Transcript Type of Networks (Continued)

CIS 4100 Systems Performance and Evaluation
Lecture 2
by
Zornitza Genova Prodanoff
Lect2..ppt - 08/11/04
Lecture Outline:
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Types of networks
Network protocols
Client/Server Model
Peer-to-Peer Model
Web services protocols
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Type of Networks
• Computer and Communications industries converge in the 70s
• The infrastructure underlying the Internet has evolved since
• Web is an overlay network on the Internet
• Web services offered trough Web sites are based on
− Client/Server model
− Peer-to-Peer model
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Type of Networks
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Wide Area Networks (WANs)
• The Internet is a WAN 
• High speed links are 45Mbps to Gbps-s
• Technologies: x25, ISDN, Frame Relay,SMDS, ATM
Internet
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• IP addresses: an ID unique for each computer
129.52.6.3
129
10000001
Octet
(8 bit)
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52
6
3
00110100 00000110 00000011
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IP Addresses are written using dotted decimal notation:
4 groups of 8 bits
Each group can be 0 – 255
Example:
Source:
D. E. Comer, “Computer Networks and Internets”, 3rd edition,
Purdue University
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Client
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Source and
Destination
address
Server
Ethernet
(cable, link)
Packets
(server to client)
• packets can get lost, duplicated, delayed, and corrupted
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Type of Networks
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Header
Source and
Destination
IP address
Data
10000000
00001010
…
Octet
(8 bit)
bit rate of link
packet rate (pkts/sec) 
packet length in bytes  8
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00011110
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~22ms (=0.02sec)
• Electromagnetic signal speed ~ 300, 000km/sec
• Network links have capacity (data rate)
– dial-in link: 56-Kbps
– Ethernet: 10 or 100 (or 1000) Mbps
– “T1”:
1.544 Mbps
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• A router connects two or more physical networks
• A router is part of each network (has 2+ IP addresses)
• Forwarding packets to the right destination
• Glue smaller networks together
• Data travel packet by packet
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Type of Networks
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• Local Area Networks (LANs)
• Technologies:
− Ethernet (10 Mbps, 100 Mbps)
− Gigabit Ethernet
− Token Ring (4-16 Mbps)
− FDDI (100 Mbps)
• Ethernet
− No central node
− Access to a shared medium (called the ether – coaxial cable )
− Carrier Sense Multiple Access with Collision Detection
(CDMA/CD) – a distributed algorithm to determine which
station uses the shared medium next
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Type of Networks
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• It is possible, however, for two NICs that are far apart in the
cable to attempt a transmission at about the same time, detect a
free medium, transmit their packets and interfere with each
other. This interference is called a collision.
• Collisions – As traffic on the Ethernet LAN increases the
probability
• Ethernet NICs are equipped to detect collisions and stop
transmitting a packet when collisions are detected
• NICs wait for a randomly selected time period before attempting
to retransmit their packets
• As traffic on an Ethernet network increases, the probability of a
collision increases and the network throughput decreases as more
of the bandwidth is spent on collisions and retransmissions
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Type of Networks
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Token Ring
• Invented at IBM Research Labs and, as the name indicates, is
based on a ring topology as shown in Fig. 2.1b.
• A sender inserts the bits of its packet into the ring
• The packet goes around the ring and is copied by the NIC
specified in the destination address field of the packet
• The packet continues its flow around the ring back to the sender
which removes the packet and compares the received packet with
the packet sent for error control
• If two or more NICs attempt to transmit simultaneously
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Fiber Distributed Data Interface (FDDI)
• a set of ANSI protocols for sending digital data over fiber optic
cable
• FDDI networks use token-passing
• support data rates of up to 100 Mbps (100 million bits) per
second
• used as backbones for WANs
• An improvement over token ring is the introduction of a second
ring – better fault tolerance
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Wireless LANs
• Use the Radio Frequency as a medium
• Use the data to be transmitted to modulate a carrier signal
(wave) transmitted by the RF transmitters
• Physical layer
− provides a carrier sense signal which indicates if there is a
transmission in progress
− data sent by one station can be received by all stations in its
area of coverage
− communication may suffer from the "hidden terminal
problem“:
 walls or other structures obstruct the RF signals
 station C receives signals from A and B, but for A and B
cannot communicate due to an obstacle
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Type of Networks
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• Figure 2.2 shows the architecture of IEEE 802.11 based LANs
• Stations communicate, within
− their cells or basic service set (BSS),
− with one another
− and with a base station called an access point (AP)
• AP is used to allow stations in different BSSs to communicate
with one another.
