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Tuesday, January 31, 2017

NTP_vs_SNTP by Spectracom Corporation.

What is the difference between NTP and SNTP?
(The following is a response to a question made by Mr. Keith Wing, Supervisor for Customer Service, at Spectracom Corporation.)
Question: As far as we can tell, the Time Server we have only supports SNTP. The Web site information we found says NTP, our manual isn’t too specific and the documentation to set up a windows client says to use SNTP. We tried unsuccessfully to get our routers to connect via NTP. Can you help us?
Mr. Wing’s response: NTP (Network Time Protocol) and SNTP (Simple Network Time Protocol) are very similar TCP/IP protocols in that they use the same time packet from the Ethernet Time Server message to compute accurate time. The time stamp that the Time Server sends out and the procedures we use are the same whether NTP (i.e. full implementation NTP) is being used, or if SNTP is being used.


              The difference between NTP and SNTP is in the time synchronization program running on each individual PC (Server or Workstations). The time program, whether it is a Windows built-in program like W32Time (Which uses the SNTP protocol) or a third party add-on, determines which protocol is being used - not the Spectracom Ethernet Time Server. The Ethernet Time Server does not care which protocol is being used. The difference between NTP and SNTP is in the error checking and the actual correction to the time itself.


The NTP algorithm is much more complicated than the SNTP algorithm. NTP normally uses multiple time servers to verify the time and then controls the slew rate of the PC. The algorithm determines if the values are accurate using several methods including fudge factors and identifying time servers that don't agree with the other time servers. It then speeds up or slows down the PC's drift rate so that (1) the PC's time is always correct and (2) there won't be any subsequent time jumps after the initial correction. Unlike NTP, SNTP usually uses just one Ethernet Time Server to calculate the time and then it "jumps" the system time to the calculated time. It can, however, have back-up Ethernet Time Servers in case one is not available. During each interval, it determines whether the time is off enough to make a correction and if it is, it applies the correction. Specific to routers, there are a couple of things to keep in mind and a couple of things you can do. First of all, to synchronize to an Ethernet Time Server, the router must be able to “see” the IP address of the
time server. If there are multiple gateways on the network, the immediate gateway needs to be programmed into the Spectracom Time Server. All gateways, after the immediate one has been programmed, have to be added to the routing tables. To make sure that the routers can see the IP address of the time server, try pinging the IP address. It will most likely respond to the ping. Pings usually get through even if the tables are not set
correctly.
The next thing to try is to make sure Telnet is enabled, at least for this test, in the Time Server. From the router, using the command prompt, try to telnet to the time server using the command: telnet (IP address of time Server) 9999 <enter>, where 9999 is the port. The response from the unit should be the software version and a prompt for the user to hit the enter key to go into the setup mode. If it does not respond to telnet, the port was not entered (9999 at the end of the command), Telnet is disabled in the time server or the IP address is not getting through the gateways. Try this command on a PC in the same subnet as the Ethernet Time Server and see if it responds. If it does, the issue is with the address getting through the Gateways. If a firewall is installed, port 123 has to be open to let NTP/SNTP packets through.The last thing that I can think of is if these are Cisco routers, you may need to enable MD5 encryption in the time server. Cisco uses MD5 but MD5 encryption may not be a requirement. The Spectracom Time Server manual includes instructions on how to set up and use the MD5 feature.

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Relationship Between Ethernet and TCPIP

Ethernet is a physical thing. It's about wires, voltages and connectors.

IP is about how messages get routed from place to place. When the data leaves my computer, how does it know to how to get to Google?
TCP is about making sure the conversation has some rules about how to talk to each other, including making sure messages don't get lost along the way.

Sending data using the TCP/IP protocol suite over Ethernet is known as "Ethernet TCP/IP."Another way to look at it; if you make a telephone call you get an awful lot accomplished. You're sending data to someone far away, and he's receiving it. But if they speak Mandarin and you speak French neither of you will get much out of the conversation. "Ethernet" is like the phone lines. An "IP address" is like a phone number. "TCP" is like the language.Ethernet does not need TCP/IP, and vice versa.Ethernet is defined by IEEE 802.3 standard, that is also ISO 8802-3. This defines the physical (layer 1) and the MAC (media access control) part of the data link layer (layer 2). These are the lowest layers of the OSI 7-layer communications stack.

