Computer Communication Networks

7A and 7B PESSE Instructor: Hari Computer Communication Networks

Books • Textbook: Data Communications and Networking (B. Forouzan)‏ • Reference: Computer Networks (J.F. Kurose, K.W. Ross)‏ • Reference: Introduction to Data Communication and Networking (W. Tomasi)‏

Introduction • Some terms related to networking: • Network, Telecommunication, Data communication, Jitter in transmission, Data representation, Simplex, Half-duplex, Full-duplex transmission • Network topology (mesh, star, bus, ring, hybrid)‏ • Categories of networks: LAN (Local Area Network), WAN (Wide Area Network), MAN (Metropolitan Area Network), wireless sensor networks, RAN (Radio Area Network – connects between UE and core network), VPN (Virtual Private Network)‏ • internet: when 2 or more networks are connected together • “The” Internet: a huge internet of hundreds of thousands of networks

History of the Internet • History of the internet: see http://en.wikipedia.org/wiki/History_of_the_Internet • 1960s: Email, Telnet • 1971: FTP • 1979: Usenet • 1983: Internet, based on TCP/IP protocol suite • 1988: IRC • 1992: WWW • 1993: Blog • 1994: Yahoo! Web directory • 1995: Wikis were developed • 1998: Google • 2001: Wikipedia • 2004: Facebook • 2005: Youtube • 2006: Twitter

The Syllabus • Networking models (OSI and TCP/IP), various types of transmission media • Data link control: Ensuring that data is reliably sent across physical channels • Multiple Access: Controlling simultaneous access to the medium • IEEE Standards for wired and wireless LANs • Connecting LANs together • Network layer and addressing • Routing protocols • UDP, TCP, DNS

Layered Tasks • The task of a communication network is to transmit data from one point to another • Although the entire process could be implemented in hardware, this would become very complex and tedious • Hence there is a division of tasks into different software “layers”, with each layer using the services of the other layers

Layered Tasks • Layering is common even in application software on computers. • For example, the layers might be Application, Operating System (OS), Basic Input Output System (BIOS), and computer hardware • The Application uses the services of the OS, the OS uses the services of the BIOS, and the BIOS controls the system hardware • Layering can be seen even in conventional systems like the postal system etc • What are other examples of layered systems?

Layered Tasks: The Postal System

OSI model • A layered architecture for data communication over networks • Never really became popular, because TCP/IP overtook it • Called “International Standards Organisation Open Systems Interconnection (ISO OSI) model” • Allows complete interoperability between otherwise incompatible systems

Layers of the OSI model • Application • Presentation • Session • Transport • Network • Data link • Physical

How OSI Works • Each layer provides some “services” (routines)‏ • On a single node (computer), each layer uses the services of the layer below it; there is an interface between adjacent layers • Communication across nodes can be effected at the same layer; this is called a “peer-to-peer process” (peer means having the same rank in the hierarchy of layers)‏ • The set of layers is called the “protocol stack” sometimes

How OSI Works • To actually transmit data, the data must be propagated all the way down the stack of layers onto the physical medium • Higher layers are not engaged on the intermediate nodes

OSI: Physical Layer • Physical Layer deals with mechanical and electrical specificationsof the interface and transmission medium • Also defines the procedures and functions that physical devices and interfaces have to perform for transmission to occur • Defines encodings of bits into waveforms suitable for transmission onto the channel • Other functions: Data rate setting, bit-synchronization, network topology, transmission mode (simplex, half-duplex, full-duplex)‏

OSI: Data Link Layer • The physical layer is a raw communication facility. The Data Link Layer transforms this into a reliable communication link that appears error free to the higher layers

OSI: Data Link Layer • Framing: division of higher layer data into frames suitable for transmission by the physical medium • Physical Addressing: to correctly differentiate between destinations on the network, a header is added to the frame • Flow Control: To ensure that data buffers don't overflow • Error control: Retransmission of lost or damaged frames; addition of an error-control trailer to the frame • Access Control: This is also called Medium Access Control (MAC) and decides which device has control of the link at any time

OSI: Network Layer • Data link layer: responsible for data transmission from source to destination within the same network • Network layer: responsible for end-to-end delivery across possibly multiple interconnected networks • Internetworks: networks of networks • Handles logical addressing (like the IP address)‏ • Handles routing across the network • Transmission is in packets. Why packetize?

