By Bradley Mitchell, About.com
Definition: A network protocol defines rules and conventions for communication between network devices. Protocols for computer networking all generally use packet switching techniques to send and receive messages in the form of packets.
Network protocols include mechanisms for devices to identify and make connections with each other, as well as formatting rules that specify how data is packaged into messages sent and received. Some protocols also support message acknowledgement and data compression designed for reliable and/or high-performance network communication. Hundreds of different computer network protocols have been developed each designed for specific purposes and environments.
Internet Protocols
The Internet Protocol family contains a set of related (and among the most widely used network protocols. Besides Internet Protocol (IP) itself, higher-level protocols like TCP, UDP, HTTP, and FTP all integrate with IP to provide additional capabilities. Similarly, lower-level Internet Protocols like ARP and ICMP also co-exist with IP. These higher level protocols interact more closely with applications like Web browsers while lower-level protocols interact with network adapters and other computer hardware.
Routing Protocols
Routing protocols are special-purpose protocols designed specifically for use by network routers on the Internet. Common routing protocols include EIGRP, OSPF and BGP.
How Network Protocols Are Implemented
Modern operating systems like Microsoft Windows contain built-in services or daemons that implement support for some network protocols. Applications like Web browsers contain software libraries that support the high level protocols necessary for that application to function. For some lower level TCP/IP and routing protocols, support is implemented in directly hardware (silicon chipsets) for improved performance.
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Sunday, June 15, 2008
Conclusion
6. CONCLUSION
While the age-old concept of the network is foundational in virtually all areas of society, Computer Networks and Protocols have forever changed the way humans will work, play, and communicate. Forging powerfully into areas of our lives that no one had expected, digital networking is further empowering us for the future. New protocols and standards will emerge, new applications will be conceived, and our lives will be further changed and enhanced. While the new will only be better, the majority of digital networking's current technologies are not cutting-edge, but rather are protocols and standards conceived at the dawn of the digital networking age that have stood solid for over thirty years.
While the age-old concept of the network is foundational in virtually all areas of society, Computer Networks and Protocols have forever changed the way humans will work, play, and communicate. Forging powerfully into areas of our lives that no one had expected, digital networking is further empowering us for the future. New protocols and standards will emerge, new applications will be conceived, and our lives will be further changed and enhanced. While the new will only be better, the majority of digital networking's current technologies are not cutting-edge, but rather are protocols and standards conceived at the dawn of the digital networking age that have stood solid for over thirty years.
The Internet: The Ultimate Network
5. THE INTERNET: THE ULTIMATE NETWORK
The Internet, currently an essential part of a fast-growing number of areas of life, has truly changed the world. Purely a network and entirely based upon OSI Reference Model compliant protocols, the Internet takes everything discussed here and scales it unimaginably. The success of the internet truly shows the flexibility and power of concepts that, when compared in this context, seem relatively simple.
Due to its scale and utilization of many different technologies and concepts, the design of the internet can not be credited to one particular individual. Rather, over time many researchers, students, corporate organizations, and individuals have contributed many technologies and ideas to form what the Internet is today. While the modern day typical internet user thinks of the internet as being web pages and email, in reality the name 'Internet' is referring simply the system on top of which web browsing, email, and innumerable other communication systems operate. The Internet, simply put, is a global network of networks. It is unquestionably the largest switched/routed IP network in existence.
The Internet, currently an essential part of a fast-growing number of areas of life, has truly changed the world. Purely a network and entirely based upon OSI Reference Model compliant protocols, the Internet takes everything discussed here and scales it unimaginably. The success of the internet truly shows the flexibility and power of concepts that, when compared in this context, seem relatively simple.
Due to its scale and utilization of many different technologies and concepts, the design of the internet can not be credited to one particular individual. Rather, over time many researchers, students, corporate organizations, and individuals have contributed many technologies and ideas to form what the Internet is today. While the modern day typical internet user thinks of the internet as being web pages and email, in reality the name 'Internet' is referring simply the system on top of which web browsing, email, and innumerable other communication systems operate. The Internet, simply put, is a global network of networks. It is unquestionably the largest switched/routed IP network in existence.
The Internet Protocol Suite:
4. THE INTERNET PROTOCOL SUITE: A LESSON IN PROTOCOL STACKS
A Protocol Stack is a group of protocols that follows the specification of several consecutive OSI Layers. This section will provide an example of a Protocol Stack/Suite by dissecting the Internet Protocol Suite.
4.1 INTRODUCTION TO THE INTERNET PROTOCOL SUITE
The Internet Protocol Suite is a stack of protocols based on the OSI Reference Model. Undeniably the single most used Protocol Stack in the world, the IP Suite is the primary power behind the internet and a large number of other networks of all sizes. This suite is known as the TCP/IP suite or the IP Suite, despite the fact that it is actually a suite of specifications and consists of more than just the TCP and IP protocols. To make things easier to understand, the TCP/IP suite is often explained using just four layers, each of which represents multiple OSI layers.
4.2 THE INTERNET PROTOCOL SUITE LINK LAYER
While not technically a part of the Internet Protocol Suite, the IP Suite relies on a link layer, just as any other protocol stack would. Without the Link Layer, which represents OSI Layers One and Two, the higher protocols defined in the TCP/IP stack would not function.
An interesting advanced topic that can be considered here is the concept of a Virtual Private Network (VPN) or network "tunnel". A network tunnel links two remote local area networks as if they were one local area network. This operates by running a VPN stack with a TCP/IP stack on top. While this concept may seem complex, the same principles discussed earlier in this document in relation to stacking apply not only to protocols, but to stacks of protocols. While theoretically this concept could extend without limits, it never really does due to protocol overhead (the space consumed by packet headers) and the fact that no widespread practical use has ever existed for more than two or three nested tunnels.
4.3 THE INTERNET PROTOCOL SUITE INTERNETWORK LAYER
Also known as the Internet Layer, due to its almost exclusive use on the medium, this is the level at which packets are routed and switched on networks. The Internet Protocol (IP, not to be confused with the IP Suite) is responsible for getting this job done. As shown when being used for an example on routing in Diagram 3, IP addresses are determined and assigned to each node by IP. IP is an OSI Layer 3 protocol.
