Lect2..ppt - 08/11/04

CIS 4100 Systems Performance and Evaluation

Lecture 2

byZornitza Genova Prodanoff

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Lecture Outline:

• 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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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

Type of Networks (Continued)

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10000001 00110100 00000011

Octet(8 bit)

129 52 6 3

00000110

• IP addresses: an ID unique for each computer

129.52.6.3

Type of Networks (Continued)

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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

Type of Networks (Continued)

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• packets can get lost, duplicated, delayed, and corrupted

Packets(server to client)

Client Server

Ethernet(cable, link)

Source andDestinationaddress

Type of Networks (Continued)

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Source andDestinationIP address

Header Data

10000000 00001010 00011110

Octet(8 bit)

…

8 bytes in lengthpacket

link of ratebit (pkts/sec) ratepacket

Type of Networks (Continued)

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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

Type of Networks (Continued)

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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

Type of Networks (Continued)

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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

Type of Networks (Continued)

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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

Type of Networks (Continued)

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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

Type of Networks (Continued)

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Type of Networks (Continued)

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

Type of Networks (Continued)

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 signalsstation C receives signals from A and B, but for A and

B cannot communicate due to an obstacle

Type of Networks (Continued)

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Type of Networks (Continued)

• 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

Type of Networks (Continued)

• 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

Type of Networks (Continued)

• 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.

Type of Networks (Continued)

• 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.

Type of Networks (Continued)

Ad hoc wireless networks

• Stations organize themselves into networks without an access point

Type of Networks (Continued)

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Type of Networks (Continued)

• 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

Type of Networks (Continued)

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

Type of Networks (Continued)

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Type of Networks (Continued)

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

Type of Networks (Continued)

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Type of Networks (Continued)