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NJIT

Local and Wide Area Networksand the Internet

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Overview of Networks

Distributed applications run over networks that link computers together.

In information Technology, A network is a series of points or nodes interconnected by communication paths.

Networks can be characterized in terms of either the topology or spatial distance.

A network can also be characterized by the type of data transmission technology it uses.

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Networks by size LAN (Local Area Network)

Unswitched (Does not use routers) Usually covers a building

MAN (Metropolitan Area Network) Unswitched Covers a City

WAN (Wide Area Network) Switched (Uses routers) May cover a county, state, country or the

whole world

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Reasons for Networks

Resource Sharing High Reliability Save Money Improve Corporate Communication

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Performance Issues for Distributed Networks Performance issues are concerned with the

speed at which messages can be transferred over the network. There are two key concerns:

Latency is the delay after a send operation is executed and before data starts to arrive at the destination node.

Data Transfer Rate is the speed that data can be transferred over the network, usually defined in bits-per-second (bps1).1 In abbreviations, b is used for bits and B for bytes.

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Network speed

The time required to transmit a message is:

transmission time=latency+length/data transfer rate The total system bandwidth is a way of measuring

the capacity or throughput of a network. It refers to the volume of traffic that can be transferred across the network in a period of time, usually 1 second. It is frequently expressed in millions or billions of bits per second, as Mbps or Gbps.

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Latency on large networks

Switching delays at routers and the time required to find and set up a communication path can cause latency delays on large networks such as the Internet that are several orders of magnitude larger than local area networks.

Most of us are familiar with the Internet 404 errors that occur when the time to access information is longer than the timeout allowed for searching. Some of this is from latency delays, although most of it is from busy hosts or missing nodes.

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Scalability

As a network grows in the number of nodes, it is a severe challenge to maintain performance. Bottlenecks and complexity often degrade throughput.

Scalability refers to the ability of a network to grow in size without substantial loss of performance.

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Reliability

The original DARPANET, from which the Internet evolved, was designed as a military network to survive a nuclear attack. A key concern was reliability, in this case the ability to continue to transfer messages in the event of failures on the network. Error tolerant and error free communications are key reliability concerns.

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Security

Networks are also concerned with protection from unauthorized use, loss or compromise of data and with external threats to the ability to transfer messages or the integrity of messages. These topics are the subject of future lectures on distributed system security.

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Mobility

Personal Digital Assistants, Lap Top Computers and cellular phones are examples of mobile devices that may need access to a network at different locations. Networks must be designed to allow this to occur in an efficient, secure and productive manner.

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Quality of Service

Metrics have been established to measure the reliability and bandwidth of networks. These often focus on throughput or bandwidth adjusted for possible failures and expressed as a minimum acceptable quality of service and a desired level.

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Multicasting

Most networks are designed for point to point transfer of information between two nodes. There may also be a requirement for one-to-many communication, and some networks are designed to do this in an efficient manner.

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Local Area Network Topologies

Bus Topology Ring Topology Star Topology Tree Topology

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Network TopologiesSource: Tanenbaum, Computer Networks, Figure 1.6

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Network Performance Coulouris et al, Table 3-1

Type Example Range BW Mbps Latency ms

LAN Ethernet 1-2 km 10-1000 1-10

WAN IP routing worldwide 0.01-600 100-500

MAN ATM 2-50 km 1-150 10

Internet Internet worldwide 0.5-600 100-500

WPAN Bluetooth 10-30 m 0.5-2 5-20

WLAN WiFi 150-1500m 2-54 5-20

WMAN WiMax 5-50 km 1.5-20 5-20

WWAN GSM, 3G worldwide 0.01-2 100-500

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Network applications increase the demands of using high-resolution graphics, video and other rich media data types, pressure is growing at the desktop, the server, the hub and the switch for increased bandwidth. There are various categories of bandwidth-intensive applications, for example:

Mission-critical business applications

Data warehousing and backup applications

Internet and intranet applications

Scientific modeling, publication, and medical imaging applications

Local Area Network Bandwidth

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1- IEEE 802.3z 2- IEEE 802.3ab

1- Fiber Cabling Specifications 2- Copper Cabling Specifications

Gigabit Ethernet Standards

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Due to collisions in CD/MA and signal attenuation, all versions of Ethernet, including Gigabit Ethernet as well as Ethernet and Fast Ethernet including have a limited working distance, making them more suitable for local area networks than wide area networks..