OSI: Transport Layer • Network layer oversees source-to-destination delivery of individual packets • The transport layer applies higher-level error control and flow control to reassemble the packets in the correct order into the original message. Ensures process-to-process delivery of entire message • Other functions: service point (port) addressing, segmentation and reassembly, connection control, flow control, error control

OSI: Session Layer • Dialog control: allows 2 systems to enter into a dialog, allows communication to take place in either half or full duplex mode • Synchronization: Allows addition of synchronization points to a data stream.

OSI: Presentation Layer • “Presentation” of data • Translation: of one machine's hardware data format to another machine's hardware data format (e.g. Big-Endian to Little-Endian number representation)‏ • Encryption • Data compression

OSI: Application Layer • Enables the user (human or software) to access network services. • Applications: email, chat, file transfer, web browsing, etc

TCP/IP Protocol Suite • Layers in the TCP/IP protocol suite do not exactly match the OSI model (TCP/IP was developed before OSI)‏ • 5 layers may be assumed: Physical, Data Link, Network, Transport, Application • Application = Application + Presentation + Session of the OSI model

Comparison of TCP/IP and OSI models • Application + Presentation + Session = Application (SMTP, FTP, HTTP, DNS, SNMP, TELNET)‏ • Transport = SCTP, TCP, UDP • Network = IP, ICMP, IGMP, RARP, ARP • Data Link + Physical = Protocols defined by the underlying networks • See Figure on page 43 of Furuozan

TCP/IP: Physical and Data Link Layers • No specific protocol defined at this level by TCP/IP • Supports all standard and proprietary protocols (like IEEE 802.1, 802.11)‏

TCP/IP: Network Layer • At the network layer, TCP/IP supports IP: Internetworking protocol. IP depends on ARP, RARP, ICMP, IGMP • IP: Internetworking Protocol • A “bare-bones” unreliable and connectionless protocol that provides necessary infrastructure for the higher layers • Data is sent in packets called datagrams • Datagrams may travel along different routes and arrive out of sequence or be duplicated • IP cannot reorder datagrams and recombine into data once the datagrams have been received • Is a host-to-host protocol – delivers from one physical device to another

TCP/IP: Address Resolution Protocol • Every node on a network has a physical address • Nodes on a network also have logical addresses i.e. IP addresses that may change • A lookup needs to be performed to find out the physical or hardware address in a network (eg. a LAN) corresponding to a given logical or internet address. • This is performed using the mechanism of ARP

TCP/IP: RARP • Reverse ARP • Enables a node to determine its logical address given its physical address. • Application: when a node is newly added to a network, it sends a RARP request to determine its logical address

TCP/IP: ICMP • ICMP = Internet Control Message Protocol • Chiefly used by the operating systems of networked computers to send error messages • Not used to send data between computers • Not usually used in applications; exceptions are Ping and TraceRoute (try these!)‏

TraceRoute output • hari@hari-laptop:~$ traceroute www.google.com • traceroute to www.google.com (209.85.231.104), 30 hops max, 40 byte packets • 1 192.168.1.1 (192.168.1.1) 1.561 ms 2.110 ms 2.645 ms • 2 117.192.224.1 (117.192.224.1) 243.950 ms 245.212 ms 248.918 ms • 3 218.248.160.234 (218.248.160.234) 251.153 ms 252.183 ms * • 4 * * * • 5 * * * • 6 * * 59.163.25.242.static.vsnl.net.in (59.163.25.242) 94.500 ms • 7 121.240.0.5.static-Mumbai.vsnl.net.in (121.240.0.5) 485.301 ms 487.769 ms 490.202 ms • 8 216.239.43.214 (216.239.43.214) 492.746 ms 495.336 ms 497.729 ms • 9 72.14.232.93 (72.14.232.93) 502.103 ms 209.85.241.52 (209.85.241.52) 519.332 ms 72.14.232.93 (72.14.232.93) 507.087 ms • 10 72.14.238.74 (72.14.238.74) 531.687 ms 66.249.94.90 (66.249.94.90) 510.220 ms 72.14.238.74 (72.14.238.74) 532.143 ms • 11 maa03s01-in-f104.1e100.net (209.85.231.104) 529.126 ms 530.559 ms 531.391 ms

TCP/IP: IGMP • IGMP = Internet Group Messaging Protocol • A mechanism for multicast (the simultaneous transmission of data to a group of recipients)‏ • What kind of applications would require multicast?

