Computer Networks

The standard for Metropolitan Area Networks (MAN) is IEEE 802.6, which is based on the Distributed Queue Dual Bus (DQDB) architecture. Ethernet is primarily a LAN standard (802.3), and Token Ring is also a LAN standard (802.5). DQDB was specifically designed to support data, voice, and video over a city-wide area.

In network architecture, a MAN typically uses a broadcast or shared medium approach rather than a point-to-point switched subnet. Wide Area Networks (WANs) rely on switching elements like routers to direct traffic between nodes. Because MANs like DQDB use a bus structure, they do not require internal switching elements to function.

The Token Ring network technology is standardized by the IEEE 802.5 committee. IEEE 802.3 refers to Ethernet, while IEEE 802.6 refers to MANs (DQDB), and IEEE 802.11 refers to Wireless LANs (Wi-Fi). Token Ring uses a special three-byte frame called a token that travels around a logical ring.

A Wide Area Network (WAN) is designed to span large geographical distances, often covering an entire country or continent. Local Area Networks (LANs) are restricted to rooms or buildings, and Metropolitan Area Networks (MANs) are restricted to cities. WANs use long-haul transmission lines to connect these smaller networks.

The IEEE 802.6 standard was developed to define the protocols for Metropolitan Area Networks. It utilizes the Distributed Queue Dual Bus (DQDB) technology to provide high-speed connectivity across a city. Other standards like 802.3 and 802.5 are intended for local area connectivity.

DQDB (Distributed Queue Dual Bus) consists of two unidirectional buses that carry data in opposite directions. This dual-bus configuration allows nodes to transmit data to any other node on the network by choosing the appropriate bus. Consequently, data transfer can be viewed as occurring in both directions across the network span.

The acronym DQDB stands for Distributed Queue Dual Bus. As the name implies, the architecture relies on exactly two unidirectional buses to facilitate communication between all connected nodes. One bus handles traffic in one direction, while the second bus handles traffic in the opposite direction.

A Metropolitan Area Network (MAN) is categorized by its ability to cover a city-sized area. It serves as an intermediate scale between a Local Area Network (LAN), which covers a building, and a Wide Area Network (WAN), which covers a country. Common examples include cable television networks or city-wide fiber loops.

Local Area Networks (LANs) are designed for small-scale environments such as a single room, an office, or a building. They provide high-speed connectivity with low delay over short distances. Larger areas like cities or countries require MANs or WANs respectively.

Token Ring is a standard protocol for Local Area Networks (LANs), defined by IEEE 802.5. It uses a logical ring topology where a token is passed between stations to grant permission to transmit. While it was once a major competitor to Ethernet, it remains a classic example of LAN technology.

In a DQDB (MAN) architecture, the head end is a specialized device at the start of each bus. It is responsible for generating a steady stream of empty slots that other nodes on the bus can then use to transmit data. This centralized slot generation is a key feature of the DQDB protocol.

A router is essentially a specialized computer equipped with a processor, memory, and specialized input/output hardware for networking. It runs software (often a specialized operating system) to manage routing tables and forward packets. It is not merely a line or a bus, but an active processing device.

The primary function of a router is to receive a packet on an incoming line and determine the best path forward. It then selects the appropriate outgoing line to send the packet toward its final destination. This decision is based on the destination IP address and the router's internal routing table.

In Wide Area Network terminology, the subnet is the collection of transmission lines and switching elements (routers) that move packets from the source host to the destination host. The hosts themselves are considered external to the subnet. Therefore, the subnet consists of both the physical lines and the routers.

Transmission lines are the physical media, such as copper wires, fiber optic cables, or radio links, that carry bits from one location to another. While routers and hosts process the data, the actual movement of bits across space is performed by the transmission lines. They form the physical layer of the network.

Wide Area Networks (WANs) are typically designed with a mesh-like topology to ensure high availability and reliability. This means there are often many different paths a packet can take to reach its destination. If one link or router fails, the network can automatically reroute traffic through an alternative path.

In the context of a network subnet, the 'switching element' is the device that receives packets and forwards them toward their destination. In modern packet-switched networks, this device is most commonly known as a router. Hosts are the end-user devices, and transmission lines are the cables connecting them.

Routers can be interconnected using a variety of physical and logical topologies depending on the network's requirements for cost, performance, and redundancy. Common topologies include star (central hub), ring (closed loop), and tree (hierarchical). In practice, large networks often use a combination of these or a partial mesh.

Wireless networks utilize electromagnetic waves to transmit data through the air, eliminating the need for physical cables. This technology enables mobile communication across diverse environments, including on land, at sea (via satellite or maritime radio), and in the air (via satellite or air-to-ground links).

During natural disasters, wired infrastructure like telephone lines and fiber optic cables is often physically damaged or destroyed. Wireless networks are much more resilient and can be rapidly deployed using portable towers or satellite links to restore critical communication. This makes them indispensable for emergency response.

CDPD stands for Cellular Digital Packet Data. It was a data transmission technology used on analog cellular networks (AMPS) in the 1990s. It allowed for packet-switched data to be sent during the idle periods of voice channels, providing an early form of mobile internet access.

The original IEEE 802.11 wireless standard, released in 1997, specified data transfer rates of 1 or 2 Megabits per second (Mbps). While modern Wi-Fi standards like 802.11ax are much faster, the foundational wireless LAN technology operated at these lower speeds. This range is the standard historical answer for early wireless data rates.

In the Distributed Queue Dual Bus (DQDB) protocol used for MANs, the head end is the node at the beginning of the bus that initiates the transmission process. It continuously generates empty slots that travel down the bus. Other nodes on the network monitor these slots and fill them with data when they need to transmit.