Computer network

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Computer Networks – Data link layer

The data link layer belongs to the lower layer of the computer network. There are two types of channels used by the data link layer:

  1. Point-to-point channel. This channel uses one-to-one point-to-point communication.
  2. Broadcast channel. This channel uses one-to-many broadcast communication, so the process is more complicated. There are many hosts connected on the broadcast channel, so a dedicated shared channel protocol must be used to coordinate the data transmission of these hosts.

LAN is a network, but we don’t talk about Lans at the network layer. That’s because at the network layer you’re talking about the interconnection of multiple networks, how packets are forwarded from one network to another, through routers. In this chapter we examine how packets can be transferred from one host to another on the same LAN without being forwarded by a router. Viewed from the whole Internet, LAN still belongs to the scope of data link layer.

We first introduce point-to-point channels and PPP, the point-to-point protocol most commonly used over such channels. Then a larger section discusses the LAN and related protocols for sharing channels.

The important thing is

  1. The characteristics of the point-to-point channel and the broadcast channel at the data link layer, and the characteristics of the protocols used by the two channels (PPP and CSMA/CD).
  2. There are three basic problems in data link layer: frame encapsulation, transparent transmission and error detection.
  3. Hardware address of the Ethernet MAC layer.
  4. Functions and applications of adapters, forwarders, hubs, Bridges, and Ethernet switches.

Mind mapping

Important concepts in this chapter

  • A link is a physical line from a node to an adjacent node, while a data link is based on a link with some necessary hardware (such as network adapter) and software (such as protocol implementation).
  • The data link layer mainly uses two kinds of channels: point-to-point channel and broadcast channel. The protocol data unit transmitted by the data link layer is the frame. The three basic problems of data link layer are: frame encapsulation, transparent transmission and error detection.
  • Cyclic redundancy check CRC is a method of error detection, while frame check sequence FCS is a redundant code added to the data.
  • Point-to-point protocol (PPP) is the most widely used protocol in data link layer. Detect errors, not correct them; No serial number, no flow control; Multiple network layer protocols can be supported simultaneously.
  • PPPoE is a link layer protocol used by hosts with broadband Internet access.
  • The advantages of LAN are: broadcast function, from a site can be very convenient access to the whole network: facilitate the development of the system and gradual evolution: improve the reliability of the system, availability and survival.
  • There are two ways to share communication media resources: one is static channel division (various multiplexing technologies), the other is dynamic media access control, also known as multi-point access (random access or controlled access).
  • The IEEE 802 committee has divided the data link layer of a local city network into two sub-layers, namely, the logical link Control (LLC) sub-layer (independent of the transmission media) and the media access Control (MAC) sub-layer (independent of the transmission media). But now the LLC sub-layer is history.
  • The communication between the computer and the external network must be through a communication adapter (or network adapter), which is also called a network interface card or nic. The hardware address of the computer is in the ROM of the adapter.
  • The Ethernet works in connectionless mode and does not number data frames or require confirmation from the peer party. The destination station receives an error frame and does nothing else.
  • Ethernet uses CSMANCD, a carrier sense multipoint access with collision detection. The main points of the protocol are: listen before sending, listen while sending, and stop sending as soon as a collision is found on the bus. It then waits for a random period of time according to the retreat algorithm and sends again. As a result, each station is subject to collision for a short period of time after it has sent the data. Ethernet sites compete equally for Ethernet channels.
  • Traditional Bus Ethernet is basically twisted-pair Ethernet using a hub. This Ethernet is a star network physically, but a bus network logically. The hub operates at the physical layer, where each interface simply forwards bits without collision detection.
  • An Ethernet hardware address, or MAC address, is actually an adapter address or adapter identifier, regardless of the location of the host. Both the source address and destination address are 48 bits long.
  • Ethernet adapters have filtering capabilities that accept only unicast, broadcast, or multicast frames.
  • Using hubs, Ethernet can be extended at the physical layer (extended Ethernet is still a network).
  • Switched hubs are often referred to as Ethernet switches or Layer 2 switches (which work at the data link layer). It is a multi-interface bridge, and each interface is directly connected to a single host or another hub, and works in full duplex mode. An Ethernet switch can connect many pairs of interfaces simultaneously, enabling each pair of communicating hosts to transmit data without collision as if they had exclusive communication media.
  • High-speed Ethernet There are 100 Mbit/s Fast Ethernet, gigabit Ethernet, and 10 Gbit/s 10 Gigabit Ethernet. More recently, 100 gigabit Ethernet has been developed. In broadband access technology, high-speed Ethernet is often used for access.

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