Gigabit Ethernet limitations

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Gigabit Ethernet Data Transfer

From Table 3-1 in the text, we see that Ethernet has a latency of 5 to 10 ms. Gigabit Ethernet has a bandwidth of 1000 Mbps. How long will it take to transfer a file of 10,000 bytes? Assume no parity, so 1 byte = 8 bits. Latency 0.005 to 0.01 seconds plus data transfer (80,000 / 1,000,000,000 = 0.00008 seconds). Since the latency is several orders of magnitude slower than the transfer time, latency dominates.

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WiFi Data Transfer

Compare the Gigabit Ethernet example to WiFi at 2 Mbps. How long will it take to transfer the same file of 10,000 bytes? Latency 0.005 to 0.02 seconds plus data transfer (80,000 / 2,000,000 = 0.04 seconds) for a total transfer time of 0.045 to 0.06 seconds.

Note that both examples ignore handshaking, connection overhead, error correction and other delays that we will discuss later in the course.

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Classroom Exercise How long will it take to transfer 25 pictures

from a camera to a photo printer using Bluetooth? Assume each picture is 1 MB and that an acknowledgement of 10 bytes has to go back to the camera to acknowledge each picture before the next can transfer. Bluetooth has a bandwidth of 0.5 to 2 Mbps and a latency of 5-20 ms. Don’t forget: B = byte and b = bit

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Metropolitan Area Networks

Distributed Queue Data Bus IEEE 802.6 Broadcast medium with dual

cables

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WAN Technologies

A Wide Area Network (WAN) is a data communications network that covers a relatively broad geographic area and often uses transmission facilities provided by common carriers, such as telephone companies.

A WAN is an interconnection of LANs. A WAN functions at the lower three layers of the

OSI model.

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Wide Area Networks

Subnets (usually point-to-point, store-and forward, packet switching using routers)

Transmission Lines (circuits, channels, trunks)

Switching Elements (packet switching nodes, intermediate systems, data switching exchanges)

Routers are used to connect LANs to WANs

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Some WAN Backbones Orfali et al, Table 4-3 (abridged)

T1 (DS1) 1.54 Mbit/s North America E1 2.04 Mbit/s CCITT E3 34.36 Mbit/s CCITT T3 (DS3) 44.73 Mbit/s North America OC1 51.84 Mbit/s Sonet fiber OC3 155.52 Mbit/s Sonet fiber OC96 4.976 Gbit/s Sonet fiber OC192 10 Gbit/s Sonet fiber OC768 40 Gbit/s Sonet fiber

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WAN Capacity

David Willis compares sharing files over WANs to herding hippos through a garden hose. (Orfali et all, page 59)

I had a Terabyte of Data in Missouri and a T3 connection to my backup system in Georgia. It takes over 62 hours to send a TB over T3 with a perfect connection, 100% efficiency, no other traffic and no transmission overhead. A week is more likely. Sending tapes by Fed-Ex was faster. That is what we did.

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Wireless Networks

Mobile Stationary Cellular (CDPD)

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Network Principles:Packets

In order to send many messages across a network, individual messages can be broken into smaller chunks, called packets. Packets usually have a maximum size, to allow nodes in the network to reserve enough memory buffer space to ensure that the message can be received, to allow sharing of the network, and to improve reliability and fault detection.

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Network Principles:Contention

A basic problem in almost every network is resource contention. One of the most basic resources is connections between nodes. Unless you have a fully connected network (slide 15d), you are likely to have a time when two or more messages want to use the same connection at the same time. A fully connected network is not practical—can you imagine spending your first few months at NJIT connecting 12,000 individual wires between your computer and every other computer on campus?