4.4 THE INTERNET PROTOCOL SUITE TRANSPORT LAYER
The Internet Protocol Suite's Transport Layer is where the TCP/IP suite shows its broad diversity and capability. Supporting multiple varied mainstream protocols, the IP Suite's Transport Layer provides many options for the protocol and associated feature-set that a node's applications may use. IP Suite Transport Layer protocols fall under the specification of OSI Layers Four and Five. Here are some of the most common IP Suite Transport Layer protocols.
TCP
TCP is a reliable, connection oriented protocol, and is possibly the most commonly used IP Suite Transport Layer protocol. Its advantages are that it is reliable, meaning it will attempt to re-send packets that fail to reach their destination with the same integrity with which they left. In order assist in preventing this issue, TCP attempts to monitor the current load and free capacity based on the action of other TCP network traffic and will throttle its packet sending rate to prevent network packet overload/collision. In addition, TCP will attempt to send packets in roughly the order they originally were intended. TCP performs best when used with an application that does not require timely, ordered information, but does require the information be of good integrity. TCP is classified as an OSI Layer 4 (Transport Layer) protocol.
UDP
Often viewed as being similar to TCP, UDP begins to differ in that it is an unreliable type protocol. This does not mean that it serves its purpose poorly, but rather that UDP does not verify that its packets have reached the destination node successfully, and will not put future packets on hold to retransmit current failed ones. This means that UDP is typically utilized in applications where the integrity of transmitted information is not particularly required, but timely delivery is. UDP is useful in such applications as multimedia streaming because it does not stop to resend bad packets, thus preventing pauses in the media stream. UDP is classified as an OSI Layer 4 (Transport Layer) protocol.
RTP
RTP is a Session Layer (OSI Layer 5) protocol that lies on top of UDP (an OSI Layer 4 protocol). RTP is specifically designed to deliver streaming audio and video content on time and in order. Utilizing UDP for its unreliable time-conscious transmission methods, RTP ensures that packets reach the end node's application both in a timely manner and in the originally intended order.
RTP is a Session Layer (OSI Layer 5) protocol that lies on top of UDP (an OSI Layer 4 protocol). RTP is specifically designed to deliver streaming audio and video content on time and in order. Utilizing UDP for its unreliable time-conscious transmission methods, RTP ensures that packets reach the end node's application both in a timely manner and in the originally intended order.
4.5 THE INTERNET PROTOCOL SUITE APPLICATION LAYER
The IP Suite's Application Layer is where things the common user interacts with come into play. Representing the OSI Reference Model's Layers Six and Seven, the IP Suite has a large number of protocols commonly used on its highest layer.
THE UNIFORM RESOURCE LOCATOR CONCEPT
In order to allow the IP Suite the flexibility to operate using a variety of Transport and Application layer protocols, the need for a uniform way to reference these protocol's resources arises. The IP Suite uses a system known as the Uniform Resource Locator (URL). A URL, as shown in Diagram 5, commonly consists of three parts, but various protocols may have an expanded syntax13 to reflect expanded capability.
The first segment of a URL indicates the Application Layer protocol that will be used for this request. Common examples are http://, https://, and ftp://. The second segment of a URL is an IP address or Host Name14. This tells the IP protocol (OSI Layer 4) the IP (logical) address of the node where the requested resource is located. The third segment of the IP address, indicated in Diagram 5 by the position of the number '80', is the Port Number. The concept of Layer 4 ports is introduced in Section 2.4 of this document. The Port Number in a URL tells the IP protocol the remote port it should attempt to access. Diagram 5 is a URL telling the IP Suite that it should use the HTTP protocol to access the HTTP protocol operating on port 80 of the node located at 127.0.0.1.
HTTP
Possibly the most recognizable protocol yet discussed here, HTTP is the HyperText Transport Protocol. Following the specification of OSI Layer 7, the HTTP protocol is responsible for fetching, sending, and receiving files per the requests of the end user. HTTP is commonly used inside of computer programs called browsers15 to allow for the quick viewing of many filetypes and for ease of navigation among them.
The average person would likely recognize HTTP as being 'those four letters typed at the beginning of a web page address', and would be correct since websites operate primarily on the HTTP protocol. Thus a website's URL might look something like: http://NSGN.net. HTTP typically operates by default on TCP Port 80.
HTTPS
HTTPS operates identically in every way to HTTP except that it encrypts all packets it handles on-the-fly. HTTPS requires an encryption certificate to operate properly. A certificate is a digital document that only the two end users transferring information via HTTPS posses. The certificate contains the encryption/decryption key, thus the only end users able to make use of the information transmitted over the HTTPS connection are the two who hold certificates. HTTPS typically operates by default on TCP Port 443.
FTP
FTP is the File Transfer Protocol. It is a protocol used for the transferring of files between two nodes over a network. While FTP is far from being the only file transfer protocol designed to run on top of the IP Suite, it is one of the first and in many ways is unparalleled. The FTP protocol is commonly used through computer programs known as "FTP Clients". These software applications send and receive FTP commands and present the various information to the user. An FTP session, depending on the software application in use, may sometimes be initiated by a URL beginning with ftp:// . FTP typically operates by default on TCP Port 21.
SSH
SSH is the Secure SHell protocol. Used primarily on business or server computer operating systems, the SSH protocol allows a node to be remotely controlled or administrated. The SSH protocol typically operates by default on TCP Port 22.
4.6SUMMARY OF THE INTERNET PROTOCOL SUITE
Despite the popularity and large user base for the example IP suite protocols discussed here, numerous others exist. The specification is just as open as the OSI Reference Model, because in reality the IP Suite is simply a specification calling for OSI compliant protocols to communicate in the same method. The IP Suite is an excellent example of what is known as a de-facto standard.
13 Syntax; The rules for the construction of a command or statement.
14 Host Name; A unique name that identifies a computer or server on the Internet. In layman's terms, a name that points to an IP address. Host names are used primarily because they are easier for humans to comprehend and remember.
