Essential Networking Concepts and Technologies

Posted on Jun 19, 2025 in Computer Engineering

Ethernet Frame Format

The Ethernet frame consists of the following fields:

• Preamble: (7 bytes) Synchronization pattern.
• Start Frame Delimiter (SFD): (1 byte) Marks the start of the frame.
• Destination Address: (6 bytes) MAC address of the recipient.
• Source Address: (6 bytes) MAC address of the sender.
• Protocol ID (TPID) / Length: (2 bytes) Indicates the type of protocol or the length of the data field.
• Tag Control Information (TCI): (4 bytes, for VLAN-tagged frames) Includes:
  • Priority: (3 bits) User Priority (PCP).
  • CFI: (1 bit) Canonical Format Indicator (set to 0 for Ethernet).
  • VID: (12 bits) VLAN ID.
• Data: (46-1500 bytes) The actual payload.
• Padding (PAD): (Variable) Added to ensure minimum frame size (if data is less than 46 bytes).
• Frame Check Sequence (FCS): (4 bytes) Error detection code.

Autonegotiation Principles

Autonegotiation is a mechanism that allows connected devices to automatically agree on the best possible connection parameters, such as:

• Transmission Rate: E.g., 10 Mbps, 100 Mbps, 1 Gbps.
• Duplex Mode: Half-duplex or full-duplex.

This process resolves speed and duplex mismatches between devices, ensuring the highest common speed and optimal mode are set for efficient communication.

xDSL Network Technologies

xDSL (Digital Subscriber Line) technologies transmit data over traditional twisted-pair copper telephone lines, typically in a point-to-point configuration. They utilize a bandwidth of approximately 2700 Hz (300 Hz – 3 kHz) for voice, while higher and lower frequencies are used for data, employing frequency multiplexing.

ADSL Variants

• ADSL / ADSL Lite:
  • Downstream (Network to User): 64 kbps to 1.5 Mbps
  • Upstream (User to Network): 32 kbps to 512 kbps
  • Maximum Distance: 5.5 km
• ADSL Normal:
  • Downstream: 64 kbps to 8 Mbps
  • Upstream: 32 kbps to 1.1 Mbps
  • Maximum Distance: 5.5 km

SDSL (Symmetrical Digital Subscriber Line)

• Type: Symmetrical duplex (transmit speed equals receive speed).
• Speed: Up to 2 Mbps.
• Distance: Up to 6.7 km.

HDSL (High Bit-Rate Digital Subscriber Line)

• Type: Duplex, often uses multiple twisted pairs.
• Speed: Up to 2 Mbps.
• Distance: Up to 4.5 km.

VDSL (Very High Bit-Rate Digital Subscriber Line)

• Type: Asymmetrical.
• Speeds:
  • Downstream: 1.3 Mbps (at 52 meters) to 1.5 Mbps (at 2.3 meters).
  • Upstream: Varies based on distance.
• Distance: Up to 1.2 km.

xDSL Modulation Techniques

Common modulation techniques used in xDSL include:

• CAP (Carrierless Amplitude/Phase): A form of amplitude and phase modulation, similar to QAM (Quadrature Amplitude Modulation).
• DMT (Discrete Multi-Tone): Divides the available bandwidth into multiple sub-channels, each carrying a portion of the data.

ADSL2 and ADSL2+ Enhancements

These standards offer improved performance and features:

• Power Management Modes:
  • L0: Normal operating mode, full power.
  • L2: Reduced power mode, no data transmission (for short periods).
  • L3: Standby mode, reduced power at both ends (e.g., after 3 seconds of inactivity, transitions from L3 to L0 upon activity).
• Features: Dynamic speed adjustment, high multiplexed speeds, multi-channel support, and Voice over ADSL (VoADSL).
• Bandwidth: Up to 2.2 MHz bandwidth.

Wireless Network Technologies

Wireless networks utilize radio or optical signals for communication. Wi-Fi (802.11) is commonly used for terminal equipment, while WiMAX (802.16) is often employed for broader distribution networks.

A key characteristic of shared wireless media is that bandwidth is shared among all connected devices, and device mobility capabilities vary by standard.

Wi-Fi (802.11) Standards

• 802.11a: 5 GHz band, up to 54 Mbps.
• 802.11b: 2.4 GHz band, up to 11 Mbps.
• 802.11g: 2.4 GHz band, up to 54 Mbps.
• 802.11n: 2.4 GHz and 5 GHz bands, up to 100 Mbps (or more with MIMO).

Wi-Fi Frequency Bands and Channels

• 2.4 GHz Band:
  • Typically 14 channels available (depending on region).
  • First channel (e.g., Channel 1) starts at 2412 MHz.
  • Channel width is 22 MHz (11 MHz on each side of the center frequency).
  • Channels are spaced at 5 MHz increments.
  • Non-overlapping channels: 1, 6, and 11 are commonly used to avoid interference.
  • Channel 14 (2484 MHz) is often restricted to specific regions (e.g., Japan).
• 5 GHz Band:
  • Offers more channels (e.g., 8 non-DFS channels, more with DFS).
  • First channel (e.g., Channel 36) starts at 5.18 GHz.
  • Channel width is typically 20 MHz (10 MHz on each side).
  • Channels are spaced at 2 MHz increments.

