regional or country-wide, with nodes in major cities

Describe Ethernet protocol (addressing, switching and speeds)!

Addressing: Uses MAC addresses for interfaces, with each MAC address being 6 bytes long, indicating source and destination addresses within the Ethernet frame. Switching: Ethernet switches route frames based on the destination MAC address, learning addresses as frames are received, without considering anything beyond the Ethernet header. Speeds: Ethernet speeds range from 10 Mbps to 100 Gbps. Terms like "1000BaseFX" indicate the speed (1000 Mbps), signaling type (baseband), and cable type (F for fiber) no provisioning for QoS or bandwidth allocation

Describe the basic properties and structure of the radio interface in LTE. Describe the PRB, derive the available physical bitrates in LTE.

Basic Properties of LTE Radio Interface: OFDM (Orthogonal Frequency Division Multiplexing): Underpins LTE's physical layer. Downlink: Uses OFDMA, suitable for high data rates. Uplink: Employs Single Carrier-FDMA, optimized for low Peak-to-Average Power Ratio (PAPR). Support: Compatible with various bandwidths, FDD, and TDD. Adaptive Modulation and Coding: Includes QPSK, 16QAM, and 64QAM, adapting to channel conditions. Additional Features: Incorporates HARQ and MIMO for enhanced data transmission reliability and capacity. ---------------------- Structure of LTE Radio Interface: Subcarriers: Each 15 kHz wide, enabling fine-grained frequency division. Frames and Subframes: A frame spans 10 ms, divided into 10 subframes (1 ms each), further split into 2 timeslots (0.5 ms each). ------------------------ PRB Description: Basic Unit: 12 subcarriers over one slot (0.5 ms), 180 kHz wide. Contains: 84 modulation symbols per slot. ---------------------- Bitrates per PRB: QPSK: 84 symbols * 2 bits/symbol = 168 bits/slot (336 bits/ms). 16QAM: 84 symbols * 4 bits/symbol = 336 bits/slot (672 bits/ms). 64QAM: 84 symbols * 6 bits/symbol = 504 bits/slot (1008 bits/ms). Calculating Bitrates: Multiply the number of bits per ms by the number of PRBs and the frame duration (10 ms) for total bitrate.

Describe the elements, functionalities and notions of WiFi networks.

Basic Service Set (BSS): One cell. Independent BSS (IBSS) for ad-hoc mode without network connection. Station (STA): Wireless station using 802.11 protocol. Access Point (AP): Enables STA to access a network. Distribution System (DS): Network connecting APs. Extended Service Set (ESS): Appears as a single 802.11 network combining BSS, AP, and DS

Sketch the basic structure of mobile networks. Describe the principle of frequency reuse: presentation of the basic idea through an example. Outline why mobile networks are based on radio cells

Basic Structure of Mobile Networks: Cells: Geographical areas each covered by a base station's transceiver. Transport Network: Composed of routers, switches, and central elements specific to mobile networks. Gateway: Connects to the Public Switched Telephone Network (PSTN). Principle of Frequency Reuse: Limited frequency channels used in each cell. Same frequencies reused in distant cells to minimize interference, increasing capacity. Example: Hexagonal cells, each using frequencies f1,f2,f3 supporting n users, thus serving more than 4n customers over a larger area. Why Based on Radio Cells: To manage signal propagation loss over distances. To adhere to protocol limitations on cell size. To enhance network capacity through efficient frequency reuse. To manage and minimize interference for better network performance

Describe the main concept of CDMA. Show the basic principle of CDMA using an example. Why is it working, what is code orthogonality?

CDMA (Code Division Multiple Access) transmits a sequence of codes at a rate faster than the bitrate, resulting in a wider signal spectrum, known as spread spectrum technology. Users share the same frequency band simultaneously, with unique codes distinguishing each user's information. The channel carries the sum of all users' codes, and the receiver can extract individual user information from this sum by knowing the user's unique code. CDMA works due to code orthogonality, where the codes are orthogonal to each other, meaning they do not interfere with one another when properly aligned

Describe the basic challenges of mobile communications and solution areas (mobility, radio channel). Outline the properties of radio channels.

Challenges: Radio channel as a shared medium leads to interference, security risks, and physical disturbances. Solutions: Use of MAC procedures, policies, and technologies to mitigate risks. Radio Channel Properties: Prone to thermal noise, attenuation, multipath propagation, fading, and frequency-selective fading.

Describe the basic properties of circuit switched and packet switched communications.