15 Browser; A computing program with a graphical user interface for displaying HTML files, used to navigate the World Wide Web : a Web browser.
Common Network Hardware
MODERN NETWORK HARDWARE & INFRASTRUCTURE STANDARDS
Since the OSI Model is a reference rather than a standard, it leaves room for standards of many different designs and applications to be made in its image. Based upon OSI Layers One and Two, these standards fill the gaps that exist in both technology and geographical distance allowing digital communication to spreader farther, carry information faster, and cost less.
3.1 ETHERNET (IEEE 802.3)
Ethernet is used to link computers in both small residential and large commercial situations and is the most widely used Network Hardware Standard today. It often delivers internet access from other longer range hardware standards to multiple computers within a home or workplace. Ethernet equipment is relatively small, affordable, and can carry data at high speeds. The original specification called for speeds of 10mbps9, while newer technologies have brought that speed today to lie between 100mbps and 1000mbps. Ethernet is rarely used outside the of local area networks found inside of business or homes due to it's range limitations. Ethernet installations typically can not run for more than a few thousand feet. This leaves other network infrastructures to link computers over great distances. Ethernet most commonly forms what is known as the LAN, or Local Area Network. A LAN is a collection of computers in close proximity that are linked together to form a network. This network may then be linked to other LANs via network infrastructure standards with long distance capabilities.
Originally developed at Xerox's PARC10 facility, the project was predominately conceived and headed by a man named Robert Metcalfe. In 1976, Metcalfe and his team published a paper entitled Ethernet: DIstributed Packet Switching For Local Computer Networks which drew out conceptual specifications for the Ethernet standard. Though the standard defined in his paper was for 3mbps 8bit communication, Ethernet would soon evolve into its more modern-day form when Metcalfe left Xerox to form 3Com. In 1980, he encouraged major companies such as DEC, Intel, and Xerox to participate in a standard he called "DIX" (for DEC, Intel, Xerox). This standard defined Ethernet as having 10mbps speeds and would end up competing directly against the day's largest proprietary systems.
The Ethernet standard demonstrates its flexibility by supporting multiple transmission mediums. The original medium, known as 10BASE-2, utilized BNC11 type coaxial connections and coaxial cabling. This was the standard for many years, transmitting data at a rate of 10mbps. 10BASE-2, however, became increasingly cumbersome, requiring high maintenance. A complete circuit was required for proper operation, meaning that a single failed node or cable break on a large network cause a cease of proper operation. In the early 90s, the newer 10BASE-T standard emerged. 10BASE-T utilizes twisted pair cable, which is similar to copper phone lines but differs in that it carries four twisted pairs instead of one or two. Operating at speeds of either 10mbps, or later 100mbps (100BASE-T), this standard has become and remains the most widely used network standard in the world. In the late 90s, Gigabit Ethernet came into existence, allowing for transfer speeds of up to 1000mbps over the same twisted pair cabling. The Gigabit Ethernet standard is also capable of transmitting over optical cable, though this ability has not gained a following due to the existence of superior high end fiber optic network standards.
While there are numerous other standards operating under the IEEE 802.3 specification, most others are used in niche markets or private deployments for very large network backbones. Ethernet has been and will remain for years the most used standard for the transmission of digital information over short distances.
3.2 WI-FI (802.11x)
Wi-Fi is a standard developed to perform nearly the same role as Ethernet does in consumer settings, but without the wires. Taking to the air, Wi-Fi allows a node to lie anywhere within a 100 to 1000 foot range of a Wi-Fi enabled router and have a constant, secure connection to the Local Area Network. Wi-Fi originated with speeds of just 11mbps in the form of IEEE 802.11b, but today can achieve speeds between 54mbps and 108mbps.
In 1991, the original Wi-Fi standard was developed at AT&T by a man named Vic Hayes. It was initially designed to provide wireless communication for cashier systems in retail locations and operated at speeds of 1 or 2mbps. In the late 90s, the IEEE ratified the 802.11b specification, providing wireless ethernet-like connectivity for nodes at speed steps between 1 and 11mbps. The varied speeds allowed a node's hardware to switch to a lower transmission speed when further from the access point12 in order to maintain the connection over a longer distance.
In 2003 and 2004 IEEE 802.11g, a newer standard based on the Wi-Fi specification, emerged. 802.11g provides speeds between 11 and 54mbps while still maintaining backward compatibility with the older 802.11b standard and it's 1 to 11mbps speed range. This means 802.11g is able to offer superior speeds while still capable of reverting to lower speed, long range transmission rates when necessary.
In 2005, an even newer standard began to emerge known as 802.11n. While not officially ratified to date, so called "Pre-N" devices have begun to be sold in the consumer marketplace based upon the 802.11n standard that is still in the ratification process at the IEEE. The 802.11n specification calls for transmission speeds of 108 to 540mbps while still maintaining full support for the 802.11b/g standards speeds between 1 and 54mbps. While in the 802.11g standard the longer range, lower speed backward compatible 802.11b standard was utilized to increase the range and connection stability nodes received when further from the 'g' access point, 802.11n uses previous standards almost exclusively for compatibility with older equipment. This is due to the fact that 802.11n devices are able to communicate at 54 to 108mbps speeds at ranges greater than those offered by 802.11b when operating at 1 to 5mbps. 802.11n is not expected to begin to receive widespread adoption until late 2006 or 2007, both because it has not yet received IEEE certification and it has the current standard's enormous market saturation to attempt to replace.
The issue of range has greatly marred the performance of 802.11 equipment for years. This problem is obvious when it is considered that most 802.11b equipment actually only operates at 5 to 7mbps, and 802.11g equipment at 24 to 36mbps during real- world use. Fortunately, with the improvements brought by the 802.11n and future standards, this problem will begin to fade as the speeds achieved during everyday use close in on the technical maximum speeds offered by emerging standards.