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Connection Sharing There are two basic approaches to sharing

connections; switching and multiplexing. Switching, like the public switched telephone

network, (PSTN) sets up a circuit—a reserved path through the network for the duration of the message.

Multiplexing allows multiple messages to use the same connection. There are two basic forms—time division multiplexing and frequency division multiplexing.

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Frequency Division Multiplexing

Frequency Division Multiplexing (FDM) works like radio. Each radio station uses a different carrier frequency and imposes a signal on that frequency. You tune your radio to the frequency of the station you want to hear. The PSTN used to depend heavily on FDM for analog circuits, sending 24 calls over a T1 line at different frequencies. Frequencies of light have different colors. Today’s laser optic circuits can have many different colors of laser light traveling on the same circuit at the same time with FDM.

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Time Division Multiplexing The alternative to FDM is Time Division

Multiplexing, or TDM. During the Second World War, the French Resistance got coded messages over BBC radio by TDM. For example, at 7:35 pm “The next song is dedicated to Clara,” might mean one thing while “This song request is from Harry” might be a different request. But the particular resistance unit listens for its message at specific times. There are several different forms of TDM, including CDSM, Tokens and Frame Relay.

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CDSM In Collision Detection Shared Multiplexing, or CDSM,

a node wishing to send a message over a connection first “listens” for traffic, and if it does not “hear” any, tries to send its message. As it tries it continues to listen so that it can detect another node that also tries at the same time, creating a collision that garbles the message. If it detects a collision, it stops sending and waits for a period of time before trying again. Each node waits a random amount of time so that the same two nodes don’t continue to collide indefinitely.

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Tokens

Token ring networks, (slide 15b) pass a token around the network from station to station. The token can either have a message attached or not. A node that wishes to send a message must wait until it receives a token without a message, then it attaches the message to the token and passes it on. It works something like trying to find an unoccupied taxi in New York City during rush hour.

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Packet Switching Frame relay and ATM are among several

technologies that send messages across a connection with very precise timing. Messages are broken into packets, sending a packet from one message, then another packet from the same or a different message. Messages can be reassembled from packets at their destination.

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Addresses

Nodes on a network need to have an identifier, so that messages can be sent to the proper node. These identifiers are called addresses. A telephone number is an address. So is a Uniform Resource Locator (URL). The Internet uses IPV4 and IPV6 addresses as primary identifiers. IP addresses are covered in the Sockets lecture.

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Packet Delivery

There are two ways that packets can be delivered to their destination

Datagram packet delivery Virtual circuit (or stream) packet delivery Both are explained in the socket lecture.

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Digital TDM

Several packets can be sent across a connection in the same time period by sending a byte from one message, followed by a byte from the next.

In the PSTN, a DS1 line is the digital equivalent of an analog T1 line. It sends 24 messages at a time, alternating bytes, and converts them back into individual messages at the destination by reassembly.

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Network Principles:Protocols

In order to accomplish communications, networks need standard rules to define what is to be done and how to do it. These rules are formally specified in documents called protocols. A protocol must be specific enough that two nodes, technologies, systems or other parties can communicate without any difficulty.

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Parts of a Protocol

Protocols have two parts: A specification of the sequence of the

messages that must be exchanged. A specification of the format of the data

in each part of the message. Protocols are implemented with a pair of

software modules on the sending and receiving ends.

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Protocols and Interfaces Source: Tanenbaum, Computer Networks, Figure 1.9

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The OSI Protocol Model

In 1992, the International Standards Organization (ISO) defined a protocol suite defining a seven layer reference model for open systems. By specifying agreed upon layer boundaries, it is possible to divide network tasks into common segments such that collections of cooperating behaviors can accomplish the tasks performed by each segment. These seven layers are shown in the diagram on the next slide.

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The Seven OSI Layers

Application Layer defines the communication needs of specific applications such as FTP or HTTP.

Presentation Layer includes encryption and such tasks as placement of fields in a display.

Session Layer includes reliability and adaptation such as failure detection and automatic recovery.

Transport layer defines connection oriented and connectionless protocols at the message level.

