Department of Electronics & communication COMPUTER NETWORKS(CS1302) by A.Asha AIM:
– To introduce the concept ,terminologies and technologies used in modern data communication and computer networking.
• OBJECTIVES: – To introduce the students the functions of different layers. – To introduce IEEE standard employed in computer networking. – To make students to get familiarized with different protocols and network components
Unit I • DATA COMMUNICATIONS
8 • Components – Direction of Data flow – networks – Components and Categories – types of Connections – Topologies –Protocols and Standards – ISO / OSI model – Transmission Media – Coaxial Cable – Fiber Optics – Line Coding – Modems – RS232 Interfacing sequences
Line Configuration Topology • physical arrangement of stations on medium
– point to point - two stations • such as between two routers / computers – multi point - multiple stations • traditionally mainframe computer and terminals • now typically a local area network (LAN)
Line Configuration - Duplex • simplex – one direction eg. television
• half duplex (two-way alternate) – only one station may transmit at a time – requires one data path
• full duplex (two-way simultaneous) – simultaneous transmission and reception between two stations – requires two data paths • separate media or frequencies used for each direction or echo canceling
Transmission Terminology • data transmission occurs between a transmitter & receiver via some medium • guided medium
– eg. twisted pair, coaxial cable, optical fiber
• unguided / wireless medium – eg. air, water, vacuum
Transmission MediaOverview • guided - wire / optical fibre • unguided - wireless • characteristics and quality
determined by medium and signal – in unguided media - bandwidth produced by the antenna is more important – in guided media - medium is more important
Transmission Characteristics of Guided Media
Frequency Range
Twisted pair (with loading) Twisted pairs (multipair cables) Coaxial cable
0 to 3.5 kHz
Optical fiber
Typical Attenuatio n 0.2 dB/km
Typical Delay
Repeater Spacing
50 µs/km
2 km
@ 1 kHz 0 to 1 MHz
0.7 dB/km @ 1 kHz
5 µs/km
2 km
0 to 500 MHz
7 dB/km @ 10 MHz
4 µs/km
1 to 9 km
186 to 370 THz
0.2 to 0.5 dB/km
5 µs/km
40 km
Twisted Pair - Transmission Characteristics • analog – needs amplifiers every 5km to 6km
• digital – can use either analog or digital signals – needs a repeater every 2-3km
• limited distance • limited bandwidth (1MHz) • limited data rate (100MHz) • susceptible to interference and noise
Unshielded vs Shielded • unshielded Twisted Pair (UTP) – – – –
ordinary telephone wire cheapest easiest to install suffers from external EM interference
• shielded Twisted Pair (STP)
– metal braid or sheathing that reduces interference – more expensive – harder to handle (thick, heavy)
• in a variety of categories - see EIA-568
Near End Crosstalk • coupling of signal from one pair to
another • occurs when transmit signal entering the link couples back to receiving pair • ie. near transmitted signal is picked up by near receiving pair
Coaxial Cable
Optical Fiber - Benefits • greater capacity – data rates of hundreds of Gbps
• smaller size & weight • lower attenuation • electromagnetic isolation • greater repeater spacing – 10s of km at least
Optical Fiber - Transmission Characteristics • uses total internal reflection to transmit light
– effectively acts as wave guide for 1014 to 1015 Hz
• can use several different light sources – Light Emitting Diode (LED)
• cheaper, wider operating temp range, lasts longer
– Injection Laser Diode (ILD)
• more efficient, has greater data rate
• relation of wavelength, type & data rate
Cable Modems • dedicate two cable TV channels to data transfer • each channel shared by number of subscribers, using statistical TDM • Downstream – – –
cable scheduler delivers data in small packets active subscribers share downstream capacity also allocates upstream time slots to subscribers
• Upstream – user requests timeslots on shared upstream
Cable Modem Scheme
UNIT II 12 • DATA LINK LAYER • Error – detection and correction –
Parity – LRC – CRC – Hamming code – Flow Control and Error control: stop and wait – go back N ARQ – selective repeat ARQ- sliding window techniques – HDLC. • LAN: Ethernet IEEE 802.3, IEEE 802.4, and IEEE 802.5 – IEEE 802.11–
responsibilities of data link layer • a) • b) • c) • d) • e)
Framing Physical addressing Flow control Error control Access control
2.1 Error – detection and correction
• 2 types of errors • a) Single-bit error. • b) Burst-bit error. • parity – parity bit set so character has even (even parity) or odd (odd parity) number of ones – even number of bit errors goes undetected
Error Detection Process
4 types of redundancy checks • a) Vertical redundancy checks (VRC). The •
most common and least expensive mechanism for error detection is the vertical redundancy check (VRC) often called a parity check. In this technique a redundant bit 3 called a parity bit, is appended to every data unit so, that the total number of 0’s in the unit (including the parity bit) becomes even.