3.3 BLUETOOTH (802.15.1)
Bluetooth, IEEE 802.15.1, is a short range wireless network standard originally developed by Ericsson Corporation. Designed to allow nodes to participate in a network using the lowest possible amount of power, Bluetooth supports three power/range levels: 1mW/10cm, 2.5mW/10m, and 100mW/100m. Bluetooth's current maximum transmission rate is 2.1mbps. While this seems very low compared to much older Wi-Fi standards such as 802.11b, Bluetooth is designed to fit a special section of the market, rather than to be a widespread, high-performance technology. Bluetooth is almost always used in a paired or "ad-hoc" type network. In an ad-hoc network, no router exists, but the nodes are simply responsible for negotiating communication among themselves automatically. A paired network is simply an ad-hoc network with only two nodes.
Common Bluetooth devices and applications include mobile phone headsets, PC-to-organizer/PDA synchronization, and other situations in which small devices need low power, short range communication capability. Bluetooth is a staple feature on most of today's newest and smallest portable information and communication devices.
9 mbps; Megabits per second. Equal to roughly one million bits per second.
10 PARC; The Xerox Palo Alto Research Center located in Palo Alto, California.
11 BNC; A type of RF connector used for terminating coaxial cable. The connector was named after its "Bayonet" locking mechanism and its two inventors "Neill" and "Concelman".
12 Access Point; Commonly the term for a wirelessly enabled network router or switch.
Since the OSI Model is a reference rather than a standard, it leaves room for standards of many different designs and applications to be made in its image. Based upon OSI Layers One and Two, these standards fill the gaps that exist in both technology and geographical distance allowing digital communication to spreader farther, carry information faster, and cost less.
3.1 ETHERNET (IEEE 802.3)
Ethernet is used to link computers in both small residential and large commercial situations and is the most widely used Network Hardware Standard today. It often delivers internet access from other longer range hardware standards to multiple computers within a home or workplace. Ethernet equipment is relatively small, affordable, and can carry data at high speeds. The original specification called for speeds of 10mbps9, while newer technologies have brought that speed today to lie between 100mbps and 1000mbps. Ethernet is rarely used outside the of local area networks found inside of business or homes due to it's range limitations. Ethernet installations typically can not run for more than a few thousand feet. This leaves other network infrastructures to link computers over great distances. Ethernet most commonly forms what is known as the LAN, or Local Area Network. A LAN is a collection of computers in close proximity that are linked together to form a network. This network may then be linked to other LANs via network infrastructure standards with long distance capabilities.
Originally developed at Xerox's PARC10 facility, the project was predominately conceived and headed by a man named Robert Metcalfe. In 1976, Metcalfe and his team published a paper entitled Ethernet: DIstributed Packet Switching For Local Computer Networks which drew out conceptual specifications for the Ethernet standard. Though the standard defined in his paper was for 3mbps 8bit communication, Ethernet would soon evolve into its more modern-day form when Metcalfe left Xerox to form 3Com. In 1980, he encouraged major companies such as DEC, Intel, and Xerox to participate in a standard he called "DIX" (for DEC, Intel, Xerox). This standard defined Ethernet as having 10mbps speeds and would end up competing directly against the day's largest proprietary systems.
The Ethernet standard demonstrates its flexibility by supporting multiple transmission mediums. The original medium, known as 10BASE-2, utilized BNC11 type coaxial connections and coaxial cabling. This was the standard for many years, transmitting data at a rate of 10mbps. 10BASE-2, however, became increasingly cumbersome, requiring high maintenance. A complete circuit was required for proper operation, meaning that a single failed node or cable break on a large network cause a cease of proper operation. In the early 90s, the newer 10BASE-T standard emerged. 10BASE-T utilizes twisted pair cable, which is similar to copper phone lines but differs in that it carries four twisted pairs instead of one or two. Operating at speeds of either 10mbps, or later 100mbps (100BASE-T), this standard has become and remains the most widely used network standard in the world. In the late 90s, Gigabit Ethernet came into existence, allowing for transfer speeds of up to 1000mbps over the same twisted pair cabling. The Gigabit Ethernet standard is also capable of transmitting over optical cable, though this ability has not gained a following due to the existence of superior high end fiber optic network standards.
While there are numerous other standards operating under the IEEE 802.3 specification, most others are used in niche markets or private deployments for very large network backbones. Ethernet has been and will remain for years the most used standard for the transmission of digital information over short distances.
3.2 WI-FI (802.11x)
Wi-Fi is a standard developed to perform nearly the same role as Ethernet does in consumer settings, but without the wires. Taking to the air, Wi-Fi allows a node to lie anywhere within a 100 to 1000 foot range of a Wi-Fi enabled router and have a constant, secure connection to the Local Area Network. Wi-Fi originated with speeds of just 11mbps in the form of IEEE 802.11b, but today can achieve speeds between 54mbps and 108mbps.
In 1991, the original Wi-Fi standard was developed at AT&T by a man named Vic Hayes. It was initially designed to provide wireless communication for cashier systems in retail locations and operated at speeds of 1 or 2mbps. In the late 90s, the IEEE ratified the 802.11b specification, providing wireless ethernet-like connectivity for nodes at speed steps between 1 and 11mbps. The varied speeds allowed a node's hardware to switch to a lower transmission speed when further from the access point12 in order to maintain the connection over a longer distance.
In 2003 and 2004 IEEE 802.11g, a newer standard based on the Wi-Fi specification, emerged. 802.11g provides speeds between 11 and 54mbps while still maintaining backward compatibility with the older 802.11b standard and it's 1 to 11mbps speed range. This means 802.11g is able to offer superior speeds while still capable of reverting to lower speed, long range transmission rates when necessary.
