Setting up a Network

What is a Wide Area Network?

Introduction

A network is a number of computers that communicate with one another using a variety of media and software. Wide in the term Wide Area Network (WAN) basically indicates that the area covered can be varying enormously. Wide is only limited by the distance a digital signal can be sent. WANs can cover an area as far as a satellite cruising the universe can be sent to and can be reached with a digital signal. A WAN may mean two LANs, each covering a building connected together or may mean an organisation that has branches all around the globe.

Basically, the difference with a LAN is:

· A LAN covers an office or building, which means a limited geographical area

· A LAN usually uses one transmission media type

· A WAN might be multiple LANs

· A WAN might include a multitude of transmission media, hardware, software and protocols

· LANs use proprietary network components, WANs uses Public Networks.

A WAN can be one of two types:

· Enterprise WAN, nowadays referred to as an INTRANET

· Global WAN, which is known as the INTERNET

There are other categories of networks apart from LANs and WANs. They fit between LANs and WANs and are know as MANs (Metropolitan Area Networks) and CANs (Country Area Networks). They are restricted to a specific geographical area, but use public network components and use fast links.

Internetworking

The term 'Internetworking' refers to connecting computers or networks together into a larger network. An Internetwork is usually called a WAN.

Public data Networks are nearly always used to build WANs, especially in the case of the Internet.

A Public Data Network is a network that is owned the company that owns the WAN. It uses the infrastructure owned and put in place by public PTTs (Public Telegraph and telephone) companies such as TELSTRA, British Telephone, Bell Telephone, AT & T etc. The PTTs grew as an offshoot of national postal services, and PTT originally meant Post, Telegraph and Telephone. These bodies became very powerful and political and formed worldwide monopolies on the communications infrastructure on this planet. The US was the first to break AT & T up in smaller companies, mainly based on geographical areas. Currently most Western World countries are deregulating the provision of these services, which in turn introduces competition resulting in better and cheaper services.

The first Data Network was gradually established using the PSTN (Public, Switched, Telephone, and Network). Telecom Australia introduced a service called DATEL (DATa over the TELephone line), which is still used today, followed by a service called DDS (Digital Data Service) and AUSPAC (AUStralian PACket switching network).

The PSTN is what still carries most of the services, being the network that uses components such as:

· Local Loops - carries a analogue signal from business or personal premised to the nearest switch

· Switch - establishes the connection to another switch in order to go from the source to the destination. Many switches can be involved in this circuit between source and destination. The nearest switch to sender and the switch nearest to the receiver (the last ones in the circuit) translate signal between digital and analog)

· Trunk Lines - carries the digital signal from one switch to another switch over a larger geographical area

The computers that communicate are connected by modems to a telephone line. Once a connection is established via the telephone network, data can be transmitted and received by the two computers. There are many communications packages that can establish such a path.

With digital networks, you don't need a modem. Your computer is capable of directly transmitting digital data onto the network without having to modulate it. The digital network is separate from the PSTN. It uses very different techniques for transmission and much faster and more reliable.

Packet switched networks involve a digital data service that does not establish a physical connection between the sender and the receiver. Each packet of data is individually addressed and sent independently from the other packets. The network software will assemble and disassemble the packets and each end. This facilitates network performance, reliability and reduces costs.

You can have a private network, which means that you have a permanent link that you lease, called a leased line or dedicated line. You do not have to 'dial-up' this line, it always exists.

The INTERNET is the largest internetwork in the world. It is a collection of non-homogenous networks and connects many subnets. The World Wide Web is an important component of the Internet - sometimes referred to as The Net

An IntraNet is a form of Internet that is used within the boundaries of an organisation. A firewall protects the Intranet from the outside world. Extranet is a network used exclusively by an organisation and its customers and/or suppliers. A good discussion is found at: Intranet & Extranet

Host Access and Terminal Emulation

The term HOST dates back to the days when computing power was only available in mainframes and the stations connected to it were "dumb" terminals. Dumb terminals have no intelligence, that is no significant processing power and all they are able to do is receive data, display or print it and send and buffer data entered via keyboard or other devices.

The host would provide all services such processing, database, and communication services.

With the arrival of PC's, IT staff quickly required a PC to behave like a terminal which eliminate the need for a terminal and a PC. The PC would use software to appear to the host as a "dumb" terminal. This software is known and terminal emulation software. Examples are 3270 emulation for IBM SNA networks and VT320 for Digital (DEC) Decent.

Exercise: Identify some of the current Terminals and Emulation on the Webster of IBM (IBM.COM), HP (HP.COM) and DEC (DEC.COM). Tip: Use their search engines to search for "Terminal emulation".