Circuit Switched: Establishes a dedicated circuit for each communication session. Involves a signaling phase to set up connections and allocate network resources. Ensures a continuous, stable connection for the duration of the session. Packet Switched: Divides data into packets, each processed independently by network nodes. No fixed connection; packets contain all necessary forwarding information. Offers flexibility and efficiency in data handling, allowing dynamic routing

What is the DNS Domain Name System?

DNS, the Domain Name System, translates human-readable domain names like into IP addresses that networks use to deliver information. It's a hierarchical and distributed directory of names with domains within domains, managed by authorities with name servers in each domain. The system is overseen by the Internet Engineering Task Force (IETF), with root name servers at the top of the hierarchy

Outline what kind of frames are used and for what purposes in WiFi.

Data Frame: Carries actual data. Control Frame: Controls functions for processes. Management Frame: Similar to data frame but carries network process management information

Digital transmission: binary data, basic element, frames, parts of a frame.

Digital Transmission: Converts information (like pictures, text, voice) into binary data for transmission over the network. Binary Data: A series of zeros and ones. Basic Element: Bit (eight bits form a byte or octet). Frames/Packets: Data is organized into frames or packets with headers (containing instructions for network elements) and bodies (containing the actual data)

Describe the operation of the WiFi MAC protocol's DCF function.

Distributed Coordination Function (DCF): Uses carrier sense mechanism for STAs to access shared radio channel without central coordination. Point Coordination Function (PCF): AP controls which terminal transmits, based on polling. Contention Period (CP) and Contention-free Period (CFP): CPs (DCF) and CFPs (PCF) alternate in the network

What kind of databases can be found in the GSM system and what are the roles of these?

GSM databases include the Home Location Register (HLR) for permanent subscriber information, the Visitor Location Register (VLR) for temporary data, the Authentication Center (AuC) for security, and the Equipment Identity Register (EIR) for tracking devices

What are the subsystems of the GSM network? What kind of functional entities (equipments) are present in the GSM system and what functions do these provide?

GSM network consists of four subsystems: Mobile Station (MS), Base Station Subsystem (BSS), Network Switching Subsystem (NSS), and Operation SubSystem (OSS). Functional entities include the SIM card and Mobile Equipment (ME) in the MS, devices allowing mobiles to reach the network and control the radio network in BSS, and devices for switching and maintaining voice connections in NSS. OSS supervises and monitors the network

Briefly describe High Speed Circuit Switched Data (HSCSD)!

HSCSD is an extension of GSM for higher bitrates, allowing up to 4 timeslots in parallel for a single data connection

Describe the features and solutions of HSDPA and HSUPA.

HSDPA (High Speed Downlink Packet Access) increases bitrate and reduces latency, uses 2 ms frames, and adaptive modulation and coding based on channel quality. HSUPA enhances uplink speeds, likely employing similar adaptive techniques as HSDPA, focused on uplink efficiency

Describe the hidden terminal problem and how to mitigate it. Describe the basic ideas of virtual carrier sense.

Hidden Terminal Problem: Occurs when two stations (e.g., A and C) simultaneously transmit to a common receiver (B), but cannot detect each other's transmission. This leads to collisions at B. Mitigation: RTS/CTS Mechanism: Before transmitting, a station sends a Request to Send (RTS) with the recipient address and transmission duration. The recipient (B) responds with Clear to Send (CTS), also indicating the duration. Stations hearing CTS (including C) set their Network Allocation Value (NAV) and refrain from transmitting for the specified duration. Helps prevent collisions as it informs all nearby stations about the ongoing transmission. ---------------- Virtual Carrier Sense: Uses MAC frame headers to indicate transmission duration. Stations set their NAV based on this duration, avoiding channel access until it ends. Particularly useful in situations where direct carrier sensing is insufficient due to distance or obstacles.

Describe the ICMP (with an example)!

ICMP: Internet Control Message Protocol is used for sending control messages at the IP layer within the payload of IP packets. Functions: Signaling of errors Messages for discovering routes and paths. Examples: Ping: Uses ICMP "echo" messages to check the availability of a device.

Describe the Internet Protocol`s addressing and routing!

IP Addressing: Internet Protocol uses a unified global addressing method. Devices connected to the network can be reached through their unique IP addresses. IP Datagram: The standard packet format in IP communications. Routing and Forwarding: IP includes specific methods for packet forwarding and routing, along with supporting mechanisms to facilitate these processes

What is the Internet Protocol`s packet format? What is IP tunneling?