In 2005, an even newer standard began to emerge known as 802.11n. While not officially ratified to date, so called "Pre-N" devices have begun to be sold in the consumer marketplace based upon the 802.11n standard that is still in the ratification process at the IEEE. The 802.11n specification calls for transmission speeds of 108 to 540mbps while still maintaining full support for the 802.11b/g standards speeds between 1 and 54mbps. While in the 802.11g standard the longer range, lower speed backward compatible 802.11b standard was utilized to increase the range and connection stability nodes received when further from the 'g' access point, 802.11n uses previous standards almost exclusively for compatibility with older equipment. This is due to the fact that 802.11n devices are able to communicate at 54 to 108mbps speeds at ranges greater than those offered by 802.11b when operating at 1 to 5mbps. 802.11n is not expected to begin to receive widespread adoption until late 2006 or 2007, both because it has not yet received IEEE certification and it has the current standard's enormous market saturation to attempt to replace.
The issue of range has greatly marred the performance of 802.11 equipment for years. This problem is obvious when it is considered that most 802.11b equipment actually only operates at 5 to 7mbps, and 802.11g equipment at 24 to 36mbps during real- world use. Fortunately, with the improvements brought by the 802.11n and future standards, this problem will begin to fade as the speeds achieved during everyday use close in on the technical maximum speeds offered by emerging standards.
3.3 BLUETOOTH (802.15.1)
Bluetooth, IEEE 802.15.1, is a short range wireless network standard originally developed by Ericsson Corporation. Designed to allow nodes to participate in a network using the lowest possible amount of power, Bluetooth supports three power/range levels: 1mW/10cm, 2.5mW/10m, and 100mW/100m. Bluetooth's current maximum transmission rate is 2.1mbps. While this seems very low compared to much older Wi-Fi standards such as 802.11b, Bluetooth is designed to fit a special section of the market, rather than to be a widespread, high-performance technology. Bluetooth is almost always used in a paired or "ad-hoc" type network. In an ad-hoc network, no router exists, but the nodes are simply responsible for negotiating communication among themselves automatically. A paired network is simply an ad-hoc network with only two nodes.
Common Bluetooth devices and applications include mobile phone headsets, PC-to-organizer/PDA synchronization, and other situations in which small devices need low power, short range communication capability. Bluetooth is a staple feature on most of today's newest and smallest portable information and communication devices.
9 mbps; Megabits per second. Equal to roughly one million bits per second.
10 PARC; The Xerox Palo Alto Research Center located in Palo Alto, California.
11 BNC; A type of RF connector used for terminating coaxial cable. The connector was named after its "Bayonet" locking mechanism and its two inventors "Neill" and "Concelman".
12 Access Point; Commonly the term for a wirelessly enabled network router or switch.
The OSI Reference Model
2. THE OSI REFERENCE MODEL
The second layer in the OSI Model is the Data Link Layer. The primary focus of the Data Link Layer is revealed in its common nickname, The Physical Address Layer. The only layer in the OSI Model that specifically addresses both hardware and software, the Data Link Layer receives information on its software side from higher layers, places this information inside of "frames", and finally gives this frame to the Physical Layer, Layer 1, for transmission as pure binary. A frame essentially takes the information passed down from a higher layer and surrounds it with Physical Addressing information. This information is important to the Data Link Layer on the receiving end of the transmission. When the frame, in binary form, arrives at the destination node3, it is passed from the transmission medium to the Data Link Layer (Layer 2) by the Physical Layer (Layer 1). The Data Link Layer on the receiving node then checks the frame surrounding the information received to see if it's Physical Address matches that of its own. If the Physical Address does not match, the frame and its encapsulated data is discarded. If the Physical Address is a match, then the information is removed from the frame and passed up to the next highest layer in the OSI Model. The Physical Address check is Obviously not of much use if there are only two nodes on a network, but suddenly becomes extremely valuable when three or more nodes exist. The Physical Addressing system allows multiple nodes to be on the same network medium, but retain the ability to address only a specific node with a transmission. On the simplest networks, all nodes receive every frame transmitted on the network, but discard frames not specifically addressed to them.
The Physical Address used in the Data Link Layer's Physical Addressing system is known as a MAC4 address and is embedded physically into the node's Network Interface Card during manufacturing. Every NIC's MAC address is unique in order to prevent addressing conflicts. It is this relationship that causes the Data Link Layer to be known as the only layer that addresses both hardware and software. This layer is where the information on the network makes the move from the physical infrastructure of the network into the software realm. The remainder of the OSI Reference Model's layers are entirely software.
OSI Layer 3 is known as the Network Layer. The first layer to deal entirely in software, the purpose of the Network Layer is to direct network traffic to a destination node who's Physical Address is not known. This is achieved through a system known as Logical Addressing. Logical Addresses are software addresses assigned to a node at Layer 3 of the OSI Model. Since these addresses are able to be defined by software rather than being random and permanent like Physical Addresses, Logical Addresses are able to be hierarchical. This allows extremely large networks to be possible. Up until this point, only small networks would be possible since all traffic was addressed to all nodes. This works fine until more than one person attempts to utilize the network at once, at which point a data "collision" occurs. While OSI Layer 4 protocols may attempt to compensate for this collision by retransmitting packets until they have reached the destination node without issue, this degrades network performance exponentially as the number of nodes on a network grows. The larger the network is the greater this issue becomes. OSI Layer 3 takes on this problem by its Logical Addressing system and a concept known as routing.
The Oxford American Dictionary defines routing as "Sending or directing along a specified course". Layer 3 routing on a network takes this foundational definition and puts it to use to enable millions of computers, rather than just a handful, to communicate at once without interference. This is achieved by having a smart device working at Layer 3 that handles network signals from each node directly rather than nodes just blindly repeating packets at Layer 1 until they happen to reach their destination. Such a device is known as a network router. A network router sits in the center of a network with all nodes having a direct link to it rather than being linked to each other. This strategic position allows the router to intercept and direct all traffic on the network. A routed network can be illustrated by a star formation, as shown in Diagram 1. On a routed network, Layer 3 packets are no longer broadcasted to all nodes, but rather received by the router and passed on only to the appropriate node. This is a valuable concept because it allows for the collision free-transport of packets across a network.