TCP/IP: Transport Level Protocols • UDP: User Datagram Protocol • Process-to-process protocol • Adds port address, checksum error control and length information to data from upper layer • TCP: Transmission Control Protocol • Is a “stream” transport protocol. This means that a connection must be established between both ends of a transmission before either can transmit data • TCP segments data at the transmitting end • The segments are numbered and carried within IP datagrams

TCP/IP: Application Layer • TCP/IP Application Layer = OSI Session + Presentation + Application • Sample applications: SMTP (Simple Mail Transfer Protocol), FTP (File Transfer Protocol), HTTP (Hypertext Transfer Protocol), DNS (Domain Name System), SNMP (Simple Network Management Protocol), TELNET

TCP/IP: Addressing • Four levels of addressing: Physical, Logical, Port and specific address • Why the need for a hierarchy of addressing? • Physical address: the address of the node in its LAN or WAN; included in the frame at the data link layer • Logical address: a universal addressing system. Called the IP address. • ARP is used for address conversion

TCP/IP: Addressing • Port Address: Once data arrives on a particular node, it must be routed to the correct application, or process. This is effected using the port address. Eg. Port address for HTTP transactions is 80. • Specific addressing: User-friendly address formats used by applications; these are converted to the lower level formats as required • Eg: yourname@gmail.com • http://www.google.com

Telephone and Cable Networks for Data Transmission • Telephone networks: for voice communication • Digital data: transferred over telephone lines using dial-up MODEMs (Modulator-Demodulator)‏ • DSL: Digital Subscriber Line, a service provided for access to the Internet through telephone lines. Is much faster than dialup modems • Cable networks: used for providing TV signals to subscribers. Also possible to use Cable networks for giving access to the Internet

Telephone networks • Telephone network: invented in 1800s • Called POTS: Plain Old Telephone System • POTS was originally all-analog, but is now mixed digital and analog • Components: Local loops, Trunks, Switching offices (end offices, tandem offices, regional offices)‏ • Local loop: a twisted pair cable that connects the subscriber telephone to the nearest end office. Bandwidth = 4 khz

Telephone networks • Trunks: Transmission media that handle the communication between offices. Many connections are handled (hundreds to thousands)‏ • Switching office: To avoid having a permanent physical link between subscribers, switches are needed to set up the connection. This is done in the switching office. A switch connects several local loops or trunks and allows a connection between subscribers

Telephone networks: Signalling • Signalling: The use of signals for controlling communication; the information exchange concerning the establishment and control of a telecommunication circuit and the management of the network • In-band signalling: Use of voice bandwidth for signalling. Eg: Rotary dialling, DTMF • Out-of-band signalling: The voice bandwidth and signalling bandwidth are separate (eg SS7)‏

Telephone networks: Signalling • Signalling system: provides dial tone, maintains and monitors call, keeps billing information, caller ID, voice mail, etc • Signalling System 7: Out of band system (avoids security problems of SS5, which used in-band multifrequency signalling)‏ • SS7 is a layered protocol (5 layers). An example of the physical layer is “T1” (1.544 Mbps) carrier. T1: (24 channels/frame * 8 bits/channel)*8000 frames/second = 1.544 Mbps

Dialup modem • Traditional telephone line frequency range: 300-3300 Hz • Baud rate: number of symbols per second transferred • Modem: Modulator + Demodulator. Modulator: conversion of binary data to modulated waveforms. Demodulator: Conversion of Waveforms to binary data. • Modem standards: V32 (Trellis coded modulation) 32 QAM, of which 1 bit is error correcting. So 4 bits/symbol*2400 baud =9600 bps • Also: V32bis, V34bis,V90, V92

DSL • Digital Subscriber Line – supports high speed data communication over local loops • ADSL: Asymmetric DSL (Asymmetric means unequal upstream and downstream rates, ~ .5 Mbps and 8Mbps). Existing local loops can actually handle upto 1.1 Mhz, but are filtered to 4 kHz at the end office (why?). This filter is removed for ADSL • ADSL is adaptive: actual data rate depends on condition of local loop line • ADSL Modulation: DMT (Discrete Multitone Technique) = QAM + FDM. Here, the total bandwidth of 1.1 Mhz is divided into 256 channels or bins of 4.3125 kHz (FDM). Based on the SNR for each channel, the data rate is set on a channel by channel basis – some of the carriers are deleted in the bins where noise is large • ADSL uses interleaving of Datalink frames; QAM is the modulation scheme used in each channel (15 bits/baud)‏

