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EECS122 – Lecture 2
Department of Electrical Engineering and Computer Sciences
University of CaliforniaBerkeley
EECS 122 - UCB 3
Applications: Web Browsing
ExampleLocating Resource: DNSConnectionEnd-to-endPacketsBitsPoints to remember
EECS 122 - UCB 4
Click Link or URL get content from localor remote computerURL:
http://www.google.com/stringSpecifies- Protocol: http- Computer: www.google.com- StringComputer (server) selects contents based on string
Web: Example
EECS 122 - UCB 5
Web: Locating Resourcewww.google.com is the name of a computerNetwork uses IP addressesTo find the IP address, the application uses a hierarchical directory service called theDomain Name System
local
com
host
www.google.com?IP = a.b.c.d
IP = a.b.c.d
www.google.com?
EECS 122 - UCB 6
Web: ConnectionThe protocol (http) sets up a connection between the host and cnn.com to transfer the pageThe connection transfers the page as a byte stream, without errors: pacing + error control
H ost cnn.com
EECS 122 - UCB 7
Web: End-to-end
The byte stream flows from end to end across many links and switches: routing (+ addressing)That stream is regulated and controlled by both ends: retransmission of erroneous or missing bytes; flow control
End-to-end pacing andflow control
Routing
www.google.com
host
EECS 122 - UCB 8
Web: PacketsThe network transports bytes grouped into packetsThe packets are “self-contained” and routers handle them one by oneThe end hosts worry about errors and flow control:
Destination checks packet for errors (using error detection code CKS) and sends ACKs with sequence number #
Source retransmits packets that were not ACKed and adjusts rate of transmissions
C
A | B | # , CKS | bytes
B C
www.google.comIP address: A
HostIP address: B
Destination
Next Hop
EECS 122 - UCB 9
Web: Bits
Equipment in each node sends the packets as a string of bitsThat equipment is not aware of the meaning of the bits
01011...011...110
Transmitter Physical Medium Receiver
01011...011...110
OpticalCopperWireless
EECS 122 - UCB 10
Web: Points to remember
Separation of tasks send bits on a link: transmitter/receiver [clock,
modulation,…] send packet on each hop [framing, error detection,…]
send packet end to end [addressing, routing]
pace transmissions [detect congestion]
retransmit erroneous or missing packets [acks, timeout]
find destination address from name [DNS]
Scalability routers don’t know about connections names and addresses are hierarchical
EECS 122 - UCB 11
Applications: Telephone
Telephone NetworkDialing a NumberSetting up a CircuitPhone ConversationReleasing the Circuit
EECS 122 - UCB 17
Telephone: Setting Up a Circuit
A
Bring
Circuit = capacity to carry one phone call (shown by thin lines)Circuit is allocated to the call between A and BCircuits are not shared; they are dedicated.
EECS 122 - UCB 20
Design: Multiplexing
OverviewOperationsTDM/FDMStatistical MultiplexingAnalysis of TDM/FDMAnalysis of Statistical Multiplexing
EECS 122 - UCB 21
Multiplexing: Overview
Networks are shared resourcesSharing via multiplexingFundamental Question:
how to achieve controlled sharing
EECS 122 - UCB 22
Multiplexing: Operations
Methods for sharing a communication channelTradeoff between utilization and predictabilityCommon Approaches: TDM (time-division multiplexing) Statistical Multiplexing
EECS 122 - UCB 23
MPX: Time Division Multiplexing
Multiplexern linksrate r bpseach
1 link, rate nr bps
Frame:
Time “slots” are reservedbps = bits per second
EECS 122 - UCB 24
MPX: Statistical Multiplexing
Multiplexern linksany rate 1 link, any rate
TraceExcerpt:
Variable-sized “packets” of data are interleavedbased on the statistics of the senders
EECS 122 - UCB 25
MPX: Analysis of TDM/FDM
TDM, FDM (frequency division multiplexing), and WDM (wavelength) may under-utilize channel with idle sendersApplicable only to fixed numbers of flowsRequires precise timer (or oscillator and guard bands for FDM)Resources are guaranteed
EECS 122 - UCB 26
MPX: Analysis of SM
Traffic is sent on demand, so channel is fully utilized if there is traffic to sendAny number of flowsNeed to control sharing: packets are limited in size prevents domination of single sender
Resources are not guaranteed
EECS 122 - UCB 28
Protocols: Definition
Agreement dictating the form and function of data exchanged between two (or more) parties to effect a communicationTwo parts: syntax and semantics syntax: where bits go semantics: what they mean and what to
do with them
EECS 122 - UCB 29
Protocols: Examples
Internet Protocol (IP) if you can generate and understand
IP, you can be on the Internet media, OS, data rate independent
TCP and HTTP if you can do these, you are on the
web
EECS 122 - UCB 30
Protocols: Standards
New functions require new protocolsThus there are many (e.g. IP, TCP, UDP, HTTP, RIP, OSPF, IS-IS, SMTP, SNMP, Telnet, FTP, DNS, NNTP, NTP, BGP, PIM, DVMRP, ARP, NFS, ICMP, IGMP)Specifications do not change frequentlyOrganizations: IETF, IEEE, ITU