MITM753: Advanced Computer Networks Chapter 1 Overview of Computer Networks and the Internet

Introduction What you have learned in the undergraduate networking class? Introduction to networking concepts. OSI layers: application, transport, network, data link and physical layers. Overview of important network applications and protocols. Network addressing. Network devices and hardware. Network cabling.

Introduction What will we learn in this class? More detail discussion on selected network protocols. Several important network-related algorithms. Wireless and mobile network. Hands-on networking skills as exercises and assignments: packet sniffing, socket programming. Use the public Internet as the principle for discussion.

What is the Internet? An interconnection of networks (a network of networks). Consists of both hardware and software. Devices connected to the Internet are called hosts or end systems. These devices run network applications. End systems are connected together by a communication links and packet switches (switches and routers). Different links transmit data at different transmission rate. End systems access the Internet through ISPs (Internet Service Providers). All Internet hardware run protocols that control the sending and receiving of information.

What is the Internet? The Internet’s principal protocols are collectively known as TCP/IP. IETF (Internet Engineering Task Force) is the body that develop and maintain Internet standards. Internet standard documents are called RFCs (requests for comments). The Internet can also be viewed as an infrastructure that provide services to applications. Network applications are distributed across multiple end systems (distributed applications). Different pieces of an application communicate with each other by sending messages through the Internet.

What is the Internet?

Network Protocols Protocol defines the rules used in communication. Similar to the rules we use in a language. More formal definition: A protocol defines the format and the order of messages exchanged between two or more communicating entities, as well as the actions taken on the transmission and/or receipt of a message or other event. In the Internet, there are lots of protocols used for different communication tasks.

The Network Edge

The Network Edge People access the Internet using end systems. Desktop PCs and workstations. Mobile phones and tablets. Web cams, sensors, toaster, refrigerator, etc. End systems run network applications. Two parts of network applications: Client program: A program that requests and receives a service from a server program. Server program: A program that responds to request from a client program. Communication is done over the Internet by exchanging messages.

The Network Edge An end system is connected to an edge router through an access network. Three categories of access networks: Residential access network Example: Dialup, ADSL, FTTH. Company access network Example: Ethernet LAN. Wireless access network Example: Wireless LAN (WiFi), WiMAX, UMTS, LTE.

The Network Edge Data in a network is transmitted over a physical media. Two categories of physical media: Guided media (wired) Waves are guided through a solid medium. Example: Twisted-pair copper wire, coaxial cable, fiber optics. Unguided media (wireless) Waves propagate in atmosphere or outer space. Example: Radio wave, microwave, infrared (all are part of electromagnetic spectrum).

Network Core Two fundamental approaches to move data through a network: Circuit switching Packet switching

The Network Core Circuit switching: Before data can be sent, a connection (called a ‘circuit’) must be established between sender and receiver. Switches along the path maintains connection state of the connection. Resources (buffers, bandwidth) needed along the path are reserved. The resources are solely used by this particular sender/receiver. Transmission rate is constant. Suitable for real-time services (e.g. phone calls). Example: telephone network.

The Network Core Packet switching. Data is divided into small packets. Resources in the network are shared. Resources are used on demand. It is possible for congestion to occur. If any link is congested, delay (or even packet loss) can occur. Therefore, transmission rate may vary depending on the network condition. Suitable for non-real-time data transfer. Two types: Virtual circuit – packet carries path number. Datagram – packet carries receiver’s address. Example: Internet, frame relay, ATM.

Delay and Loss in Packet-Switched Networks Packets traveling in the network suffer from several types of delay at each node. Nodal processing delay: Time required to examine the packet’s header and determine where to direct the packet. In the order of microseconds or less. Queuing delay: Delay experienced by the packet while waiting to be transmitted onto the link. Depends on the length of queue. In the order of microseconds to milliseconds.

Delay and Loss in Packet-Switched Networks Transmission delay: The amount of time required to transmit all the packet’s bits into the link. If the packet length is L bits and the transmission rate is R bits/sec, dtrans = L/R. Propagation delay: The amount of time taken by one bit to propagate from the beginning of the link to the end of the link. Depends on the physical medium used. Typically around 2x108 m/s to 3x108 m/s.

Delay and Loss in Packet-Switched Networks

Delay and Loss in Packet-Switched Networks Total node delay: dnodal = dproc + dqueue + dtrans + dprop If there are N-1 routers between source and destination hosts and assuming that the network is not congested (all dqueue = 0), the end-to-end delay is: dend-end = N(dproc + dtrans + dprop) Try ping or tracert in Microsoft Windows command prompts.

Throughput in Computer Networks Throughput measures the rate at which data is received. Measured in bits per second. The instantaneous throughput is the rate at which a host is receiving a file at any point in time. This is the rate shown by most applications when downloading file. Will vary depending on network condition. If a file size is F bits, and the transfer takes T seconds: Average throughput = F / T bits per second.

Throughput in Computer Networks Throughput is related to the data rate of physical links. Throughput between two hosts cannot be higher than the slowest link between the two hosts. Such link is called the bottleneck link. The bottleneck link can also be a high-capacity link that is shared by multiple client-server pairs. Each client-server pair will only get a fraction of the link’s rate. Given the file size, F, and the rate of the links between sender and receiver (R1, R2 … RN), the file transfer time, T, can be estimated: T = F / min {R1, R2 … RN)

ISPs and Internet Backbones

ISPs and Internet Backbones Access networks are connected to the Internet through a tiered hierarchy of ISPs (Internet Service Providers). Tier-1 ISPs: Also known as Internet backbone networks. Located at the top of the hierarchy. Very high link speed (up to 10Gbps). Directly connected to each of the other tier-1 ISPs. Connected to a large number of tier-2 ISPs. International coverage. Example: Sprint, MCI, AT&T (mostly US companies).

ISPs and Internet Backbones Tier-2 ISPs: Connected to a few of the tier-1 ISPs. Customer to tier-1 ISPs. Provide Internet access to users or lower-tier ISPs. Regional or national coverage. Most users and content providers are connected to lower-tier ISPs. Within an ISP network, the points at which an ISP connects to other ISPs are known as Point of Presence (POPs): A group of one or more routers in the ISP’s network at which routers from other ISPs can connect to.

Protocol Layers The tasks involved in communication can be very complex. To make things easier, the communication tasks are broken up into modules. A protocol architecture is the layered structure of these modules. Each layer performs subset of the tasks required for communication. Makes it easier for design and development. Examples of protocol architecture: OSI reference model TCP/IP protocol stack

Protocol Layers The Internet uses the TCP/IP protocol stack.

Protocol Data Units

The Bigger Picture