• b) Longitudinal redundancy checks (LRC). •
In longitudinal redundancy check (LRC), a block of bits is divided into rows and a redundant row of bits is added to the whole block.
• c) Cyclic redundancy checks (CRC). A CRC
checker functions exactly like a generator. After receiving the data appended with the CRC it does the same modulo-2 division. If the remainder is all 0’s the CRC is dropped and the data accepted. Otherwise, the received stream of bits is discarded and
Cyclic Redundancy Check • • • • • •
one of most common and powerful checks The sender follows these steps a) The units are divided into k sections each of n bits. b) All sections are added together using 2’s complement to get the sum. c) The sum is complemented and become the checksum. d) The checksum is sent with the data.
Error Correction Process
Flow Control • ensure sending entity does not overwhelm receiving entity
– by preventing buffer overflow
• influenced by: – transmission time • time taken to emit all bits into medium – propagation time • time for a bit to traverse the link
• assume here no errors but varying
Stop and Wait • source transmits frame • destination receives frame and
replies with acknowledgement (ACK) • source waits for ACK before sending next • destination can stop flow by not send ACK • works well for a few large frames • Stop and wait becomes inadequate if
Stop and Wait Link Utilization
Sliding Windows Flow Control • allows multiple numbered frames to be in
transit • receiver has buffer W long • transmitter sends up to W frames without ACK • ACK includes number of next frame expected • sequence number is bounded by size of field (k) – frames are numbered modulo 2k – giving max window size of up to 2k - 1
Sliding Window Diagram
Sliding Window Example
Error Control • detection and correction of errors such as:
– lost frames – damaged frames
• common techniques use: – – – –
error detection positive acknowledgment retransmission after timeout negative acknowledgement &
Automatic Repeat Request (ARQ) • collective name for such error control mechanisms, including: • stop and wait • go back N • selective reject (selective retransmission)
Stop and Wait • source transmits single frame • wait for ACK • if received frame damaged, discard it – transmitter has timeout – if no ACK within timeout, retransmit
• if ACK damaged,transmitter will not recognize it – – –
transmitter will retransmit receive gets two copies of frame use alternate numbering and ACK0 /
Stop and wait see example with both types of errors pros and cons simple inefficient
Go Back N • based on sliding window • if no error, ACK as usual • use window to control number of outstanding frames • if error, reply with rejection
– discard that frame and all future frames until error frame received correctly – transmitter must go back and retransmit that frame and all subsequent frames
Go Back N - Handling • Damaged Frame – error in frame i so receiver rejects frame i – transmitter retransmits frames from i
• Lost Frame – frame i lost and either • transmitter sends i+1 and receiver gets
frame i+1 out of seq and rejects frame i • or transmitter times out and send ACK with P bit set which receiver responds to with
Go Back N - Handling • Damaged Acknowledgement – receiver gets frame i, sends ack (i+1) which is lost – acks are cumulative, so next ack (i+n) may arrive before transmitter times out on frame i – if transmitter times out, it sends ack with P bit set – can be repeated a number of times before a reset procedure is initiated
• Damaged Rejection – reject for damaged frame is lost
Selective Reject • also called selective retransmission • only rejected frames are retransmitted • subsequent frames are accepted by the receiver and buffered • minimizes retransmission • receiver must maintain large enough buffer • more complex logic in transmitter • hence less widely used • useful for satellite links with long
Go Back N vs Selective Reject
High Level Data Link Control (HDLC) • an important data link control
protocol • specified as ISO 33009, ISO 4335 • station types: – Primary - controls operation of link – Secondary - under control of primary station – Combined - issues commands and responses
HDLC Transfer Modes • Normal Response Mode (NRM) – unbalanced config, primary initiates transfer – used on multi-drop lines, eg host + terminals
• Asynchronous Balanced Mode (ABM) – balanced config, either station initiates transmission, has no polling overhead, widely used