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Open Systems Interconnection Model

The seven layers

in the OSI model

from the

Abdus Salam Center

Network tutorial

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OSI Low Level Layers

Network Layer transfers packets between nodes using a protocol specific to the particular network. This may include setting up connections between routers

Data Link Layer manages the transfer of packets between nodes connected by a physical link.

Physical Layer specifies the circuits and hardware that carry electrical, light or electromagnetic signals between nodes.

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How OSI Works Source: Tanenbaum, Computer Networks, Figure 1.19

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Headers and trailers

Each level is packaged as data to other levels with a header attached.

Headers Trailer

Message

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Physical Layer

The physical layer just sends bits that might be encoded as different voltage layers for a specified instant of time on an electric wire or as pulses of light on a fiber optic line. There are many possible ways to distinguish a 1 or a 0 on a communications medium.

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Data Link Layer

The Data Link layer groups bits into frames or other units and adds additional bits of information to group the bits, indicate the beginning and end of a character, assign sequence numbers for ordering, and provide for error detection and correction with parity and checksums. Sequence numbers, parity bits and checksums are all examples of overhead.

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Network Layer The Network Layer adds information that

allows the receiver of a message to identify traffic that belongs to it and allows intermediate devices to route information to the proper destination. The most common form uses Internet Protocol, which uses IP addresses and ports to identify clients and servers. Each message contains the addresses and port numbers of both the client and the server as overhead.

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Transport Layer

The Transport Layer can add information for synchronization, breaking messages into chunks, acknowledgement of receipt, timeouts and retransmission of data not acknowledged. The most common transport protocols are TCP and UDP.

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Session Layer

The Session Layer is an enhancement of the Transport Layer and can add information for dialog control, synchronization, error recovery, and similar functions. The session and lower levels are all concerned with getting a bit stream across a connection reliably.

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Presentation Layer

The Presentation Layer is the lowest layer that is concerned with the meaning of the bits transmitted. It identifies collections of bits with identifiers so that they can be assigned meaning. Data can be collected into fields and records and assigned labels.

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Application Layer

While the Application Layer was originally designed to contain a collection of standardized network applications like electronic mail and file transfer, it has become a general purpose container for applications and protocols that do not fit into the lower layers. It lacks a clear separation between applications, application specific protocols, and general purpose protocols such as File Transfer Protocol.

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Packet Assembly The Network layer is responsible for preparing

packets to move across the network. One requirement is to break up messages into packets that can be no larger than the Maximum Transfer Unit (MTU), including both the header and the data field. For example, the MTU for Ethernet is 1500 bytes. The IP protocol MTU is 64 KB, although most systems are set for 8 KB to allow for smaller I/O buffers. If IP packets are sent over Ethernet, they must be fragmented to the Ethernet MTU size.

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Transmission Control Protocol Layers

The early specifications of network layers were defined before the OSI model and only include four layers, as shown on the next slide.

This was done for the Defense Advanced Research Projects Agency (for DARPANET). When anti-war sentiment was common on college campuses during the Vietnam war, it was renamed the Advanced Research Projects Agency (and ARPANET).

When portions of ARPANET were opened to public use, those portions became the Internet.

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Comparing TCP/IP to OSI

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Initial TCP/IP Networks and Protocols

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TCP Finite State Machine

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Classroom Exercise

Use the state chart on the previous page. What is the minimum time to transfer one packet using TCP with a latency of 10 ms? Assume small messages and fast transmission like Gigabit Ethernet, so that data transfer time is negligible compared to connection latency.

Note that actual timing is affected by other network traffic and timeout settings. There are multiple timeout settings in TCP.

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Bibliography George Coularis, Jean Dollimore and Tim

Kindberg, Distributed Systems, Concepts and Design, Addison Wesley, Fourth Edition, 2005

Andrew Tannenbaum and Maarten Steen, Distributed Systems, Principles and Paradigms, Prentice Hall, 2002

DARPA RFC 793, September 1981, figure 6, page 23

Orfali, R., Harkey, D., Edwards, J, The Essential Client Server Survival Guide, Second Edition, Wiley, 1996