HDSL • HDSL, SDSL, VDSL • HDSL: High bit rate DSL. T1 uses AMI (Alternate Mark Inversion) coding; this is susceptible to attenuation at high frequencies. HDSL uses 2B1Q encoding to achieve greater repeater distance at 1.544 Mbps. T1 line ~ 1km. HDSL line ~ 3.86 km. Two twisted pairs used for full-duplex transmission (2B1Q). • SDSL: Symmetric Digital Subscriber Line. Symmetric means same data rate in both directions (768 kbps in each direction). Is HDSL with one twisted pair cable, but still supports full duplex. (2B1Q). • VDSL: Very high bit rate DSL; uses coaxial, fiber-optic or twisted-pair cabling for short distances. Downstream: 25-55Mbps. Upstream: 3.2 Mbps

Cable TV for data transmission • DSL: uses existing unshielded twisted pair cable, susceptible to noise. So we use cable TV (a coaxial system from end to end)‏ • Hybrid Fiber-Coaxial N/w used: Fiber from Regional Cable Head (RCH) to fiber node; coaxial cable through the neighbourhood • RCH: serves up to 400,000 subscribers • Communication in the traditional cable network is unidirectional. But we make this bidirectional by using bidirectional amplifiers • Bandwidth: 5 to 750 Mhz. Divided into video, upstream and downstream bandwidth • Downstream: 64 QAM modulation, ~30 Mbps • Upstream: QPSK modulation, ~12 Mbps • Sharing of bandwidth needs to be done both upstream and downstream. • Read up on cable nws from page 257 (assignment)‏

Cable TV for data transmission • Downstream: 64 QAM modulation, ~30 Mbps • Upstream: QPSK modulation, ~12 Mbps • Sharing of bandwidth needs to be done both upstream and downstream. • Read up on cable nws from page 257 (assignment)‏

Unit 2: Data Link Control • Data link layer has 2 main functions: • Data link control (handles adjacent node-to-node communication (WAN) or communication within the same network segment (LAN))‏ • Medium access control (how to share the data link)‏ • Data link control: framing, flow and error control

Framing • Physical layer functions: mechanical & electrical specifications, modulation or encoding scheme, data rate setting, synchronization, topology, transmission mode (simplex, half- or full-duplex)‏ • Most important functions: modulation scheme, synchronization • Physical layer: gets bits across the channel • Data link layer: packs bits into frames suitable for transport across the medium

Framing • Framing: in the postal system, in the form of an envelope • Framing: other examples of framing. Written text? • Framing in the data link layer: a source address and a destination address are added. Why is the source address needed? • Basic reason for framing: error in a frame affects only that frame, not the whole message

Framing • Types of framing: Fixed-size and variable size • Fixed-size framing: ATM WAN • Variable sized framing: frame size needs to be specified • Character-oriented protocol: special characters are present at the beginning and end of the frame. Drawback: This is OK for text applications, but not OK when a general stream of binary data is used, because the marker may show up as data. To guard against this, an escape character is used to signify that the next character (possibly marker) in the frame is actually data (byte stuffing). The escape character is removed from the payload. To represent Esc character in data, Esc Esc is used in the frame. This prevents big messup when Esc+Flag is part of the data. Character oriented framing is used in PPP protocol for dialup connection to the Internet.

Framing • Variable-Size framing • Bit-oriented protocol: Zero-bit insertion • First used in IBM's SDLC (later called HDLC). Also used in USB to prevent transmission of too many 1s. • Now we work with the actual bitstream rather than a stream of characters. • The pattern 01111110 is used at the beginning and end of the frame • If 011111 is encountered in the data, mindlessly insert 0 • So: 0111111 --> 01111101 • 0111110-->01111100, so that receiver can distinguish between stuffed and unstuffed bits • Main disadvantage: data rate is unpredictable & depends on data

Flow and Error Control • Flow control + Error control = Data link control • Flow control • The flow of data must not be allowed to overwhelm the receiver. How is it possible for the receiver to be overwhelmed? (Limited processing speed, limited buffers)‏ • How much data to send before waiting for an acknowledgement? • If too much data: please send fewer frames or stop • Flow control: a set of procedures used to restrict the amount of data that the sender can send before waiting for ACK. (Remember, network congestion and congestion control are at the network level)‏

Flow and Error Control • Error control • Error control = error detection + error correction • If error, please retransmit frame: this is called ARQ (Automatic Repeat Request)‏

Computer Communication Networks