• Asynchronous Response Mode (ARM) – unbalanced config, secondary may initiate transmit without permission from primary, rarely used
HDLC Frame Structure • synchronous transmission of frames • single frame format used
Address Field • identifies secondary station that sent or will receive frame • usually 8 bits long • may be extended to multiples of 7 bits
– LSB indicates if is the last octet (1) or not (0)
• all ones address 11111111 is broadcast
Control Field • different for different frame type – Information - data transmitted to user (next layer up) •Flow and error control piggybacked on information frames – Supervisory - ARQ when piggyback not used – Unnumbered - supplementary link control
• first 1-2 bits of control field identify frame type
Control Field • use of Poll/Final bit depends on context • in command frame is P bit set to1 to solicit (poll) response • •
from peer in response frame is F bit set to 1 to indicate response to soliciting command seq number usually 3 bits – can extend to 8 bits as shown below
Information & FCS Fields • Information Field – in information and some unnumbered frames – must contain integral number of octets – variable length
• Frame Check Sequence Field (FCS) – used for error detection – either 16 bit CRC or 32 bit CRC
HDLC Operation • consists of exchange of information,
supervisory and unnumbered frames • have three phases – initialization • by either side, set mode & seq – data transfer • with flow and error control • using both I & S-frames (RR, RNR, REJ, SREJ) – disconnect • when ready or fault noted
Timers and time registers in FDDI. • Time registers – – –
Synchronous allocation(SA) Target token rotation time(TTRT) Absolute maximum time(AMT)
• Timers – –
Token rotation timer(TRT) Token holding timer(THT)
Ethernet. • Access method :CSMA/CD • Addressing • Electrical specification • Frame format • Implementation • 10 base 5 :Thick Ethernet • 10 base 2 :Thin Ethernet • 10 base T :Twisted-pair Ethernet • 1 base 5 :Star LAN
UNIT III 10 • NETWORK LAYER • Internetworks - Packet Switching and Datagram approach – IP addressing methods – Subnetting – Routing – Distance Vector Routing – Link State Routing – Routers
Packet Switching • circuit switching was designed for
voice • packet switching was designed for data • transmitted in small packets • packets contains user data and control info – user data may be part of a larger message
Advantages • line efficiency – single link shared by many packets over time – packets queued and transmitted as fast as possible
• data rate conversion – stations connects to local node at own speed – nodes buffer data if required to equalize rates
Switching Techniques • Datagram approach • Virtual circuit approach • Switched virtual circuit(SVC) • Permanent virtual circuit(PVC)
• Circuit – switched connection versus virtual – circuit connection – Path versus route – Dedicated versus shared
Virtual Circuits v Datagram • virtual circuits – network can provide sequencing and error control – packets are forwarded more quickly – less reliable
• datagram – no call setup phase – more flexible – more reliable
Routing in Packet Switched Network • key design issue for (packet) switched
networks • select route across network between end nodes • characteristics required: – – – – – –
correctness simplicity robustness stability fairness optimality
Routing Strategies - Fixed Routing • use a single permanent route for each source to destination pair • determined using a least cost algorithm • route is fixed
– at least until a change in network topology – hence cannot respond to traffic changes
• advantage is simplicity
Distance vector routing and link state routing. • Distance vector routing – – – – –
Sharing information Routing table Creating the table Updating the table Updating algorithm
– – – – – –
Information sharing Packet cost Link state packet Getting information about neighbors Initialization Link state database
• Link state routing
Bridges • Types of bridges – – –
Simple bridge Multiport bridge Transparent bridge
Subnetting • Three levels of hierarchy • Masking – Masks without subnetting – Masks with subnetting
• Finding the subnetwork address – Boundary level masking – Non-boundary level masking
UNIT IV • TRANSPORT LAYER
8 • Duties of transport layer – Multiplexing – Demultiplexing – Sockets – User Datagram Protocol (UDP) – Transmission Control Protocol (TCP) – Congestion Control – Quality of services (QOS) – Integrated Services.