2.4 OSI LAYER 4: THE TRANSPORT LAYER
OSI Layer 4 is known as the Transport Layer. Since we are now above Layer 3, all information transfered is assumed to be at the correct destination node and is being passed up to Layer 4. The Transport Layer is responsible for the reliability of the link between two end users and for dividing the data that is being transmitted by assigning port numbers to its Layer 4 packages, known as segments. Ports can be thought of as virtual destination mailboxes or outlets. When information reaches a Layer 4 protocol, the segment is examined to determine the destination port of the data it contains. Once the port is determined, just as all of the past layers have done, the wrapper is discarded and the payload data passed up to the next layer's protocol.
Ports allow more than one set of Layer 5-7 protocols to exist on a single node. This is important if the node has more than one purpose. Modern home computers utilize many ports during everyday use, because the modern computer user demands that a computer serve many purposes at once. Higher layer protocols that provide services such as email, web browsing, text chat, file transfer and more each operate on their own unique Layer 4 port, allowing all of these protocols to be operated at once without interference.
On the reliability front, Transport Layer protocols can be capable of running a checksum7 on the payload data they carry. This allows the protocol to determine the integrity of incoming payload data. If this data has been corrupted or its integrity compromised, the Layer 4 protocol will request the segment be retransmitted.
2.5 OSI LAYER 5: THE SESSION LAYER
While being an optional layer in most protocol packages today, OSI Layer 5, known as the Session Layer, still serves a purpose in the OSI Reference Model. The Session Layer draws the outline for protocols that manage the combination and synchronization of data from two separate higher layers. Layer 5 protocols are responsible for ensuring that the data is synced and consistent before transmitted. A good example situation is the streaming of live multimedia audio and video, where near perfect synchronization between video and audio is desired.
2.6 OSI LAYERS 6 & 7: THE PRESENTATION AND APPLICATION LAYERS
The sixth and seventh layers in the OSI Reference Model are the Presentation Layer and the Application Layer. The primary purpose of these layers is to facilitate the movement of formatted information between applications interacting with end users on nodes by way of the lower layer protocols. Commonly used top layer protocols are HTTPS (for the secure transfer of web page related files), File Transfer Protocol (FTP), Simple Mail Transfer Protocol (SMTP, used for the sending of email messages), and SSH (Secure Shell, used for secure remote shell8 access to a computer operating system).
2.7 SUMMARY OF THE OSI REFERENCE MODEL CONCEPT
The concept of how a modern day network operates can be understood by dissecting it into seven layers. This seven layer model is known as the OSI Reference Model and defines how the vast majority of the digital networks on earth function. OSI is the acronym for Open Systems Interconnection, which was an effort formed by the International Organization for Standardization in 1982 with the goal of producing a standard reference model for the hardware and software connection of digital equipment. The important concept to realize about the OSI Reference Model is that it does not define a network standard, but rather provides guidelines for the creation of network standards. The OSI has become so accurate a concept that almost all major network standards in use today conform entirely to it's seven layer model. Though seven layers may at first appear to make a network seem overly complex, the seven layer OSI Model has been proven over the past twenty years to be the most efficient and effective way to understand this extremely complex subject.
2.1 OSI LAYER 1: THE PHYSICAL LAYER
The first and foundational layer of a network is the Physical Layer. The Physical Layer is literally what it's name implies: the physical infrastructure of a network. This includes the cabling or other transmission medium and the network interface hardware placed inside of computers and other devices which enable them to connect to the transmission medium. The purpose of the Physical Layer is to take binary1 information from higher layers, translate it into a transmission signal or frequency, transmit the information across the transmission medium, receive this information at the destination, and finally translate it back into binary before passing it up to higher layers. Transmission signals or frequencies vary between network standards and can be as simple as pulses of electricity over copper wiring or as complex as flickers of light on optical lines or amplified radio frequency transmissions. The information that enters and exits the Physical Layer must be bits; either 0s or 1s in binary. The higher layers are responsible for providing the Physical Layer with binary information. Since nearly all information inside of a computer is already digital2, this is not difficult to achieve. The Physical Layer does not examine the binary information nor does it validate it or make changes to it. The Physical Layer is simply intended to transport the binary information between higher layers located at points A and B.
2.2 OSI LAYER 2: The DATA LINK LAYER
The second layer in the OSI Model is the Data Link Layer. The primary focus of the Data Link Layer is revealed in its common nickname, The Physical Address Layer. The only layer in the OSI Model that specifically addresses both hardware and software, the Data Link Layer receives information on its software side from higher layers, places this information inside of "frames", and finally gives this frame to the Physical Layer, Layer 1, for transmission as pure binary. A frame essentially takes the information passed down from a higher layer and surrounds it with Physical Addressing information. This information is important to the Data Link Layer on the receiving end of the transmission. When the frame, in binary form, arrives at the destination node3, it is passed from the transmission medium to the Data Link Layer (Layer 2) by the Physical Layer (Layer 1). The Data Link Layer on the receiving node then checks the frame surrounding the information received to see if it's Physical Address matches that of its own. If the Physical Address does not match, the frame and its encapsulated data is discarded. If the Physical Address is a match, then the information is removed from the frame and passed up to the next highest layer in the OSI Model. The Physical Address check is Obviously not of much use if there are only two nodes on a network, but suddenly becomes extremely valuable when three or more nodes exist. The Physical Addressing system allows multiple nodes to be on the same network medium, but retain the ability to address only a specific node with a transmission. On the simplest networks, all nodes receive every frame transmitted on the network, but discard frames not specifically addressed to them.
The Physical Address used in the Data Link Layer's Physical Addressing system is known as a MAC4 address and is embedded physically into the node's Network Interface Card during manufacturing. Every NIC's MAC address is unique in order to prevent addressing conflicts. It is this relationship that causes the Data Link Layer to be known as the only layer that addresses both hardware and software. This layer is where the information on the network makes the move from the physical infrastructure of the network into the software realm. The remainder of the OSI Reference Model's layers are entirely software.
