Chapter 1: Overview of Computer

1 Computer Networks and the Internet

1.1 What is the Internet?

• A Nuts-and-Bolts Description

All the devices that connect to the internet, like laptops, smartphones, cars, are called hosts or end systems.

End systems are connected together by a network of communication links and packet switches

End systems access the Internet through Internet Service Providers(ISP)

End systems, packet switches and other pieces of the Internet run protocols that control the sending and receiving of information within internet. The Transmission Control Protocol(TCP) and the Internet Protocol (IP) are the most important protocols.

• A Services Description
  Internet is is an infrastructure that provides services to applications.

End systems attached to Internet provide a socket interface that specifies how a program running on one end system asks the Internet infrastructure to deliver data to a specific destination program running on another end system.

• What is protocol
  A protocol defines the format and the order of messages exchanged between 2 or more communicating entities, as well as the actions taken on the transmission and/or receipt of a message or other event

1.2 The Network Edge

hosts or end systems are sometimes further divided into 2 categories: clients and servers.

• Access Networks

  • DSL
    digital subscriber line. A residence typically obtains DSL Internet access from the same local telephone company that provides its wired local phone access.
    When DSL is used, a customer's telco is also its ISP. each customers' DSL modem uses the telephone line to exchange data with digital subscriber line access multiplexer(DSLAM) at telco’s local central company(CO)
    The residential telephone line carries both data and traditional telephone signals. which encoded in different frequencies.

    1. high-speed downstream channel, in 50 kHz to 1MHz band
    2. medium-speed upstream channel, in 4 kHz to 50kHz band

    3. two-way telephone channel, 0 to 4kHz band
       Since the 3 channels locate in different band, a phone call and internet connection can share the DSL link at the same time.

       
       DSL Internet

  • Cable
    Use the cable television infrastructure.
    Because both fiber and coaxial cable are employed in this system, it is often referred to as hybrid fiber coax(HFC)
    Cable internet requires special modems, called cable modems. It is typically an external devices connects to the home PC through an Ethernet port.
    cable Internet access is a shared broadcast medium. If several users are simultaneously downloading a video file on the downstream channel, the actual rate at which each user receives its video file will be significantly lower than the aggregate cable downstream rate. But for web surfing, Since the users will rarely request a web page at exactly same time. They will receive the web page at the full cable downstream rate.
    

    Hybrid fiber-coaxial access network

  • FTTH
    Fiber to the Home
    

    FTTH Internel access

  • Ethernet
    a local area network (LAN) is used to connect an end system to the edge router. Ethernet is the by far the most prevalent access tech.
    Users user twisted-pair copper wire to connect to an Ethernet switch, is then in turn connected into larger internet.
    

    Ethernet Internet access

  • WiFi
    Wireless LAN

1.3 The Network Core

1.3.1 Packet Switching

To send a message from a source end system to a destination end system, the source breaks long messages into smaller chunks of data known as packets. Each packet travels through communication links and packet switches (include router and link-layer switches).
Packets are transmitted over each communication link at a rate equal to the full transmission rate of the link.

• Store-and-forward Transmission
  Most packet switches use store-and-forward transmission at the inputs to the links.

The packet switch must receive the entire packet before it can begin to transmit the first bit of the packet onto the outbound link.

Consider a case that sending 1 packet from source to destination over a path consisting of N links each of rate R. Then the end-to-end delay is

d = N * L / R

• Queuing delays and packet loss
  The packet switch has an output buffer. If an arriving packet need to be transmitted onto a link busy with transmission of another packet, the arriving packet must waits in the output buffer, which causes queuing delays. If the buffer is full and a packet arrives, packet loss will occur.

1.3.2 Circuit Switching

The resources needed along path for communication between the end systems are reserved for the duration of communication session.

Packet vs circuit switching

1.3.3 A network of networks

The first network provider, Network Structure I, inter connects all if the access ISPs with a single global transit ISP(imaginary). It is a network of routers and communications links that not only spans the globe, but also has at least one router near each of the thousands of access ISP. Actually it is formed with Tier1 ISPs. Global ISP is costly and charge every access ISPs

Since a global transit ISP is costly, other companies want to build their own ISPs. Network Structure 2, consists of millions of access ISPs and multiple transit ISPs. The global transit ISPs are connected. Else the access ISPs cannot communicate. Also, in any given region, there may be a regional ISP to which access ISPs in the region connect.

There might be multiple levels of regional ISP. For instance, in china there will be City ISP, Provincial ISP, National ISP. These ISPs forms Network Structure 3

To build a network that more closely resembles today’s Internet, we must points of presence(PoPs), multi-homing, peering, and Internet exchange points(IXPs). These things, together with access ISPs, form Network Structure 4.

A PoP is simply a group of one or more routers at the same location in the provider’s network to connect to a provider’s PoP. IXP is a meeting point where multiple ISPs can peer together. An IXP is typically a stand-alone building with its own switches.

Network Structure 5 is content-provider networks which is builds on top of Network Structure 4. Google is the leading examples. It is a private network that connects it data center to the internet, by pass tier-1, regional ISPs

Interconnection of ISPs

1.4 Delay, Loss and Throughput in Packet-Switched Networks

• Processing Delay
  The time required to examine the packets’ header and determine where to direct the packet. Check for bit-level errors and other factors should also be included.

• Queuing Delay
  The time waits to be transmitted onto the link at the queue. It depends on the number of earlier-arriving packets.
  Denote that the length of the packet by L bits, the transmission rate is R bits/sec, every sec A number of packets will arrive. Then if

L*A/R > 1

The packets will be queued more and more, and the queuing delay will finally become infinite

• Transmission Delay
  The time required to push(transmit) all of the packet’s bits into the link.
  Denote the length of the packet by L bits, denoting the transmission rate of the link from router A to router B by R bits/sec. The transmission delay is

dt = L/R

Typically it is on the order of us ~ ms

• Propagation Delay
  The time required to propagate from the beginning of the link to router B. d is the length of the link, s is the propagation speed, which depends on the physical material, closed to light speed. The propagation delay is

dprop = d/s

• end-to-end delay
  Suppose there are N -1 routers in this route
  d end-end = N(dproc + dtrans + dprop)

• Throughput
  If the file consists of F bits and the transfer takes T seconds for Host B to receive all F bits. Then the average throughput of the file transfer is

Throughput = F/T

The throughput is always depends on the bottleneck part in the link

1.5 Protocol Layers and Their Service Models

1.5.1 Layered Architecture

The Internet protocol stack and OSI reference model

- **Application Layer**
Where network applications and their application-layer protocols reside.  Including `HTTP`(for web document request and transfer), `SMTP`(for the transfer of e-mail messages), `FTP`(for the transfer of files between two end systems). DNS(domain name system) is also located in application layer

- **Transport Layer**
IN the Internet there are 2 transport protocols, `TCP` and `UDP`, either of which can transport application-layer messages. 

TCP is a connection-oriented service to its applications. TCP includes guaranteed delivery of applications-layer messages to the destination and flow control.
UDP provides a connectionless service to its applications. This is a no-frills service that provides no reliability, no flow control, no congestion control.

- **Network Layer**
Responsible for moving network-layer packets known as **datagram** from one host to another. Although the network layer contains both the IP protocol and numerous routing protocols, it is often simply referred to as the IP layer.

- **Link Layer**
Responsible for moving a packet to another route. Examples of link-layer protocols include Ethernet, WiFi, and cable access networks’ DOCSIS protocol. The link-layer packets are referred as **frames**

- **Physical Layer**
real physical layer.

1.5.2 Encapsulation