Duties of transport layer • end-to-end data transfer service • shield upper layers from network details • reliable, connection oriented – has greater complexity – eg. TCP
• best effort, connectionless – datagram – eg. UDP
Multiplexing • of upper layers (downward multiplexing)
– so multiple users employ same transport protocol – user identified by port number or service access point
• may also multiplex with respect to network services used (upward multiplexing)
– eg. multiplexing a single virtual X.25
Sockets • process sends/receives messages to/from its socket
• socket analogous to mailbox • sending process relies on transport
infrastructure which brings message to socket at receiving process
User Datagram Protocol (UDP) • connectionless service for application level procedures specified in RFC 768
– unreliable – delivery & duplication control not guaranteed
• reduced overhead • least common denominator service • uses: – – – –
inward data collection outward data dissemination request-response real time application
TCP • Transmission Control Protocol (RFC 793) • connection oriented, reliable communication • over reliable and unreliable (inter)networks • two ways of labeling data: • data stream push
– user requires transmission of all data up to push flag – receiver will deliver in same manner – avoids waiting for full buffers
TCP Services • a complex set of primitives: – incl. passive & active open, active open with data, send, allocate, close, abort, status – passive open indicates will accept connections – active open with data sends data with open
• and parameters: – incl. source port, destination port &
TCP Header
TCP and IP • not all parameters used by TCP are in its header • TCP passes some parameters down to IP – – – – –
precedence normal delay/low delay normal throughput/high throughput normal reliability/high reliability security
TCP Mechanisms Connection Establishment • three way handshake – SYN, SYN-ACK, ACK
• connection determined by source
and destination sockets (host, port) • can only have a single connection between any unique pairs of ports • but one port can connect to multiple different destinations (different ports)
TCP Mechanisms Data Transfer • data transfer a logical stream of octets • octets numbered modulo 223 • flow control uses credit allocation of
number of octets • data buffered at transmitter and receiver – sent when transport entity ready – unless PUSH flag used to force send
• can flag data as URGENT, sent
immediately • if receive data not for current connection,
TCP Mechanisms Connection Termination • graceful close – –
TCP user issues CLOSE primitive transport entity sets FIN flag on last segment sent with last of data
• abrupt termination by ABORT primitive
– entity abandons all attempts to send or receive data – RST segment transmitted to other end
TCP Implementation Options • TCP standard precisely specifies
protocol • have some implementation policy options: – – – – –
send deliver accept retransmit acknowledge
Congestion Control • flow control also used for congestion control
– recognize increased transit times & dropped packets – react by reducing flow of data
• RFC’s 1122 & 2581 detail extensions – Tahoe, Reno & NewReno implementations
• two categories of extensions:
Retransmission Timer Management • static timer likely too long or too
short • estimate round trip delay by observing pattern of delay for recent segments • set time to value a bit greater than estimate • simple average over a number of segments
Exponential RTO Backoff • timeout probably due to congestion – dropped packet or long round trip time
• hence maintaining RTO is not good idea • better to increase RTO each time a segment is re-transmitted – RTO = q*RTO – commonly q=2 (binary exponential
Karn’s Algorithm • if segment is re-transmitted, ACK may be for:
– first copy of the segment (longer RTT than expected) – second copy
• no way to tell • don’t measure RTT for re-transmitted
segments • calculate backoff when re-transmission occurs
Window Management • slow start – larger windows cause problem on connection created – at start limit TCP to 1 segment – increase when data ACK, exponential growth
• dynamic windows sizing on congestion – when a timeout occurs perhaps due to congestion – set slow start threshold to half current congestion window – set window to 1 and slow start until threshold
Window Management
Fast Retransmit Fast Recovery • retransmit timer rather longer than RTT • if segment lost TCP slow to retransmit • fast retransmit
– if receive 4 ACKs for same segment then immediately retransmit since likely lost
• fast recovery – lost segment means some congestion
Effects of Congestion
Mechanisms for Congestion Control
Backpressure • if node becomes congested it can slow
down or halt flow of packets from other nodes – cf. backpressure in blocked fluid pipe – may mean that other nodes have to apply control on incoming packet rates – propagates back to source
• can restrict to high traffic logical
connections • used in connection oriented nets that allow hop by hop congestion control (eg.
