CS2302- COMPUTER NETWORKS. RAJALAKSHMI ENGINEERING COLLEGE DEPARTMENT OF INFORMATION TECHNOLOGY. UNIT I. INTRODUCTION: A computer network is a group of interconnected computers A collection of computers and devices connected to each other.
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RAJALAKSHMI ENGINEERING COLLEGE
DEPARTMENT OF INFORMATION TECHNOLOGY
Identify the set of constraints and requirements based on
How system resource is shared effectively by multiple users
OSI model defines the communications process into Layers
Provides a standards for communication in the
Primary architectural model for inter-computing and Inter networking communications.
network communication protocols have a structure based on OSI Model
Binary Synchronous Communication
Digital Data Communication Message Protocol
BISYNC FRAME FORMAT
PPP Frame Format
High-Level Data Link Control
2.Closed Based Framing(SONET)
Synchronous Optical Network
After 5 consecutive 1s insert 0
Next bit is 0 – stuffed removed
Next bit is 1 –end of frame or erorr
9 rows of 90 byte each
First 3 byte for overhead rest contains data
Payload bytes scrambled- exclusive OR
Electrical Interference, thermal noise
Two Dimensional Parity
Internet Checksum Algorithm
Cyclic Redundancy Check
Deliver Frames Reliably
Accomplished by Acknowledgements and Timeouts
ARQ-Automatic Repeat Request
Stop and Wait
Concurrent Logical Channels
LFASliding Window: Receiver
Describes the way in which the components in a Local
Area Network are connected
All nodes on the LAN are connected in a loop and their
Network Interface Cards (NIC) are working as repeaters.
No starting or ending point.
Each node will repeat any signal that is on the network
regardless its destination.
The destination station recognizes its address and copies
the frame into a local buffer.
The frame continues to circulate until it returns to the
source station, where it is removed.
Example:Token Ring (IEEE 802.5)
FDDI (IEEE 802.6) another protocol used in the
1. the physical medium used to carry Ethernet signals between computers,
2. a set of medium access control rules embedded in each Ethernet interface that allow multiple computers to fairly arbitrate access to the shared Ethernet channel,
3. an Ethernet frame that consists of a standardized set of bits used to carry data over the system
no explicit feedback from network
congestion inferred from end-system observed loss, delay
approach taken by TCP
Network-assisted congestion control:
routers provide feedback to end systems
single bit indicating congestion (SNA, DECbit, TCP/IP ECN, ATM)
explicit rate sender should send atApproaches towards congestion control
Two broad approaches towards congestion control:
sender limits transmission:
CongWin is dynamic, function of perceived network congestion
How does sender perceive congestion?
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
conservative after timeout events
RTTTCP Congestion Control
additive increase: increase CongWin by 1 MSS every RTT in the absence of loss events: probing
Long-lived TCP connection
double CongWin every RTT
done by incrementing CongWin for every ACK received
Summary: initial rate is slow but ramps up exponentially fast
timeTCP Slow Start (more)
A: When CongWin gets to 1/2 of its value before timeout.
At loss event, Threshold is set to 1/2 of CongWin just before loss eventRefinement (more)
Hypertext and Hypermedia
FTP uses the services of TCP. It needs two TCP connections. The well-known port 21 is used for the control connection, and the well-known port 20 is used for the data connection.