TCP/IP refers to the full suite of protocols defined for the Internet by the IETF (Internet engineering task force), the standards body for the Internet. TCP refers to a protocol used at layer 4 of the OSI stack (transportation), and IP refers to Internet protocol defined at layer 3 of the OSI stack. Actually, TCP/IP includes many other protocols, some of which are at the top of the OSI stack, the Application layer.

This protocol layering is done to allow the upper layers to be implemented on any lower layer. For example, Ethernet is only one such lower layer, but IEEE 802.11 a/b/g/n is another lower layer that happens to be wireless. Both support TCP/IP.
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Distributed Control System

 

A Distributed Control System (DCS) is a computerisedcontrol system for a process or plant, in which autonomous controllers are distributed throughout the system, but there is central operator supervisory control. This is in contrast to non-distributed control systems that use centralised controllers; either discrete controllers located at a central control room or within a central computer. The DCS concept increases reliability and reduces installation costs by localising control functions near the process plant, but enables monitoring and supervisory control of the process remotely.

Distributed Control Systems first emerged in large, high value, safety critical process industries, and were attractive because the DCS manufacturer would supply both the local control level and central supervisory equipment as an integrated package, thus reducing design integration risk. Today the functionality of SCADA and DCS systems are very similar, but DCS tends to be used on large continuous process plants where high reliability and security is important, and the control room is not geographically remote.

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Monday, January 30, 2017

Networking Fundamentals

Introduction to PC Networking
  • A computer network allows users to communicate with other users on the same network by transmitting data on the cables used to connect them.
  • A computer network is defined as having two or more devices (such as workstations, printers, or servers) that are linked together for the purpose of sharing information, resources, or both.
  • The phrase “information superhighway” describes the benefit of the Internet to business and private communication.
  • The Internet breaks down barriers of time and space, enabling the sharing of information around the globe almost instantaneously.

Simplex Transmission
  • Simplex transmission is a single one-way baseband transmission.
  • It is also called unidirectional because the signal travels in only one direction.
  • An example of simplex transmission is the signal sent from the cable TV station to the home television.

Half-Duplex Transmission
  • This means that only one side can transmit at a time.
  • Two-way radios, such as Citizens Band (CB) and police/emergency communications mobile radios, work with half-duplex transmissions.

Full-Duplex Transmission
  • Traffic can travel in both directions at the same time.
  • A regular telephone conversation is an example of full-duplex communication. Both parties can talk at the same time, and the person talking on the other end can still be heard by the other party while they are talking.

Peer-to-Peer Networks
  • In a peer-to-peer network, the networked computers act as equal partners, or peers, to each other.
  • As peers, each computer can take on the client function or the server function alternately.

Client/Server Networks
  • In a client/server network arrangement, network services are located in a dedicated computer whose only function is to respond to the requests of clients.
  • The server contains the file, print, application, security, and other services in a central computer that is continuously available to respond to client requests.

Local-Area Networks (LANs)


  • A local-area network (LAN) can connect many computers in a relatively small geographical area such as a home, an office, or a campus.
  • It allows users to access high bandwidth media like the Internet and allows users to share devices such as printers.
  • The general shape or layout of a LAN is called its topology.
  • WLAN, wireless LAN (WLAN) is a wireless local area network that links two or more computers or devices using spread-spectrum
  • metropolitan area network (MAN).



Wide-Area Networks (WANs)
  • A WAN, as the name implies, is designed to work over a larger area than a LAN.
  • Point-to-point lines connect only two locations, one on each side of the line. Point-to-multipoint lines connect one location on one side of the line to multiple locations on the other side.

Networking Media- Coaxial cable
  • Networking media can be defined simply as the means by which signals (data) are sent from one computer to another (either by cable or wireless means).
  • Coaxial cable is a copper-cored cable surrounded by a heavy shielding and is used to connect computers in a network.
  • There are several types of coaxial cable, including thicknet, thinnet, RG-59 (standard cable for cable TV), and RG-6 (used in video distribution).

Networking Media- Twisted-Pair
                                 
  • Twisted-pair is a type of cabling that is used for telephone communications and most modern Ethernet networks.
  • A pair of wires forms a circuit that can transmit data. The pairs are twisted to provide protection against crosstalk, the noise generated by adjacent pairs.
  • There are two basic types, shielded twisted-pair (STP) and unshielded twisted-pair (UTP).