Choke Packet • a control packet – – –
generated at congested node sent to source node eg. ICMP source quench • from router or destination • source cuts back until no more source quench message • sent for every discarded packet, or anticipated
• is a rather crude mechanism
Implicit Congestion Signaling • transmission delay increases with
congestion • hence a packet may be discarded • source detects this implicit congestion indication • useful on connectionless (datagram) networks – eg. IP based • (TCP includes congestion and flow control see chapter 17)
Explicit Congestion Signaling • network alerts end systems of
increasing congestion • end systems take steps to reduce offered load • Backwards – congestion avoidance notification in opposite direction to packet required
• Forwards
– congestion avoidance notification in same direction as packet required
Integrated Services • changes in traffic demands require variety of quality of service
– eg. internet phone, multimedia, multicast
• new functionality required in routers • new means of requesting QoS • IETF developing a suite of Integrated
Services Architecture (ISA) standards • RFC 1633 defines overall view of ISA
ISA Approach • IP nets control congestion by – routing algorithms – packet discard
• ISA provides enhancements to
traditional IP • in ISA associate each packet with a flow • ISA functions: – admission control – routing algorithm
ISA in Router
ISA Services • Guaranteed – assured data rate – upper bound on queuing delay – no queuing loss
• Controlled load – approximates best effort behavior on unloaded net – no specific upper bound on queuing delay – very high delivery success
• Best Effort – traditional IP service
Token Bucket Scheme
Queuing Discipline • traditionally FIFO – no special treatment for high priority flow packets – large packet can hold up smaller packets – greedy connection can crowd out less greedy connection
• need some form of fair queuing – – – –
multiple queues used on each output port packet is placed in queue for its flow round robin servicing of queues can have weighted fair queuing
UNIT V • APPLICATION LAYER
7 • Domain Name Space (DNS) • SMTP • FDP • HTTP • WWW • Security
5. 1 DNS The Internet Directory Service • the Domain Name Service (DNS) provides mapping between host name & IP address • defined in RFCs 1034 / 1035 • key elements – – – –
domain name space DNS database name servers name resolvers
Domain Names
DNS Database • hierarchical database • containing resource records (RRs) • features – – –
variable-depth hierarchy for names distributed database distribution controlled by database
• provides name-to-address directory service for network applications
Resource Records (RRs)
DNS Operation
DNS Server Hierarchy • DNS database is distributed hierarchically
– may extend as deep as needed
• any organization owning a domain
can run name servers • each server manages authoritative name data for a zone • 13 root name servers at top of hierarchy share responsibility for top
Name Resolution • query begins with name resolver on host • knows name/address of local DNS server • given a name request, the resolver can: – return name from cache if already known – send DNS query to local server which may return answer, or query other servers
5.2 SMTP • RFC 821 • not concerned with format of messages or data
– covered in RFC 822 (see later)
• SMTP uses info written on envelope of mail – message header
• does not look at contents – message body
• except: – standardize message character set to 7 bit
Basic Operation • email message is created by user agent program (mail client), and consists of:
– header with recipient’s address and other info – body containing user data
• messages queued and sent as input to SMTP sender program
– yypically a server process (daemon on UNIX)
SMTP Mail Flow
Mail Message Contents • each queued message has two parts • message text – RFC 822 header with envelope and list of recipients – message body, composed by user
• list of mail destinations – – – –
derived by user agent from header may be listed in header may require expansion of mailing lists may need replacement of mnemonic names with mailbox names
• if BCCs indicated, user agent needs to
SMTP Sender • takes message from queue • transmits to proper destination host – via SMTP transaction – over one or more TCP connections to port 25
• host may have multiple senders active • host must create receivers on demand • when delivery complete, sender
SMTP Protocol - Reliability • used to transfer messages from
sender to receiver over TCP connection • attempts to provide reliable service • no guarantee to recover lost messages • no end to end acknowledgement to originator • error indication delivery not
SMTP Receiver • accepts arriving message • places in user mailbox or copies to outgoing queue for forwarding • receiver must: – verify local mail destinations – deal with errors
• sender responsible for message until receiver confirm complete transfer
– indicates mail has arrived at host, not
SMTP Forwarding • mostly direct transfer from sender host to receiver host • may go through intermediate machine via forwarding capability – sender can specify route – target user may have moved
SMTP Replies • positive completion reply (2xx) – –
e.g. 220 <domain> Service ready e.g. 250 Requested mail action okay, completed
• positive intermediate reply (3xx)
– e.g. 354 Start mail input; end with .