Sketch and describe typical WAN configurations

A WAN is conventionally drawn as a 'network cloud'. However, an organisational WAN could be drawn showing specific links to the network.

The following exercise lets you briefly review Services currently available in Australia.

Exercise1: What WAN services does Telstra Offer? What is the role of the ACA?

Protocols and standards

Protocols

Introduction

Computers can't just throw data at each other any old way. Because so many different types of computers and operating systems connect via modems or other connections, they have to follow communications rules called protocols.

· What protocols do you use to make a telephone conversation?

· Is a protocol the same as an interface?

· What reasons are there for having protocols?

Is there a difference between data and communication protocols?

The OSI Model sets protocol standards. OSI was to be an open world standard, setup by ISO. The present model only commands a small percentage of the world. TCP/IP, SNA and DECnet each have a higher percentage of usage.

The basic elements of a protocol are:

· Character sets,

· A set of rules for timing and sequence of messages constructed from the character set

· Procedures to determine when a error has occurred in the transmission

· Procedures how to correct the error

The character sets have subsets of meaningful characters (to human beings) and control characters. The characters sets can be coded in standard codes such as ACII, EBDCFIC and TTY.

A simple protocol

TTY is a very simple protocol of 58 characters, 50 printable ones and 8 control characters. It uses asynchronous data transmission.

A simple protocol looks something like this:

· A communication channel is connected between two machines (dialled or leased)

· Sending machine sends a WRU (who are you), for verification that the machine is the correct one

· The sending sends a Here is.., same sequence of chars as sent for WRU

· Sending machine sends message header (preamble) that identifies the name and address of intended message recipient, the date and time of entry, same for transmission, and message sequence number.

· At the end of the message the sending machine sends another Here is… and WRU. This is to determine whether the receiver is still there. A BELL signal could be sent to alert the receiver that a message is ready for delivery.

This simple protocol does not deal with errors very well. Only the WRU verifies that the correct machine is receiving the message.

Error Detection and Correction

Protocols support error checking as a basic function. The principle in error checking is that at the sender side, a number of control bits are generated based on an algorithm. The receiver than checks the received data and compares it with the control data. Error detection techniques to be used in simple protocols are:

· Parity checking: Odd or even. This technique adds up the number of ones in a byte. In the case of Odd parity it will add a one in the parity bit if the number of 'ones' are odd.

·

o Vertical redundancy checking

o Horizontal redundancy checking

· Checksum: Adds the bits together and adds a bit to store the sum as a control bit

These two techniques are called Block Redundancy Checks (BRC)

· Cyclic Redundancy Check (CRC) - Adds the bits and divides by a pre-set number, the remainder or the quotient is stored as the control bit

· Echoplexing - returning the complete data stream for comparison at the sender side.

Some terminology related to error handling:

error A protocol within XNS by which a station reports that it has received (and is discarding) a defective packet. Interpreted in the XNS PI suite.

error control - A technique for assuring that transmissions from a source are received at the destination without errors.

error-correcting code - A code having sufficient intelligence and incorporating sufficient signalling information to enable the detection and correction of many errors at the receiver.

error-detecting code - A code that can detect transmission errors through analysis of received data based on their adherence to appropriate structural guidelines.

error rate -In data transmission, the ratio of the number of incorrect elements transmitted to the total number of elements transmitted.

Protocols terminology

protocol - formal set of conventions governing the format and control of inputs and outputs.

protocol stack - Related layers of protocol software that function together to implement particular communications architecture. Examples include AppleTalk and DECnet.

Protocol suite - All the protocols that is available within a network system. They can all work together, but are generally not all required to transmit

protocol address - Network address - physical, device, network address, depending on the layer the protocol is working on.

protocol converter - Enables equipment with different data formats to communicate by translating the data transmission code of one device to the data transmission code of another device.

protocol interpreter - The Sniffer analyser uses its protocol interpreters to identify the protocols nested within each frame and interprets their contents.

protocol translator - A network device or software that converts one protocol into another, similar, protocol. For example, the Cisco CPT performs conversion between X.25 PAD and Telnet

Bodies responsible for standards

International Standards Organization - (ISO) - The International Standards body that encompasses ANSI (US), DIN (Germany), SAA (Australia)

Institute of Electrical and Electronic Engineers - (IEEE) - responsible for mainly 2^nd layer (Data Link and Media Access) standards

International Telecommunications Union - Telecommunications standards - (ITU-T) - mainly responsible for 1^st layer standards.