IP Packet Format: An IP packet includes a data header and the actual data payload. The data header contains essential information for routing and delivery, such as the sender and recipient's IP addresses. IP Tunneling: This is a technique where a new IP header is put before the original IP packet. The new header contains the source and destination addresses, which are the endpoints of the tunnel. Tunneling can also include additional security measures such as authentication and encryption, commonly referred to as IPsec

Summarize IPv6 addressing, packet format and protocol features!

IPv6 Addressing: Offers a vastly expanded address space with 128-bit addresses, capable of providing trillions of unique IP addresses for each square meter of the Earth's surface. IPv6 Packet Format: Features a streamlined header that removes non- essential information, facilitating faster processing and routing. The header is designed to be efficient for routers to quickly determine the packet's path. IPv6 Protocol Features: Incorporates security directly in the IP layer with IPSec, supports easy auto-configuration for devices joining networks, and includes enhancements for mobility, allowing devices to maintain their IP address while moving across different networks

What are the main differences between IPv4 and IPv6?

IPv6 can provide a lot more internet addresses than IPv4. IPv6 has better security and can automatically set up its own address when moving networks. IPv6 has a simplified header structure, which streamlines processing compared to IPv4

Describe the architecture and main features of LTE network (EUTRAN + EPC). Describe the roles of nodes in the LTE network.

LTE Network Architecture (EUTRAN + EPC): EUTRAN (Evolved UMTS Terrestrial Radio Access Network): Contains base stations known as eNodeBs or eNBs. Functions of eNodeB include radio resource management, handover functions, and security functions. No central controller as in UMTS (RNC in UMTS). --------------- EPC (Evolved Packet Core): Main components include the Mobility Management Entity (MME), Serving Gateway (SGw), and Packet Data Network Gateway (PDN Gw). MME handles control plane functions, mobility management, and UE location updating. SGw is responsible for forwarding UE data and maintaining data bearers. PDN Gw acts as a gateway to external networks, allocates IP addresses, and enforces policies. --------------- Main Features: LTE provides efficient radio network management, supports billing and authentication, manages end-to-end connections, and offers robust mobility support. --------------- Roles of Nodes in the LTE Network: eNodeB (eNB): Handles radio resource management, mobility management, scheduling, and interfacing with other eNBs and the EPC. MME (Mobility Management Entity): Manages signaling, mobility, security, and bearer management. Serving Gateway (SGw): Routes data, forwards packets during handovers, and manages bearer contexts Packet Data Network Gateway (PDN Gw): Allocates IP addresses, connects to external networks, collects charging data, and enforces QoS policies.

Describe the problem of LTE resource scheduling. Describe how the system solves this.

LTE Resource Scheduling Problem: The challenge is in determining which Physical Resource Block (PRB), which subframe, and what amount of power to allocate to each User Equipment (UE). This decision needs to meet Quality of Service (QoS) requirements, fairness, and power limitations, while considering the dynamic nature of wireless channels and interference. ---------------------- Solution: Scheduler in eNodeB: Allocates time-frequency resources to UEs. The scheduler's algorithm is not standardized, allowing for flexibility in implementation. Interference Management: The system includes mechanisms for coordinating distributed scheduling among eNodeBs to avoid interference, using the X2 interface. Adaptation: The scheduler dynamically adjusts to changes in channel quality and UE demands.

Summarize GSM radio basics (medium access, frame structure, bursts)!

Medium Access: Uses a combination of TDMA, FDMA, and FDD. FDMA divides the spectrum into 200 kHz channels, while TDMA splits the channel into 8 timeslots per frame. Frame Structure: A GSM frame consists of 8 timeslots and lasts approximately 4.615 ms. A multiframe structure is used for traffic channels (26 frames, 120 ms) and control channels (51 frames, 235.36 ms). Bursts: Each GSM transmission is made of bursts containing a training sequence for synchronization and data bits

How can the transmission errors of the radio channel corrected (FEC, ACK, interleaving)?

Methods: Forward Error Correction (FEC) adds redundancy for error correction; Acknowledgement/Retransmission for packet errors. Use: Combined in real systems for effective error correction. Acknowledgement/retransmission: Upon reception of packets or frames, the receiver sends ACKnowledgement to the sender. If erroneous packet received, NegativeACKnowledgement, or no acknowledgement is sent, the transmitter then retransmits.

What is the geographic scale of comm. networks (PAN, LAN, MAN, WAN)?