2.3 OSI LAYER 3: THE NETWORK LAYER
OSI Layer 3 is known as the Network Layer. The first layer to deal entirely in software, the purpose of the Network Layer is to direct network traffic to a destination node who's Physical Address is not known. This is achieved through a system known as Logical Addressing. Logical Addresses are software addresses assigned to a node at Layer 3 of the OSI Model. Since these addresses are able to be defined by software rather than being random and permanent like Physical Addresses, Logical Addresses are able to be hierarchical. This allows extremely large networks to be possible. Up until this point, only small networks would be possible since all traffic was addressed to all nodes. This works fine until more than one person attempts to utilize the network at once, at which point a data "collision" occurs. While OSI Layer 4 protocols may attempt to compensate for this collision by retransmitting packets until they have reached the destination node without issue, this degrades network performance exponentially as the number of nodes on a network grows. The larger the network is the greater this issue becomes. OSI Layer 3 takes on this problem by its Logical Addressing system and a concept known as routing.
The Oxford American Dictionary defines routing as "Sending or directing along a specified course". Layer 3 routing on a network takes this foundational definition and puts it to use to enable millions of computers, rather than just a handful, to communicate at once without interference. This is achieved by having a smart device working at Layer 3 that handles network signals from each node directly rather than nodes just blindly repeating packets at Layer 1 until they happen to reach their destination. Such a device is known as a network router. A network router sits in the center of a network with all nodes having a direct link to it rather than being linked to each other. This strategic position allows the router to intercept and direct all traffic on the network. A routed network can be illustrated by a star formation, as shown in Diagram 1. On a routed network, Layer 3 packets are no longer broadcasted to all nodes, but rather received by the router and passed on only to the appropriate node. This is a valuable concept because it allows for the collision free-transport of packets across a network.
As well as being linked directly to all nodes in a local network, a router can be linked directly to other routers. This allows groups of nodes separated by distance to communicate with each other in a practical way. It would not be practical to have nodes separated by a great distance all connect to a single router. The amount of cabling required would be immense, and depending on the number of nodes involved, the router may not posses the required number of physical connections. Placing a router at each group of nodes and running a single line from router to router, however, is quite practical. Routers can be chained in a line, or as shown in Diagram 2, can be connected by a central router. This concept is virtually infinitely scalable and is very efficient. 
When a node starts a transmission, the OSI Layer 3 protocol takes the information passed down from higher layers and encapsulates it with the logical address of the destination node in a unit called a packet. This packet, then passing through the remaining lower layer protocols, is transmitted over the network medium from the node to the router. This router reads the logical address that the packet contains and compares it to a list of physical addresses of nodes directly connected to it. If the packet's destination address matches an entry in this list, the packet is transmitted directly on the line that leads straight to the destination node. If the router does not know of a direct connection to the destination node, the packet is transmitted on a line leading directly to another router. This router then treats the packet much like the first router did upon receipt. The packet's logical address is checked for matches against the list of logical addresses belonging to nodes directly connected to the router. If the packet reaches a router with connections only to other routers, as shown in Diagram 2, the router uses the logical address' orderly numbering scheme to try and determine the closest router to the destination node and then transmits the packet to that router.
When a node starts a transmission, the OSI Layer 3 protocol takes the information passed down from higher layers and encapsulates it with the logical address of the destination node in a unit called a packet. This packet, then passing through the remaining lower layer protocols, is transmitted over the network medium from the node to the router. This router reads the logical address that the packet contains and compares it to a list of physical addresses of nodes directly connected to it. If the packet's destination address matches an entry in this list, the packet is transmitted directly on the line that leads straight to the destination node. If the router does not know of a direct connection to the destination node, the packet is transmitted on a line leading directly to another router. This router then treats the packet much like the first router did upon receipt. The packet's logical address is checked for matches against the list of logical addresses belonging to nodes directly connected to the router. If the packet reaches a router with connections only to other routers, as shown in Diagram 2, the router uses the logical address' orderly numbering scheme to try and determine the closest router to the destination node and then transmits the packet to that router.
As we can now see, the OSI Reference Model Layer 3 is one of the most complex, but most functionally important, parts of the modern day network. The Layer 3 protocol IP stands for Internet Protocol and is the protocol handling virtually all traffic on the internet today. The fashion in which Layer 3 protocols connect computers in a star-shaped, extensible network is much of the reason the internet is commonly called the "web".
2.4 OSI LAYER 4: THE TRANSPORT LAYER
OSI Layer 4 is known as the Transport Layer. Since we are now above Layer 3, all information transfered is assumed to be at the correct destination node and is being passed up to Layer 4. The Transport Layer is responsible for the reliability of the link between two end users and for dividing the data that is being transmitted by assigning port numbers to its Layer 4 packages, known as segments. Ports can be thought of as virtual destination mailboxes or outlets. When information reaches a Layer 4 protocol, the segment is examined to determine the destination port of the data it contains. Once the port is determined, just as all of the past layers have done, the wrapper is discarded and the payload data passed up to the next layer's protocol.
Ports allow more than one set of Layer 5-7 protocols to exist on a single node. This is important if the node has more than one purpose. Modern home computers utilize many ports during everyday use, because the modern computer user demands that a computer serve many purposes at once. Higher layer protocols that provide services such as email, web browsing, text chat, file transfer and more each operate on their own unique Layer 4 port, allowing all of these protocols to be operated at once without interference.
On the reliability front, Transport Layer protocols can be capable of running a checksum7 on the payload data they carry. This allows the protocol to determine the integrity of incoming payload data. If this data has been corrupted or its integrity compromised, the Layer 4 protocol will request the segment be retransmitted.
2.5 OSI LAYER 5: THE SESSION LAYER
While being an optional layer in most protocol packages today, OSI Layer 5, known as the Session Layer, still serves a purpose in the OSI Reference Model. The Session Layer draws the outline for protocols that manage the combination and synchronization of data from two separate higher layers. Layer 5 protocols are responsible for ensuring that the data is synced and consistent before transmitted. A good example situation is the streaming of live multimedia audio and video, where near perfect synchronization between video and audio is desired.