The Internet Society (ISOC): coordinates the use of numerous Internet protocol parameters. You can visit their site on Welcome to ISOC

The Internet Engineering Task Force ( IETF) coordinates the Internet address among other things. The location of their site is IETF Home Page

There are a variety of other bodies that control specific groups of standards.

Major Protocols

FILE TRANSFER PROTOCOLS

Xmodem

This is a protocol for transferring files during direct dial-up communications. Developed by Ward Christensen in 1977, Xmodem has basic error checking to ensure that information isn't lost or corrupted during transfer; it sends data in 128-byte blocks. Xmodem has undergone a couple of enhancements: Xmodem CRC uses a more reliable error-correction scheme, and Xmodem-1K transfers data faster by sending it in 1,024-byte blocks.

After this protocol is started, a NAK (negative acknowledge) character is sent by receiver, and every 10 seconds. After the first NAK is received, it sends messages of 128 characters, surrounded by some protocol messages. Each block has a SOH (start of header) character, followed by a block number. (ASCII), followed by the same block number with the bit inverted (1’s complement). A 128 character piece of the file is sent, followed by a checksum that is the remainder of the sum of all 128 bytes in the message divided by 255.

Start of header

block number

1’s complement

128 data characters

checksum

The receiver checks each part of the received block.

· Was the first character a SOH

· was the blocknumber one more than previous one

· was exactly 128 character received

· was locally calculated checksum the same as received one?

If receiver is satisfied it will send an Acknowledge.(ACK) back to the transmitter and the transmitter sends next block. If not, a NAK is sent and the transmitter resends the block in error. At the end of all data the transmitter sends an EOT character and the receiver replies with an ACK, session terminates.

Ymodem

This is a protocol for transferring files during direct dial-up communications. So named because it builds on the earlier Xmodem protocol, Ymodem sends data in 1,024-byte blocks and is consequently faster than Xmodem. However, it doesn't work well on noisy phone lines, unlike its successor, Zmodem. Ymodem has undergone a few enhancements: Ymodem-Batch can send several files in one session; Ymodem-G drops software error correction, which speeds up the process by leaving hardware-based error correction in modems.

Zmodem

This file transfer protocol should be your first choice for sending and receiving files using dial-up connections. Zmodem's speed and error checking recommend it, and it can resume a file transfer after a break in communications, so make sure this protocol is available in your communications software and any BBS you dial into. In case you couldn't tell, it's so named because it's intended to supersede Xmodem and Ymodem.

Interface protocols

RS-232- Recommended Standard 232

This was originally a nine-wire interface standard for Teletype machines from the Electronics Industry Association. Now in its third revision (RS-232-C), it's the standard for computer serial-port transfers. The RS-232 standard is probably the only computer component that's 40 years old and still working. One wire is used as the ground; the rest are dedicated to detecting carrier signals, managing the timing of data transfer, oh, and sending and receiving data.

X-21 - CCITT interface digital signal interface

X.21 - A CCITT recommendation that defines a protocol for communication between a circuit-switched network and user devices.

The physical layer is implemented through X-21 CCITT digital signal interface. The interface exists between the DTE and DCE software/hardware. The digital circuit is established in the network layer and is operated by the physical layer using the X-21 protocol. It specifies the functions of the physical layer for leased circuits using digital transmission and also specifies the circuit switching functions of the network layer. It performs transmit, receive, control and timing signal functions. A.21 BIS is used as the physical level interface in X.25 networks (same standards as V.24).

Polling protocols

What is the polling protocols used in single line and multidrop ? They are used in mainframe - terminal connections.

One machine in a multidrop line is controlling the traffic between the different nodes in the network. This is similar to a host controlling a star network. Polling protocols are connection oriented as they assume a circuit can be established when a connection is made.

A connection-less system, however, is based on self-contained packets of data called data grams. This does not establish a link for the duration of the transmission but sends a packet independently from all other packets, which may go by different routes.

Protocol efficiency

There are a number of techniques that be used to transmit data. Those methods can lead to different efficiencies in different situations.

Sliding Window Technique

The sliding window flow control Method of flow control in which a receiver gives transmitter permission to transmit data until a window is full. When the window is full, the transmitter must stop transmitting until the receiver advertises a larger window. The Window works as a Queue, the last frame is added to the window, and the frame that is acknowledged as received by the destination node, will be removed by the sender (First-in-First out). TCP, and other transport protocols, and several link-layer protocols use this method of flow control.

Selective re- transmission of errors

Depending on the nature of the error, the message will be re-transmitted.

Bulk re-transmission of errors

Errors will be held until a certain level of errors has occurred. All errors will then be transmitted.