• transient negative completion reply (4xx)
– e.g. 452 Requested action not taken: insufficient system storage
• permanent negative completion reply (5xx)
– e.g. 500 Syntax error, command unrecognized – e.g. 550 Requested action not taken: mailbox unavailable
FTP • Transfer a file from one system to another. • TCP connections • Basic model of FTP
5.4 Hypertext Transfer Protocol HTTP • base protocol for World Wide Web • for any hypertext client/server application • is a protocol for efficiently transmitting information to make hypertext jumps
– can transfer plain text, hypertext, audio, images, and Internet accessible information
HTTP Overview • transaction oriented client/server protocol • between Web browser (client) and Web server • uses TCP connections • stateless – – –
each transaction treated independently each new TCP connection for each transaction terminate connection when transaction complete
• flexible format handling
HTTP Operation - Caches • often have a web cache • stores previous requests/ responses • may return stored response to subsequent requests • may be a client, server or intermediary system • not all requests can be cached
Intermediate HTTP Systems
HTTP Messages
HTTP Messages BNF Format HTTP-Message = Simple-Request | SimpleResponse | Full-Request | Full-Response Full-Request = Request-Line *( General-Header | Request-Header | EntityHeader ) CRLF [ Entity-Body ] Full-Response = Status-Line *( General-Header | Response-Header | EntityHeader ) CRLF [ Entity-Body ] Simple-Request = "GET" SP Request-URL CRLF
HTTP General Header Fields • Cache-Control • Connection • Data • Forwarded • Keep-Alive • Mime-Version • Pragma • Upgrade
Request Methods • request-line has – – –
method Request URL HTTP version
– Request-Line = Method Request-URL HTTPVersion CRLF
• HTTP/1.1 methods: – OPTIONS, GET, HEAD, POST, PUT, PATCH, COPY, MOVE, DELETE, LINK, UNLINK, TRACE, WRAPPED, Extension-
Status Codes • informational - headers only • successful - headers & body if
relevant • redirection - further action needed • client error - has syntax or other error • server error - failed to satisfy valid request
Response Header Fields • Location • Proxy-Authentication • Public • Retry-After • Server • WWW-Authenticate
Entity Header Fields • Allow • Content-Encoding • Content-Language • Content-Length • Content-MD5 • Content-Range • Content-Type • Content-Version • Derived-From
• Expires • Last-Modified • Link • Title • TransferEncoding • URL-Header • Extension-
Entity Body • entity body is an arbitrary sequence of octets • HTTP can transfer any type of data including:
– text, binary data, audio, images, video
• data is content of resource identified by URL • interpretation data determined by header fields:
WWW • Hypertext & Hypermedia • Browser Architecture • Categories of Web Documents • HTML • CGI • Java
Network Security • Security Requirements • confidentiality - protect data
content/access • integrity - protect data accuracy • availability - ensure timely service • authenticity - protect data origin
Passive Attacks • eavesdropping on transmissions • to obtain information – release of possibly sensitive/confidential message contents – traffic analysis which monitors frequency and length of messages to get info on senders
• difficult to detect • can be prevented using encryption
Active Attacks • masquerade – pretending to be a different entity
• replay • modification of messages • denial of service • easy to detect – detection may lead to deterrent
• hard to prevent –
Requirements for Security • strong encryption algorithm – even known, unable to decrypt without key – even if many plaintexts & ciphertexts available
• sender and receiver must obtain secret key securely • once key is known, all communication using this key is
type of encryption/decryption method • Conventional Methods: • Character-Level Encryption:
Substitutional & Transpositional • Bit-Level Encryption: Encoding/Decoding, Permutation, Substitution, Product, • Exclusive-Or & Rotation • Public key Methods
Cryptography :RSA Security • brute force search of all keys – –
given size of parameters is infeasible but larger keys do slow calculations
• factor n to recover p & q – a hard problem – well known 129 digit challenge broken in 1994 – key size of 1024-bits (300 digits) currently secure for most apps
• TEXT BOOKS • Behrouz A. Foruzan, “Data communication and • • • • •
Networking”, Tata McGraw-Hill, 2004. REFERENCES James .F. Kurouse & W. Rouse, “Computer Networking: A Topdown Approach Featuring”, Pearson Education. Larry L.Peterson & Peter S. Davie, “COMPUTER NETWORKS”, Harcourt Asia Pvt. Ltd., Second Edition. Andrew S. Tannenbaum, “Computer Networks”, PHI, Fourth Edition, 2003. William Stallings, “Data and Computer Communication”, Sixth Edition, Pearson