2.6 OSI LAYERS 6 & 7: THE PRESENTATION AND APPLICATION LAYERS
The sixth and seventh layers in the OSI Reference Model are the Presentation Layer and the Application Layer. The primary purpose of these layers is to facilitate the movement of formatted information between applications interacting with end users on nodes by way of the lower layer protocols. Commonly used top layer protocols are HTTPS (for the secure transfer of web page related files), File Transfer Protocol (FTP), Simple Mail Transfer Protocol (SMTP, used for the sending of email messages), and SSH (Secure Shell, used for secure remote shell8 access to a computer operating system).
2.7 SUMMARY OF THE OSI REFERENCE MODEL CONCEPT
The OSI Reference Model exists not to make hard rules or to shape the industry, but to provide a logical, well-researched, and tested model after which the world's best communication protocol "stacks" are modeled. Protocol stacks are a set of two or more protocols that stack on top of each other following the lead of the OSI Reference Model's layered format. The TCP/IP stack is very well-known for being the driving force behind most of the internet, and represents the third (IP) and fourth (TCP) layers of the OSI Model. Every layer in the OSI Model is a reference for a protocol which must facilitate communication between both higher and lower layers. The "U-shaped" example shown in Diagram 4 provides a visual concept of how two users may be linked on a given network in reference to the OSI Model. Data starts and ends with the user. From the Application Layer of the first user, it must travel down through layers 7 to 1, across the transmission medium, then back up layers 1 to 7 to be presented at the Application Layer to the user on the end of the transmission. Diagram 4, of course, only shows an example of a path between two nodes. On node diagrams such as Diagram 1 and Diagram 2, each node is assumed to be operating some stack of OSI based protocols. Protocols defined by this reference are dependent on the next lowest layer protocol. So, for example, one could not run an Application Layer protocol on a node without the presence of Layer 1 through 6 protocols also being utilized on the node. A node could, however, operate with only three layer protocols if it just needed to interact with information in Network Layer (Layer 3) Packets. An example of such a node would be a router, since routers do not need to decipher payload data from layers any higher than Layer 3, the layer which caries the routing information. This stackable concept allows nodes to operate on a scalable range of complexity and capability.
1 Binary; Expressed in a system of numerical notation that has
2 rather than 10 as a base2 meaning 'in binary'
3 Node; A piece of equipment, such as a computer or other device, attached to a network.
4 MAC; Media Access Control. The lower sub-layer of the OSI data link layer. The upper and lower sub-layers of the Data Link Layer are not covered in this document.
5 IPv4 is the standard used for this example. The forthcoming IPv6 uses a different logical addressing scheme, to the same end.
6 IPv4 is the standard used for this example. These figures may not specifically apply in the forthcoming IPv6 standard.
7 Checksum; A digit or digits representing the sum of the correct digits in a piece of stored or transmitted digital data, against which later comparisons can be made to detect errors in the data.
8 The shell of an operating system is a program that presents an interface to various operating system functions and services. The shell is so called because it is an outer layer of interface between the user and the innards of the operating system, the kernel. Source: Wikipedia
Introduction
1. INTRODUCTION
Over the past forty years computer networks have dramatically changed the way that we work, play, and communicate. In more ways than ever thought possible, we now have information and personal communication literally at our fingertips. The technology has become so evolved and integrated into every area of our lives that most people are not even aware of their lifestyle's significant reliance on computer networks. Television, cellular and landline phones, the Internet, our national military forces, and even the electricity received through the walls of homes would all fail if computer network technology was to vanish from the earth. This document will cover the history, workings, and modern day implementations of computer networks and the most common data communication protocols they utilize.
1.1 HISTORY OF THE CONCEPT
The Oxford American Dictionary primarily defines the word network as "An arrangement of intersecting horizontal and vertical lines; A group of interconnected people or things". One of the simplest concepts developed by man, a network is the infrastructure that allows the efficient linking of multiple points. This term was not coined with the dawn of the computer, but rather closer to the dawn of time. Some well-known examples of networks before the digital age are interconnected irrigation canals, automobile highway systems, and utility delivery lines. Though not commonly called networks, all of these systems can be defined as networks and are excellent examples of a concept that is not new, but rather has found startling new applications in the past four to six decades. The first computers were extremely expensive, room sized machines which required a team of people to operate and performed what, by today's standards, would seem trivial computations. Despite the computer's start being so primitive, it did not take long for the first computer scientists to begin envisioning the potential power of applying the concept of networking to the new world of digital processing. In 1962, a man by the name of Licklider conceived an idea he called the "Galactic Network", and specifically exampled how it could be used for social interactions. Though his concepts were slightly ahead of their time, over the next few decades the idea that he drafted would come into reality.
Over the past forty years computer networks have dramatically changed the way that we work, play, and communicate. In more ways than ever thought possible, we now have information and personal communication literally at our fingertips. The technology has become so evolved and integrated into every area of our lives that most people are not even aware of their lifestyle's significant reliance on computer networks. Television, cellular and landline phones, the Internet, our national military forces, and even the electricity received through the walls of homes would all fail if computer network technology was to vanish from the earth. This document will cover the history, workings, and modern day implementations of computer networks and the most common data communication protocols they utilize.
1.1 HISTORY OF THE CONCEPT
The Oxford American Dictionary primarily defines the word network as "An arrangement of intersecting horizontal and vertical lines; A group of interconnected people or things". One of the simplest concepts developed by man, a network is the infrastructure that allows the efficient linking of multiple points. This term was not coined with the dawn of the computer, but rather closer to the dawn of time. Some well-known examples of networks before the digital age are interconnected irrigation canals, automobile highway systems, and utility delivery lines. Though not commonly called networks, all of these systems can be defined as networks and are excellent examples of a concept that is not new, but rather has found startling new applications in the past four to six decades. The first computers were extremely expensive, room sized machines which required a team of people to operate and performed what, by today's standards, would seem trivial computations. Despite the computer's start being so primitive, it did not take long for the first computer scientists to begin envisioning the potential power of applying the concept of networking to the new world of digital processing. In 1962, a man by the name of Licklider conceived an idea he called the "Galactic Network", and specifically exampled how it could be used for social interactions. Though his concepts were slightly ahead of their time, over the next few decades the idea that he drafted would come into reality.
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