Network Devices: Hubs and Switches

Hub Functionality

A Hub operates at the Physical Layer (Layer 1) of the OSI model. It functions as a multi-port repeater, broadcasting all incoming data to every connected device. This results in:

• Shared Bandwidth: All devices share the total bandwidth, meaning only one device can transmit at a time (creating a single collision domain).
• Visibility: All connected devices “see” all traffic, regardless of the intended recipient.

Hub Cabling (MDI/MDI-X)

• MDI-X (Medium Dependent Interface Crossover): Used to connect a hub to another hub or a switch using a straight-through cable.
• MDI (Medium Dependent Interface): Used to connect a hub to a host device (e.g., PC) using a straight-through cable, or to another hub using a crossover cable. (Note: Modern hubs/switches often have auto-MDI/MDI-X).

Switch Functionality

A Switch operates at the Data Link Layer (Layer 2) of the OSI model. Unlike a hub, a switch intelligently forwards data only to the intended destination device based on MAC addresses. This significantly improves network performance by:

• Dedicated Bandwidth: Each port typically operates in its own collision domain, allowing multiple devices to transmit simultaneously.
• Reduced Traffic: Unnecessary traffic is not broadcast to all ports.

VLANs (Virtual Local Area Networks)

VLANs enable the logical separation of traffic within a single physical switch. Each switch port can be assigned a VLAN ID, effectively creating multiple isolated broadcast domains on the same switch. This allows for:

• Traffic Segmentation: Isolating different departments or types of traffic.
• Enhanced Security: Preventing unauthorized access between segments.
• Scalability: Supports up to 4096 VLANs (802.1Q standard), though practical implementations typically use fewer active VLANs (e.g., 32-40).

802.11 Wi-Fi Frame Format

The 802.11 standard defines various frame formats for Wi-Fi communication, including different preamble types for varying speeds.

PLCP Preamble and Header (Long Preamble)

Used for lower speeds (e.g., 1, 2 Mbps) and for backward compatibility.

• Synchronization (Sync): (16 bits) Used for receiver synchronization in asynchronous transmission.
• Start Frame Delimiter (SFD): (2 bits) Marks the start of the PLCP header.
• PLCP Header: Contains system transmission information:
  • Signal (SIG): (2 bits) Indicates the transmission rate (e.g., 0.1 Mbps increments).
  • Service (SRV): (2 bits) Service field.
  • Length (LEN): (2 bits) Indicates the size of the PSDU (payload).
  • CRC (Cyclic Redundancy Check): (2 bits) For error checking of the PLCP header.

Note: This structure supports speeds up to 11 Mbps (for DSSS). The PLCP Service Data Unit (PSDU) follows, carrying the actual data (up to 2346 bytes).

PLCP Preamble and Header (Short Preamble)

Used for higher speeds (e.g., 5.5, 11 Mbps) and improved efficiency.

• Synchronization (Sync): (Shortened) For receiver synchronization.
• Start Frame Delimiter (SFD): (2 bits) Inverted SFD compared to the long preamble.

Note: This preamble type is typically used for 2 Mbps and higher data rates. The PLCP header structure is similar to the long preamble but optimized for speed.

802.11 MAC Frame Types

The PSDU (PLCP Service Data Unit) contains the 802.11 MAC frame, which can be one of three types:

• Management Frames: Used for network management tasks, such as:
  • Association and disassociation of devices.
  • Synchronization and beaconing.
  • Authentication and deauthentication.
• Control Frames: Used for media access management and confirmation, including:
  • RTS/CTS (Request To Send / Clear To Send) for controlled access.
  • ACK (Acknowledgment) for successful frame reception.
• Data Frames: Used for transmitting actual user data.

802.11 MAC Frame Fields

A typical 802.11 MAC frame includes the following fields:

• Frame Control: (2 bytes) Defines the type of frame, protocol version, etc.
• Duration/ID: (2 bytes) Indicates the duration of the transmission or a connection ID.
• Address 1 (Receiver Address): (6 bytes) MAC address of the immediate recipient.
• Address 2 (Transmitter Address): (6 bytes) MAC address of the immediate sender.
• Address 3 (Destination Address): (6 bytes) MAC address of the final destination (in some frame types).
• Sequence Control: (2 bytes) Contains fragment number and sequence number for reassembly.
• Address 4 (Source Address): (6 bytes) MAC address of the original source (in some frame types, e.g., WDS).
• Information/Data: (0-2312 bytes) The actual payload data.
• CRC (Cyclic Redundancy Check): (4 bytes) For error detection of the MAC frame.

Virtual Private Networks (VPN)

A VPN creates a secure, encrypted connection over a less secure public network, such as the internet. It functions by establishing a “tunnel” that encrypts and encapsulates data, making it appear as a private, point-to-point connection between two endpoints. This technology is crucial for protecting sensitive information transmitted across public infrastructures.

WiMAX (802.16) Technology

WiMAX (Worldwide Interoperability for Microwave Access) is a wireless broadband communication standard (802.16) that offers higher bandwidth and broader coverage compared to traditional Wi-Fi. It is designed for metropolitan area networks (MANs) and last-mile broadband access.

WiMAX Standards Comparison