Types of protocols

Media Access protocols

There are two types of protocols that prescribe access to communication media :

· Probabilistic : access to the media will be probable when a node is ready to send a message. When a message is send, it is assuming that the media is available, unless a collision occurs. The protocol is known as Carrier Sense Multiple Access/ Collision Detection (CSMA/CD).This protocol was developed by Xerox and is used by Ethernet.

· Deterministic : Access to the communications media can be predicted accurately. When a device is ready to sent a message, it will be able to determine whether access is available. The protocol is known as Token Ring and was developed by IBM.

Protocols, Software and Wide Area Networks

REVISION QUESTIONS

· Describe the differences between a LAN, MAN and WAN in terms of distance, hardware, software and media.

· Give a definition of a communications Protocol. Describe the major functionality that is prescribed in a protocol.

· Explain the terms protocol stack, multi protocol stack and protocol suite.

· Explain how CSMA/CD works

· Describe three systems of error checking used in protocols

· Describe the difference between circuit, message and packet switching. Give some advantages of one over the other

· What is host access and terminal emulation

· List some standards bodies that govern standards in network communication

TCP/IP

To keep it simple, the Internet Protocol was developed to create a Network of Networks (the "Internet"). Individual machines are first connected to a LAN (Ethernet or Token Ring). TCP/IP shares the LAN with other uses (a Novell file server, Windows for Workgroups peer systems). One device provides the TCP/IP connection between the LAN and the rest of the world.

As with all other communications protocol, TCP/IP is composed of layers:

• IP - is responsible for moving packet of data from node to node. IP forwards each packet based on a four byte destination address (the IP number). The Internet authorities assign ranges of numbers to different organizations. The organizations assign groups of their numbers to departments. IP operates on gateway machines that move data from department to organization to region and then around the world.
• TCP - is responsible for verifying the correct delivery of data from client to server. Data can be lost in the intermediate network. TCP adds support to detect errors or lost data and to trigger retransmission until the data is correctly and completely received.
• Sockets - is a name given to the package of subroutines that provide access to TCP/IP on most systems.

To insure that all types of systems from all vendors can communicate, TCP/IP is absolutely standardized on the LAN. However, larger networks based on long distances and phone lines are more volatile. In the US, many large corporations would wish to reuse large internal networks based on IBM's SNA. In Europe, the national phone companies traditionally standardize on X.25. However, the sudden explosion of high speed microprocessors, fiber optics, and digital phone systems has created a burst of new options: ISDN, frame relay, FDDI, Asynchronous Transfer Mode (ATM). New technologies arise and become obsolete within a few years. With cable TV and phone companies competing to build the National Information Superhighway, no single standard can govern citywide, nationwide, or worldwide communications.

The original design of TCP/IP as a Network of Networks fits nicely within the current technological uncertainty. TCP/IP data can be sent across a LAN, or it can be carried within an internal corporate SNA network, or it can piggyback on the cable TV service. Furthermore, machines connected to any of these networks can communicate to any other network through gateways supplied by the network vendor.

Addresses

Each technology has its own convention for transmitting messages between two machines within the same network. On a LAN, messages are sent between machines by supplying the six byte unique identifier (the "MAC" address). In an SNA network, every machine has Logical Units with their own network address. DECNET, Appletalk, and Novell IPX all have a scheme for assigning numbers to each local network and to each workstation attached to the network.

On top of these local or vendor specific network addresses, TCP/IP assigns a unique number to every workstation in the world. This "IP number" is a four byte value that, by convention, is expressed by converting each byte into a decimal number (0 to 255) and separating the bytes with a period. For example, the PC Lube and Tune server is 130.132.59.234.

An organization begins by sending electronic mail to [email protected] requesting assignment of a network number. It is still possible for almost anyone to get assignment of a number for a small "Class C" network in which the first three bytes identify the network and the last byte identifies the individual computer. The author followed this procedure and was assigned the numbers 192.35.91.* for a network of computers at his house. Larger organizations can get a "Class B" network where the first two bytes identify the network and the last two bytes identify each of up to 64 thousand individual workstations. Yale's Class B network is 130.132, so all computers with IP address 130.132.*.* are connected through Yale.

The organization then connects to the Internet through one of a dozen regional or specialized network suppliers. The network vendor is given the subscriber network number and adds it to the routing configuration in its own machines and those of the other major network suppliers.

