1.2 Network Edge
In the previous section, we gave an overview of the Internet and network protocols. In this section we will learn more about computer networks. End systems are called end systems because they are almost at the application layer of the network, which is also the very edge of the network.
In the end system, it is subdivided into client and server. The client is usually desktop PC mobile device, while the server is more powerful device, such as Google And Baidu. They have super-large data centers, and each data center has more than 100,000 servers.
1.2.1 access network
An access network is a network that connects the end system to its edge router, the one you can’t live without in your home, which is your first access network
1. Home access :DSL cable FTTH and satellite (cable)
DSL(Dight Subscriber Each subscriber’s DSL uses the telephone Line to exchange data with a digital Subscriber access Multiplexer (DSLAM) located in the phone company’s local central office (CO).
Home phone lines carry both data and traditional phone signals, but they are encoded at different frequencies so that one phone line can seem like many different lines, performing many tasks. The digital Subscriber Line Access Multiplexer (DSLAM) is tasked with separating the data signals arriving at the home from the telephone signals
DSL is only suitable for users with a short distance network connection of 5-10 miles or socableFiber optic cables connect cable ends to regional hubs and coaxial cables to individual apartments. The system is also known as a Hybrid Fiber Coax HFC system because it uses Fiber optics and coaxial cable. Each regional hub typically supports 500-5000 families. So if more than one user uses downlink channel download at the same time may cause the network speed to slow down.
A more high-speed approach is fiber to the home, as the name implies directly from CO; Connect to a home optical fiber. Fiber to the home also has a variety of different schemes, the most direct is to allocate one fiber to each user, but this cost is high and the traffic of a home is not that large, so the more common scheme is one fiber from the local central office until relatively close to the home to separate several fibers to the user’s home. This scheme can be divided into two structures: AON (Active Optical Network) and PON(Passive Optical Network)
Where DSL and cable and fiber to the home are not available, satellite links are used but are slower in some villages.
At present, most campuses and homes use local area networks (Lans) to connect end systems to edge routers. Ethernet is the most popular network access technology, and users need to use it within tens of meters of the router.
3. Wan Mobile devices use cellular networks to access the Internet within tens of thousands of meters of base stations.
1.2.2 Physical Media
When we want to send a piece of information or data, first the source transmits the bit, then the first router receives the bit and then the second router goes through a series of receivers and transmitters to get to the destination. Bits may be stored as electromagnetic waves or light pulses along the way, not necessarily of the same type. There are two types of physical media: guided media and unguided media. For guided media, radio waves travel along a solid medium, such as fiber optic twisted-pair copper or coaxial cable. For unguided media, radio waves travel through the air or outer space, such as an infinite LAN or digital satellite channel
This section summary
This section delves further into the network and discusses the process from the network edge to the router to the local central office, as well as several connection modes
Review questions
Answer: Residential access: Fiber access DSL access Corporate access: Ethernet WiFi Wide area wireless access: 3G 4G
Answer: HFC bandwidth is shared between users. No collision occurs in the downstream HFC channel. Because packets are issued by one source and accepted by different end systems, there is no conflict.
Answer: Fiber to the home. A few tens of meters.
Answer: 10M to 10G.
Answer: twisted-pair copper wire, coaxial cable, optical fiber and so on.
Answer: dial-up modem: rate 56kbps dedicated bandwidth HFC: downstream rate: 42.8Mbps upstream rate: 30.7Mbps bandwidth sharing DSL: downstream rate: 24Mbps upstream rate :2.5Mbps bandwidth sharing FTTH: Average downlink rate: 20Mbps bandwidth sharing
Answer: WiFi and 4G access. WiFi is fast but short, 4G is slow but long.