Personal Area Network (PAN): Typically extends to 10 meters, close to one person. Wired PAN includes USB and FireWire connections, while wireless PAN involves Bluetooth and infrared communication. Local Area Network (LAN): Covers a limited geographical area such as a home, school, or office building. Wired LANs often use Ethernet technology, while wireless LANs use technologies like 802.11a/ac/ad/b/g/n/ac/ax. Metropolitan Area Network (MAN): Usually spans a city or a large campus. Wide Area Network (WAN): Covers a large geographic area, such as a city, country, or even intercontinental distances, using various media like telephone lines, cables, and airwaves. WAN technologies generally function at the lower three layers of the OSI model

Outline Shannon`s formula, describe the parameters in it and its consequences

R=< W* log2 (1+ Psignal/ Pnoise Formula: Relates bitrate (R), bandwidth (W), signal power (P_signal), noise power (P_noise), and SNR. Consequences: Higher bitrate needs more bandwidth; increasing power less effective; efficiency measured by spectral efficiency (R/W).

What are real time/quasi real time and non-real time services?

Real Time Services: Require immediate transmission of information (e.g., voice calls, videotelephony) with low latency and tolerable packet loss. Quasi Real Time Services: Include live streaming of voice and video with latency potentially spanning seconds. Non-Real Time Services: Latency is not a primary concern (e.g., browsing, file transfer, email), and data loss is not permissible

What are the new nodes (compared to GSM) in GPRS system? What are the roles of these?

SGSN (Serving GPRS Support Node): Manages GPRS subscribers within its service area. GGSN (Gateway GPRS Support Node): Acts as an interface between the GPRS network and external networks. PCU (Packet Control Unit): Sits within the BSS, managing the radio resources for GPRS and handling packet data routing between the BSS and the SGSN

Define how power save mode is working in WiFi.

Sleeping Mode: Devices don't listen to the channel to save power. Beacon Frames: AP periodically informs sleeping devices of pending packets. Wake-Up Cycle: Devices periodically check these beacon frames. Power-Save Poll: Devices signal readiness to receive packets. Packet Handling: AP responds and sends stored packets to the awakened device. This mode allows devices to conserve energy by reducing active listening time

Describe the TCP transport protocol!

TCP, or Transmission Control Protocol, is designed for reliable and ordered delivery of a bytestream of data over the otherwise connectionless and potentially unreliable IP layer. It ensures data sent is the same as data received. TCP provides mechanisms for acknowledgment and retransmission, flow control, and multiplexing of different data transfers over a single connection. TCP operates by establishing a connection between endpoints, where they agree on initial sequence numbers and readiness to transmit/receive data, allowing for two-way data transfer

What is the difference between TDMA/FDMA and FDD? What is PDH and SDH?

TDMA/FDMA are methods for multiple access, dividing channels by time or frequency, while FDD uses separate frequency bands for uplink and downlink. PDH and SDH are types of multiplexing used in GSM's transport network for high bitrate and robust networking

Describe the main functionalities of the first four layers of the OSI layered model.

The OSI (Open System Interconnection) model's first four layers are primarily handled by networks, with the remaining three layers at the endpoints. Examples include SS7 over TDM in telephony networks and TCP/IP in Ethernet, which utilize all seven OSI layers

Describe the UDP transport protocol!

UDP, or User Datagram Protocol, is a connectionless transport protocol, meaning it does not establish a connection before sending data. It's designed for low overhead, without flow control or retransmission, which makes it suitable for applications like voice and video where speed is more important than reliability. UDP packets, or datagrams, include source and destination ports and error detection via checksums, but there's no guarantee of delivery, ordering, or protection against duplicates

Describe the architecture and main features of UMTS network (UTRAN + Core Network). Describe the roles of new devices in the UTRAN.

UMTS network consists of UTRAN (RNCs and Node Bs) for radio access and a Core Network with MSC, SGSN, and GGSN for data handling and supporting databases like HLR and VLR. Node Bs are similar to GSM's BTS but use new hardware and technologies

Describe main properties of UMTS radio interface. How are channels of users distinguished in UMTS? How W-H and scrambling codes are used? Derive the physical gross bitrates in UMTS. List the typical implemented bitrates.

UMTS's radio interface has a 10 ms frame structure with 15 timeslots, uses QPSK modulation, and operates at a chip rate of 3.84 Mcps. Channels for users are distinguished by channelization codes (Walsh- Hadamard codes or OVSF) and scrambling codes. OVSF codes define data or control channels, and scrambling codes differentiate signals of different cells or users. The physical gross bitrates are derived from the chip rate and code length, with downlink rates up to 1.92 Mbps for the shortest code (4)