There is no mathematical formula that translates the numbers 192.35.91 or 130.132 into "Yale University" or "New Haven, CT." The machines that manage large regional networks or the central Internet routers managed by the National Science Foundation can only locate these networks by looking each network number up in a table. There are potentially thousands of Class B networks, and millions of Class C networks, but computer memory costs are low, so the tables are reasonable. Customers that connect to the Internet, even customers as large as IBM, do not need to maintain any information on other networks. They send all external data to the regional carrier to which they subscribe, and the regional carrier maintains the tables and does the appropriate routing.

New Haven is in a border state, split 50-50 between the Yankees and the Red Sox. In this spirit, Yale recently switched its connection from the Middle Atlantic regional network to the New England carrier. When the switch occurred, tables in the other regional areas and in the national spine had to be updated, so that traffic for 130.132 was routed through Boston instead of New Jersey. The large network carriers handle the paperwork and can perform such a switch given sufficient notice. During a conversion period, the university was connected to both networks so that messages could arrive through either path.

Subnets

Although the individual subscribers do not need to tabulate network numbers or provide explicit routing, it is convenient for most Class B networks to be internally managed as a much smaller and simpler version of the larger network organizations. It is common to subdivide the two bytes available for internal assignment into a one byte department number and a one byte workstation ID.

The enterprise network is built using commercially available TCP/IP router boxes. Each router has small tables with 255 entries to translate the one byte department number into selection of a destination Ethernet connected to one of the routers. Messages to the PC Lube and Tune server (130.132.59.234) are sent through the national and New England regional networks based on the 130.132 part of the number. Arriving at Yale, the 59 department ID selects an Ethernet connector in the C& IS building. The 234 selects a particular workstation on that LAN. The Yale network must be updated as new Ethernets and departments are added, but it is not effected by changes outside the university or the movement of machines within the department.

A Uncertain Path

Every time a message arrives at an IP router, it makes an individual decision about where to send it next. There is concept of a session with a preselected path for all traffic. Consider a company with facilities in New York, Los Angeles, Chicago and Atlanta. It could build a network from four phone lines forming a loop (NY to Chicago to LA to Atlanta to NY). A message arriving at the NY router could go to LA via either Chicago or Atlanta. The reply could come back the other way.

How does the router make a decision between routes? There is no correct answer. Traffic could be routed by the "clockwise" algorithm (go NY to Atlanta, LA to Chicago). The routers could alternate, sending one message to Atlanta and the next to Chicago. More sophisticated routing measures traffic patterns and sends data through the least busy link.

If one phone line in this network breaks down, traffic can still reach its destination through a roundabout path. After losing the NY to Chicago line, data can be sent NY to Atlanta to LA to Chicago. This provides continued service though with degraded performance. This kind of recovery is the primary design feature of IP. The loss of the line is immediately detected by the routers in NY and Chicago, but somehow this information must be sent to the other nodes. Otherwise, LA could continue to send NY messages through Chicago, where they arrive at a "dead end." Each network adopts some Router Protocol which periodically updates the routing tables throughout the network with information about changes in route status.

If the size of the network grows, then the complexity of the routing updates will increase as will the cost of transmitting them. Building a single network that covers the entire US would be unreasonably complicated. Fortunately, the Internet is designed as a Network of Networks. This means that loops and redundancy are built into each regional carrier. The regional network handles its own problems and reroutes messages internally. Its Router Protocol updates the tables in its own routers, but no routing updates need to propagate from a regional carrier to the NSF spine or to the other regions (unless, of course, a subscriber switches permanently from one region to another).

Undiagnosed Problems

IBM designs its SNA networks to be centrally managed. If any error occurs, it is reported to the network authorities. By design, any error is a problem that should be corrected or repaired. IP networks, however, were designed to be robust. In battlefield conditions, the loss of a node or line is a normal circumstance. Casualties can be sorted out later on, but the network must stay up. So IP networks are robust. They automatically (and silently) reconfigure themselves when something goes wrong. If there is enough redundancy built into the system, then communication is maintained.

In 1975 when SNA was designed, such redundancy would be prohibitively expensive, or it might have been argued that only the Defense Department could afford it. Today, however, simple routers cost no more than a PC. However, the TCP/IP design that, "Errors are normal and can be largely ignored," produces problems of its own.

Data traffic is frequently organized around "hubs," much like airline traffic. One could imagine an IP router in Atlanta routing messages for smaller cities throughout the Southeast. The problem is that data arrives without a reservation. Airline companies experience the problem around major events, like the Super Bowl. Just before the game, everyone wants to fly into the city. After the game, everyone wants to fly out. Imbalance occurs on the network when something new gets advertised. Adam Curry announced the server at "mtv.com" and his regional carrier was swamped with traffic the next day. The problem is that messages come in from the entire world over high speed lines, but they go out to mvt.com over what was then a slow speed phone line.