• Access points may be connected through a wired LAN, as shown in
Figure 2.2, or through a wireless LAN
• Similar to transmission in an Ethernet, wireless stations may
interfere with one another if they transmit at the same time
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• The IEEE 802.11 protocol uses a Carrier Sense Multiple Access/
Collision Avoidance (CSMA/CA) protocol
• If the channel is sensed idle for an amount of time equal to the
Distributed Inter Frame Space (DIFS), a station may transmit
• The receiver of a correct frame sends an acknowledgement
frame to the sender after a short period of time called the Short
Inter Frame Spacing (SIFS)
• Acknowledgements (ACK) in IEEE 802.11 LANs are required, but
not in Ethernet LANs since the sender can hear its own
transmission
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• The IEEE 802.11 protocol does not use collision detection, as
Ethernet does and aims at avoiding collisions
• Any transmitted frame contains the duration of the transmission,
so that other stations can avoid transmitting collision at that
time.
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• Collisions can occur because of the hidden terminal problem.
• It is possible for two stations hidden from one another to collide
when transmitting to the same destination at about the same time
• To avoid this problem, IEEE 802.11 provides for the optional use
of a Request To Send (RTS) and Clear To Send (CTS) exchange of
frames before the data transmission
− Before a sender transmits its data frame, it sends a RTS
frame to the receiver indicating the entire duration of the
transmission including the time to send the ACK
− If the RTS is received correctly a short CTS is sent by
the intended receiver to the sender, which may proceed
with the actual transmission
− All other stations hear this exchange and avoid interfering
with the data transmission.
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Ad hoc wireless networks
• Stations organize themselves into networks without an access
point
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• The LAN to WAN Connection
• LANs usually connect to WANs through dedicated leased lines at
T1 (1.5 Mbps) or T3 (45 Mbps) speeds.
• The adequate sizing of the LAN-to-Wan link will be discussed in
Chapters 9 and 10
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Figure 2.4 shows the networking topology for a company
headquarters in Los Angeles, with branches in Chicago and New
York:
• The three locations are connected through a Frame Relay
WAN
• The headquarters has a Ethernet and a Token Ring LAN
connected to a 100-Mbps FDDI ring backbone
• The backbone connects to the Frame Relay WAN through a
T1 line
• The Chicago branch has two LAN segments connected by a
bridge
• The router to the WAN is located in one of the LAN
segments
• The New York branch has a single 16-Mbps Token Ring LAN
connected to the WAN through a router
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Alternatives, when connecting a home PC (LANs) to a WAN:
• Dial-up analog modems at speeds ranging from 14.4 to 56 Kbps
• ISDN Basic Rate Interface (BRI): requires a dial-up digital
modem and provides speeds of 128 Kbps
• ISDN Primary Rate Interface (PRI) delivers 1.544 Mbps
• Leasing a T1 line: 1.544 Mbps
• T1-like speeds can also be obtained with High-Bit-Rate Digital
Subscriber Line (HDSL) but with more flexibility
• An asymmetric version of HDSL, Asymmetric Digital Subscriber
Line (ADSL), provides 640 Kbps outbound and 6 Mbps inbound
This asymmetry is advantageous for Web access, since the
bandwidth requirements for fast image and video downloads are
higher than for sending requests to Web servers
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