Download Review of Underlying Network Technologies

Review of
Underlying Network
Technologies
Chapter 2
Network Communication
Internet is not a new kind of physical
network
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Method of interconnecting physical networks
Set of conventions for using networks
Communication networks can be divided
into two basic types
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Connection-oriented (circuit-switched)
Connectionless (packet-switched)
Connection-Oriented
Forms a dedicated connection or circuit
between two points
Like U.S. telephone system
Guarantees capacity
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Once circuit is established, no other network
activity will decrease the circuit’s capacity
Disadvantage is cost
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Circuit costs are fixed, regardless of use
Connectionless
Data to be transmitted is divided into
packets
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Usually only a few hundred bytes
Carries identification information
Allows concurrent communication
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Multiple computers over shared medium
Disadv:
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As activity increases, given pair of
computers receives less of the network
capacity
Who Wins?
Connectionless!
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Despite not being able to guarantee capacity
Wins via cost and performance
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Sharing network bandwidth means fewer
connections are required
Performance ok since we can build high
speed network hardware
Throughout the text, network means
connectionless network
WANs and LANs
WAN: spans large geographical distance
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Sometimes called long-haul networks
Usually do not have any distance limit
Slower: 1.5 Mbps to 2.4 Gbps
More delay: few ms to several tenths of a second
Usually consists of a series of packet switches
interconnected by long distance comm lines
Extend network by new switch and comm line
Computers added by attaching to a packet
switch
LAN: spans short geographical distance
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Fast: 100 Mbps to 10 Gbps
Less delay: few tenths of ms to 10 ms
Each computer usually contains a Network
Interface Card
Connects machine directly to network
Network is “dumb”; interface devices in the
computers do the work
Every computer attached to a network has
a unique address*
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Sender must know recipient’s address
Hardware technology specifies address
scheme
*Will change this statement slightly soon!
Ethernet Technology
Ethernet is a packet-switched LAN
Invented by Xerox in early 1970’s
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Standardized by Xerox, Intel, and DEC in 1978
IEEE standard number 802.3
Many variants exist
Original design known as 10Base5
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Uses coaxial cable approximately ½ inch in
diameter and up to 500 meters long
Cable is completely passive
10 Mbps
Original wiring scheme
Has been superseded
Thin-wire Ethernet
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Known as 10Base2
Some original Ethernet disadvantages:
Transceiver has non-trivial cost
Transceivers located with cable
Cable difficult to install (thick shield; hard to bend)
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Thinnet cable thinner, cheaper, more flexible
Computer has both the host interface and
connection circuitry
Easy to connect and disconnect (no
technician)
Less protection from interference; shorter
distances; fewer connections per network
Twisted Pair Ethernet
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Known as 10Base-T
Popular, current technology
Uses conventional unshielded copper wire
Cheaper and easier to install
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Each computer connects to a hub over 4 pairs of wires
Only 2 pairs of wires used
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Same communication capability as thick or thin
Ethernet; just alternate wiring scheme
Fast Ethernet
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Thick, thin, twisted pair: 10 Mbps
Faster processors  Ethernet became
bottleneck
Developed 100Base-T (100 Mbps)
Uses same twisted pair wires
Gigabit Ethernet
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Known as 1000Base-T (1 Gbps, copper))
1000Base-X – uses fiber optics
Fiber is much faster
Developing 10 and 40 Gbps Ethernet technologies
10/100/1000 Ethernet
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Allows compatibility with either 10Base-T,
100Base-T, or 1000Base-T
Can use for computer interfaces or hubs
Computer with 10/100/1000 interface card
can attach to any of the 3 configurations
Hardware automatically detects speed
No hardware or software reconfiguration
required
Power over Ethernet
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Small amount of power
Sent over same copper cable
Power does not degrade data transmission
Can power small devices with one cable
Two facts about increased capacity
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(1) Few computers can sustain 1 Gbps data
rate
(2) New versions did not change standards
Max packet size same as for 10Base-T
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Higher-speeds not optimized for highest
possible computer-to-computer throughput
Allows more stations and more total traffic
Ethernet properties
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Shared bus that supports broadcast
All stations connect to single shared channel
All stations receive every transmission
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Uses best-effort delivery
Sender gets no information about packet delivery
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Distributed access control
No central authority to grant access to shared
channel
Carrier Sense Multiple Access with Collision
Detect (CSMA/CD)
Host listens before transmit; sends if idle
Maximum packet size limits transmission time
Must observe minimum idle time between sends
Collision detection and recovery
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Signals travel at approx 70% speed of light
Stations can begin transmitting simultaneously
(or almost simultaneously)
Results in a collision
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Each station monitors cable while transmitting
If detect collision: host stops, waits, retries
Uses binary exponential backoff policy
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Sender delays random time; doubles on second
collision; quadruples on third collision; and so on
If network busy, retransmit attempts quickly
spread over a reasonably long period
Wireless Ethernet
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Several wireless standards related to Ethernet
802.11b (Wi-Fi)
Up to 11 Mbps; usually 2.5 – 4 Mbps
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802.11a and 802.11g
Up to 54 Mbps
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802.16 (Wi-Max)
Above three for point-to-point or with base
802.16 is for point-to-point only
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802.11n
540 Mbps
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802.11i
Standard for security
Ethernet addressing and frames
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Each computer has a 48-bit address
Checked in interface hardware, not computer’s CPU
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When sending, specifies destination:
Single, broadcast, or multicast
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Addresses associated with the interface
hardware
Move/change interface; new machine physical
address
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Transmitted data viewed as a frame
Variable length; 64 octets to 1518 octets
Besides data, contains:
 Preamble, destination and source addresses
 Frame type
 Cyclic Redundancy Check (CRC)
Ethernet bridges
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Bridge
Connects two Ethernets; passes frames
Operates on packets vs signals
Bridge follows CSMA/CD rules, so (almost)
arbitrary number can be added
Ethernet switch is like a multi-port bridge
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Adaptive or learning bridges
Can decide which frames to forward
Usually very sophisticated and robust
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Important point:
Bridges hide the interconnection details
Set of bridged segments acts like single Ethernet
Asynchronous Transfer Mode
ATM is connection-oriented
Designed for extremely high speed data
switching
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Operate at gigabit speeds
Needs complex, expensive hardware
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One or more high speed switches
Optical fiber for all connections
Uses fixed-size frames called cells
Connection-oriented networking
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Not like packet-switched networks
Must first establish a connection to the
destination computer
ATM switch finds path from sender to receiver
Waits on remote computer to accept the request
Local ATM switch selects identifier for the
connection and passes it to the computer
Computer sends using identifier
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When done, connection must be broken
Wide Area Point-to-Point Networks
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WANs formed by leasing data circuits
Digital circuits initially used for digitized voice
Data came later, so data rates are not powers of 10
Are powers of 64 Kbps due to PCM
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8000 samples/sec; each sample 8 bits
T1: 1.544 Mpbs; T2: 6.312; T3: 44.736; T4: 274.760
Lower data rates use copper; higher data
rates need fiber circuits
Point-to-point “network”
When system connects exactly two computers
“Network” is a stretch; viewed as such for
consistency
Main point: do not need hardware addresses
Dialup IP
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Another example of a point-to-point network
Typically from modem (residence) to ISP
TCP/IP view:
Placing call is like running a wire
Connection is made; stays as long as needed
Summary
Reviewed several network hardware
technologies
General idea:
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TCP/IP protocols are extremely flexible
Almost any underlying technology can be
used to transfer TCP/IP traffic