Occasionally a snow storm cancels flights and airports fill up with stranded passengers. Many go off to hotels in town. When data arrives at a congested router, there is no place to send the overflow. Excess packets are simply discarded. It becomes the responsibility of the sender to retry the data a few seconds later and to persist until it finally gets through. This recovery is provided by the TCP component of the Internet protocol.

TCP was designed to recover from node or line failures where the network propagates routing table changes to all router nodes. Since the update takes some time, TCP is slow to initiate recovery. The TCP algorithms are not tuned to optimally handle packet loss due to traffic congestion. Instead, the traditional Internet response to traffic problems has been to increase the speed of lines and equipment in order to say ahead of growth in demand.

TCP treats the data as a stream of bytes. It logically assigns a sequence number to each byte. The TCP packet has a header that says, in effect, "This packet starts with byte 379642 and contains 200 bytes of data." The receiver can detect missing or incorrectly sequenced packets. TCP acknowledges data that has been received and retransmits data that has been lost. The TCP design means that error recovery is done end-to-end between the Client and Server machine. There is no formal standard for tracking problems in the middle of the network, though each network has adopted some ad hoc tools.

Need to Know

There are three levels of TCP/IP knowledge. Those who administer a regional or national network must design a system of long distance phone lines, dedicated routing devices, and very large configuration files. They must know the IP numbers and physical locations of thousands of subscriber networks. They must also have a formal network monitor strategy to detect problems and respond quickly.

Each large company or university that subscribes to the Internet must have an intermediate level of network organization and expertise. A half dozen routers might be configured to connect several dozen departmental LANs in several buildings. All traffic outside the organization would typically be routed to a single connection to a regional network provider.

However, the end user can install TCP/IP on a personal computer without any knowledge of either the corporate or regional network. Three pieces of information are required:

1. The IP address assigned to this personal computer
2. The part of the IP address (the subnet mask) that distinguishes other machines on the same LAN (messages can be sent to them directly) from machines in other departments or elsewhere in the world (which are sent to a router machine)
3. The IP address of the router machine that connects this LAN to the rest of the world.

In the case of the PCLT server, the IP address is 130.132.59.234. Since the first three bytes designate this department, a "subnet mask" is defined as 255.255.255.0 (255 is the largest byte value and represents the number with all bits turned on). It is a Yale convention (which we recommend to everyone) that the router for each department have station number 1 within the department network. Thus the PCLT router is 130.132.59.1. Thus the PCLT server is configured with the values:

• My IP address: 130.132.59.234
• Subnet mask: 255.255.255.0
• Default router: 130.132.59.1

The subnet mask tells the server that any other machine with an IP address beginning 130.132.59.* is on the same department LAN, so messages are sent to it directly. Any IP address beginning with a different value is accessed indirectly by sending the message through the router at 130.132.59.1 (which is on the departmental LAN).

Requirements for LAN protocols

The requirements of a LAN protocol are not that much different from any other computer communications protocol. However, they do not carry the historical baggage developed in the days when communications systems themselves were very slow and noisy and unreliable.

LAN protocols assume reliable links and high speed. The distances in LANs are small and there is a large usage in file transfers. Each message on a LAN contains the destination node address. Each node on the LAN looks for its address on each message.

A relatively high percentage of protocols are adhering to standards generated by IEEE. IEEE committee 802 has the specific responsibility for all LAN protocols. Currently there are a large number of subcommittees. Each standard is identified by its committee number eg. IEEE 802.2 for the LLC interface standard, 802.3 for the CSMA/CD standards, 802.4 for Token Passing Bus and 802.5 for Token Passing Ring. 802.6 is a standard for MANs.

The characteristics of LANs are :

· flexibility

· speed

· reliability

· hardware and software sharing

· transparent interface

· adaptability

· Access to other LANs and Wans

· Security

· Centralised management

· Private Ownership of a LAN

The major groups of components that make up a LAN are :

· server

· LAN communication system

· workstations

· network software

Bus, ring and star topologies are all used in Lans. The LAN protocols are the rules by which the computers in a LAN communicate.

The most common protocols are still proprietary eg. SPX/IPCX and TCP/IP - and not covered by standards

Two most common protocols for LANs are for LLC (Logical Link Control) and MAC (Media Access Control). The logical Link Control protocol is bit oriented. The protocol data unit is a LLC frame, which looks as follows :

Header	Destination address	Source address	Control field	Data	Trailer
Indicates beginning of frame	Indicates the address of the receiving node	Address of the sending node	Error control etc	The contents of the message	Indicates end of frame

The MAC protocol is CSMA/CD, which uses a basic frame format. It anticipates a conflict between nodes trying to use a communication channel at the same time. It is part of ETHERNET.

