Transcript Slides for Chapter 3: Networking and Internetworking

Networks – basics
A network consists of:
Transmission media (wire, cable, …).
Hardware devices (routers, switches, …).
Software components (protocol stacks, drivers, …).
Terminology:
Host: Computers and other devices that use a network.
Node: Any computer or switching device attached to a network.
Subnet: Set of interconnected nodes.
Design issues: Performance, scalability,
reliability, security, mobility, quality of service,
multicasting.
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Networks – types
Range
LAN
1-2 kms
WAN
worldwide
MAN
2-50 kms
Wireless LAN 0.15-1.5 km
Wireless WAN worldwide
Internet
worldwide
Bandwidth (Mbps) Latency (ms)
10 – 1000
0.010 – 10000 [1]
1 – 150
2 – 54 [2]
0.010 – 2
0.010 – 2
1 – 10
100 – 500
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5 – 20
100 – 500
100 – 500
[1]: OC-192 over ATM:
http://newsroom.cisco.com/dlls/innovators/switching/eugene_wang_profile.html
[2]: IEEE 803.11a:
http://www.wi-fiplanet.com/tutorials/article.php/2109881
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Networks – packet transmission
Message: Sequence of data items of arbitrary
length.
Messages subdivided into packets.
Switching schemes:
Broadcast.
Circuit switching.
Packet switching.
ATM / Frame relay.
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Protocols – basics
Protocol: Set of rules and formats to be used for
communication between processes in order to
perform a given task.
Should include specification of:
Sequence of messages that must be exchanged.
Format of the data in the messages.
Implemented by a pair of software modules in
the sending and receiving computers.
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Protocols – layers
Message received
Message sent
Layer n
Layer 2
Layer 1
Sender
Communication
medium
Recipient
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Protocols – encapsulation and headers
Applic ation-layer mess age
Pres entation header
Ses si on header
T rans port header
Network header
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Protocols – the ISO Open Systems Interconnection (OSI) model
Mes sage s ent
Mes sage received
Layers
Applic ation
Pres entation
Ses si on
T rans port
Network
Data l ink
Physic al
Sender
Communic ati on
medium
Rec ipient
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Protocols – internetwork layers
Mes sage
Layers
Applic ation
Internetwork
protoc ols
T rans port
Internetwork
Internetwork pac kets
Network interface
Network-spec ifi c pac kets
Underlying
network
protoc ols
Underlying network
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Protocols – network layer routing (WAN)
Packet delivery: Datagram or virtual circuit?
A
Hosts
or local
networks
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B
2
3
Links
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C
5
D
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E
Routers
Routing algorithms, adaptive routing (congestion control)
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Internet protocols – Internetworking
Internetwork: Network which integrates a
number of different subnets.
Needs:
Unified internetwork addressing scheme (Internet: IP
addresses)
 Protocol defining format of internetwork packets and
specifying rules for handling (Internet: IP protocol).
Interconnecting components that route packets to
their destinations (Internet: Internet routers).
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Internet protocols – the TCP/IP protocol suite
Message
Layers
Application
Messages (UDP) or Streams (TCP)
Transport
UDP or TCP packets
Internet
IP datagrams
Network interface
Network-specific frames
Underlying network
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Internet protocols – encapsulation and headers
Application message
TCP header
port
IP header TCP
Ethernet header IP
Ethernet frame
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Internet protocols – IP
Internet Protocol.
Transmits datagrams from one host to another,
if necessary via intermediate routers.
Unreliable, best-effort delivery semantics.
Address resolution: Conversion of Internet
addresses to network addresses (for a given
network).
Routing: Each router in the Internet implements
IP-layer software to provide a routing algorithm.
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Internet protocols – IP packet layout and addressing
header
IP address of s ource
IP address of des tinati on
data
up to 64 kil obytes
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Clas s A:
Clas s B:
0
Hos t ID
Network ID
1 0
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Network ID
Hos t ID
8
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Clas s C:
1 1 0
Hos t ID
Network ID
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Clas s D (mul tic as t):
1 1 1 0
Multic as t address
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Clas s E (reserved):
1 1 1 1 0
unused
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Internet protocols – TCP and UDP
UDP features:
Transport-level replica of IP.
No guarantee of delivery.
No setup cost, no acknowledgement messages.
Message size up to 64 kbytes.
TCP features:
Reliable delivery.
Arbitrarily long sequences of bytes.
Connection-oriented.
Mechanisms: Sequencing, flow control,
retransmission, buffering, checksum.
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Ethernet (IEEE 802.3) – Basics
Carriers Sense Multiple Access with Collision
Detection (CSMA/CD)
Xerox – Ethernet
Random Access
 Stations access medium randomly
 Contention
Stations contend for time on medium
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Ethernet – ALOHA
Sender
Go ahead and send!
Retransmit if no ACK
Problems
Collisions
Low utilization (18%)
Slotted ALOHA is an improvement (max
utilization 37%)
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Ethernet – CSMA
 Carrier Sense Multiple Access
 Observations
Propagation time is much less than transmission time
All stations know that a transmission has started almost
immediately
 Sender
First listen for clear medium (carrier sense)
If medium idle, transmit
If two stations start at the same instant, collision
Wait reasonable time
Retransmit if no ACK
 Max utilization depends on propagation time (medium
length) and frame length
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Ethernet – CSMA/CD
 Carrier Sense Multiple Access – Collision Detection
 Observation: With CSMA, collision occupies medium for
duration of transmission
 Sender
If medium idle, transmit
If busy, listen for idle, then transmit
Station listens whilst transmitting
If collision detected, transmit jam signal, then cease
transmission
 After jam, wait random time then start again
Binary exponential back off
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Ethernet –
CSMA/CD
Operation
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Ethernet – Collision Detection
Bus
Collision produces much higher signal voltage than
signal
Collision detected if cable signal greater than single
station signal
Star
Activity on more than one input is collision
Special collision presence signal
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Summary
Networks.
Protocols.
Internet protocols (TCP/IP).
Ethernet
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