Major LAN Systems

There are a variety of System that provide Networking. Examples of some of the major LAN systems are :

· Novell Netware - Novell's Networking System

· LAN Manager/Windows NT - Microsoft's Networking system

· LAN Server - IBM's equivalent to :LAN Manager

· Appletalk - Apple Macintosh networking system

Let's look at each LAN system and determine what protocols they use and the services they provide.

AppleTalk

Apple developed the AppleTalk protocol suite for the networking of Macintosh systems. If provides a variety of connectivity options such as DOS/Windows. Phase II offers an increased number of networked computers and is interoperable with large heterogeneous networks and the included protocols.

Macintoshes have built in circuitry for networking. It uses a fileserver approach and is very simple to use. Appletalk has it own set of protocols but will use other (standard) ones.

AppleTalk provides 3 basic services :

· Remote Access to network files

· Communication services to printers

· File services to DOS/Windows based systems

Some of the AppleTalk protocols are :

	Name	Description	OSI Level
LLAP	LocalTalk	· CSMA/CA protocol for small networks · originally 32 devices at 230 Kbps · phase II - 16,000,000 devices, 300 metres · Ethernet cabling - EtherTalk · Token Ring : TokenTalk	1-2
AARP	AppleTalk Address Resolution	· runs on any Data Link Architecture	2-3
ATP	AppleTalk Transaction Protocol	· provides acknowledgement for delivery of data and initiates a retransmission	4
DDP	Datagram Delivery Protocol	· provides a connectionless service · performs route selection	3
ADSP	AppleTalk Data Stream Protocol	· full duplex connection oriented service that runs on DDP	3
RTMP	Routing Table Maintenance Protocol	· established and maintains routing tables	3
ZIP	Zone Information Protocol	· maintains zone information · maps network numbers to zones	5
NBP	Name binding protocol	· translates between AppleTalk names and node addresses	4
ASP	AppleTalk Session Protocol	· establishes ,maintains and releases sessions	5
PAP	Printer Access Protocol	· this protocol establishes a session between clients and devices	5
AFP	AppleTalk Filing Protocol	· provides access to files	6
	AppleShare Services	· provides shared services to printers, files and access to PC's to files	7

Novell Netware

Originally this Network System was developed for star networks, with one single fileserver. Now, Novell's Netware is a hardware independent system, supporting many topologies and many platforms. Netware supports many Network Cards running protocols such as ARCnet, Ethernet and Token Ring.

The most common client on a Novell Network is the IBM PC or IBM compatible PC.

Netware is currently the most widely used network system. It used to prefer SPX/IPX protocols, but TCP/IP is becoming the more dominant protocol.

Some of the protocols in Netware are :

	Name	functions	OSI Layer
MLID	MultiLink Interface Driver	· software that intialises NIC · Comply with ODI (Open DataLink Interface)	2
LSL	Link Support Layer	· allows MLID to communicate with the network layer protocols · forwards the datagram	2
IPX	Internetwork Packet Exchange	· addressing · route selection · connection services	3-4
RIP	Router Information Protocol	· route discovery protocol, uses hopcount · based solely on IPX packet	3
NSLP	NetWare Link Services Protocol	· route discovery protocol · based on ISO IS-IS protocol · high fault tolerance for mesh networks	3
SPX	Sequenced Packet Exchange Protocol	· provides addressing · connection services	4
NCP	NetWare Core Protocol	· connection services · session management · character and file conversion · service utilisation methods	5-7
SAP	Service Advertising Protocol	· identifies the Netware services and address every minute to the network by sending a Service Identification Packet · client can identify by sending a Service Query Packet	5-7

Windows NT/LAN Manager

Originally this protocol was the same as LAN Server. LAN Server is now the IBM equivalent of Microsoft's LAN manager.

NetBeui

This protocol is excellent for small LANs. It is fast and excellent for Peer-to-Peer Networks. It is non -routable and has very low overheads. It basically uses NetBios in Ethernet Frames.

TCP/IP

This powerful and widely used protocol suite was originally developed by the US department of Defense and some Universities in the early seventies as a general purpose network system. The original networks supported by the suite were ArpaNet and MilNet. They were eventually combined into the Internet.

TCP and IP are the two main protocols in the suite. There are many others and we will discuss a number of these below. TCP/IP is non OSI compliant, but works with (many computer platform. It deals well with non-conformed networks, but is therefore complex and not always reliable. The protocols cover layers 3-7.

IP Addresses

Every node in a TCP/IP network has to have a unique address. The format of the address is four groups of numbers, separated by periods (.) eg. 203.30.141.2 and contains a network part and a host part.

In IP, you can have three types of networks :

· A - huge - millions of hosts, but limited number of networks

· B - medium sized for 64,000 hosts

· C - small, 254 hosts

The structure of the address depends on the network class.

· A - 24 bit hostid

· B - 16 bit hostid

· c - 8 bit host id

The first bit or bits identify the type of address that is following eg. 1-127 identifies A class networks. 127-191 identifies B-class networks, 192-227 identifies C class networks.

Domains

Every network has to have a registered Domain Name. It represents the common part of the organisation's address. It is a normal language name eg. Tamtafe.nsw.edu.au. Parts of the name have a specific meaning, eg. AU means Australia, EDU means educational organisation, COM means a commercial site. The names are managed by DNS (Domain Name Service) protocol, which looks after the name to address linking. You can either use the name or the address to locate a particular site.

Packet Sizes

The packet sizes depend very much on the protocols used at the lower layers. For example :

· MAC frames hold 128 bytes

· Ethernet has 1500 bytes packets

· Token Ring has 8000 bytes packets

· IP NSDU (= Network Service Data Unit) can hold up to 65536 bytes

This means a lot of fragmenting and assembling takes place between different networks.

In TCP/IP data moves between the different protocols as follows :

The Application sends a Protocol Data Unit, including a Port address, down to TCP/IP. This is used up to the transport layer, where IP sends the data down as a IP datagram, which includes the IP address.

TCP/IP suite protocols

	Name	functions	OSI Layer
IP	Internet Protocol	· logical network addressing · packet switching · dynamic route selection · error control for connectivity	3
ICMP	Internet Control Message Protocol	· error handling · flow control	3
RIP	Router Information Protocol	· route discovery · routing decisions	3
OSPF	Open Shortest Path First	· extension to RIP	3
TCP	Transport Control Protocol	· service addressing · connection identification and establishment · sequencing · error management · flow control
UDP	User Datagram Protocol	· connectionless service · fast because of lower overhead · less reliable transfer	4
ARP	Address Resolution Protocol	· resolution of physical or MAC addresses given a logical or IP address	3
FTP	File Transfer Protocol	· connection establishment and release · file transfer · file translation	5-7
SLIP	Serial Line Internet Protocol	· connecting to Internet using modem · no error correction	3
PPP	Point-to-Point Protocol	· successor to SLIP	3
DSN	Domain Naming System	· address and name resolution · uses a distributed database system that maintains a hierarchy of names that are human language	4
SMTP	Simple Mail Transfer Protocol	· messaging services	6
TELNET		· a terminal emulation program	5-7
NFS	Network File System	· file sharing services · remote services	5-7
RPC	Remote Procedure Call	· connection establishment and release · file transfer services	5
HTTP	HyperTexT Protocol		6

A very good reference to TCP/IP, comparison to the OSI model, TCP/IP addressing etc can be found on : http://www.alexia.net.au/~www/yendor/internetinfo/index.html

Protocol Stacks

Strictly speaking, a protocol stack is a collection of protocols that work together towards oe objective . .transmitting data from sender to receiver, effective, efficient and error free. The protocols involved are layered on top of one another. In practise, the protocol stack refers to the software that is required to implement the stack.

Multiple Protocol Stacks in LAN systems allow for a use of one protocol stack for one application needing communications, while using another stack on the same LAN workstation for another application. For example the workstations in our LAB use NetBEUI for local LAN file services, while using TCP/IP to access the Internet through a gateway, using TCP/IP. The workstations are also configured to run under NOVELL Netware, using the SPX/IPX protocols.

Microsoft's NDIS and Novell's ODI both support multiprotocol stacks. This allows a station to process packets from another workstations that runs another protocol, eg. a workstation running AppleTalk could access a Novell Server. A Novell Server would normally run IPX, but with the ODI drivers installed it could process packets from stations with AppleTalk installed.

LAN Support for Terminal Emulation

LANs are high speed communications systems. They do not provide terminal emulation directly, but do support it. There are several options :

· connection to a mainframe through a gateway

· direct connection in the LAN to a host

· gateway from the LAN to the cluster controllers

· terminals servers connected to the LAN

Terminal Servers are essentially multiplexers which make the terminals think they are directly connected the host. The data from the terminals are combined into packets and passed over the LAN using the appropriate protocols. At the hosts they are disassembled into individual strings of data.