Networking Media – Optical Fiber
  • Fiber-optic cable is a networking medium capable of conducting modulated light transmissions.
  • Fiber-optic refers to cabling that has a core of strands of glass or plastic (instead of copper), through which light pulses carry signals.
  • Signals that represent data are converted into beams of light.

Common Networking Devices HUB,SWITCH
  • A hub is a device that is used to extend an Ethernet wire to allow more devices to communicate with each other.
  • Hubs are most commonly used in Ethernet 10BASE-T or 100BASE-T networks, although there are other network architectures that use them.
  • A switch is a more sophisticated device than a hub, although the basic function of the switch is deceptively simple.
  • Ethernet switches are becoming popular connectivity solutions because they increase network performance.

Common Networking Devices ROUTER,SERVER
  • Routers are slower than switches, but make “smart” decisions on how to route (or send) packets received on one port to a network on another port.
  • Server components are those components that are used exclusively with the network server. End users depend on the server to provide the services required.
  • To keep the server running at it is optimal performance, a higher level of preventive maintenance must be maintained.

Ethernet
  • The Ethernet architecture is based on the IEEE 802.3 standard. The IEEE 802.3 standard specifies that a network implements the Carrier Sense Multiple Access with Collision Detection (CSMA/CD) access control method.
  • Standard transfer rates are 10 Mbps or 100 Mbps, but new standards provide for gigabit Ethernet, which are capable of attaining speeds up to 1 Gbps over fiber-optic cable or other high-speed media.
  • Each Ethernet station is given a single 48-bit MAC address, which is used to specify both the destination and the source of each data packet

Token Ring
  • The Token Ring standards are defined in IEEE 802.5.
  • A Token Ring network uses a token (that is, a special signal) to control access to the cable.
  • A token is initially generated when the first computer on the network comes online.
  • When a computer wants to transmit, it waits for and then takes control of the token when it comes its way.
  • The token can travel in either direction around the ring, but only in one direction at a time.
  • Fiber distributed data interface (FDDI)
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Sunday, January 29, 2017

IP Address

           An Internet Protocol address (IP address) is a numerical label assigned to each device (e.g., computer, printer) participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing.
Not unique (but should be), user assigned
  •  Layer 3
  •  4 byte (32 bit)
  •  Network part + host part
  •  2564 or 4,290 million different addresses
 Normally accompanied by a subnet mask
  •  IP Address : 149.218.90.19
  •  Subnet Mask : 255.255.0.0
In addition to the subnet mask a default gateway is also added.
This is the device that the host should talk to if it
cannot find any particular IP address or MAC address on the connected network.
Most common name for this device is Router
There are 5 different classes
Every interface on an internet must have a unique IP address


Private IP address range:
  • 10.0.0.0         -    10.255.255.255
  • 172.16.0.0     -    172.31.255.255
  • 192.168.0.0   -    192.168.255.255
It cannot be Cannot be routed over the Internet
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Open Systems Interconnection model (OSI model)

 

for simpler definitions  check in bottom of page

 
 

OSI MODEL

 
The open connection model describes about the networking process and their flow
It consists of some seven layers describes about working functions of networking.
APPLICATION
PRESENTATION
SESSION
TRANSPORT
NETWORKING
DATALINK
PHYSICAL LAYER
 The top three layers are the operating system related layers and the bottom four are lower layers
In normally the end to end networking depends on the three address [Physical address(MAC  address)+logical address(IP address)+ Port address]

APPLICATION LAYER


The application layer serves as the window for users and application processes to access network services. This layer contains a variety of commonly needed functions:
  • Resource sharing and device redirection
  • Remote file access
  • Remote printer access
  • Inter-process communication
  • Network management
  • Directory services
  • Electronic messaging (such as mail)
  • Network virtual terminals
protocols used:
other protocols in application layer
 

PRESENTATION LAYER

The presentation layer formats the data to be presented to the application layer. It can be viewed as the translator for the network. This layer may translate data from a format used by the application layer into a common format at the sending station, then translate the common format to a format known to the application layer at the receiving station.

The presentation layer provides:
  • Character code translation: for example, ASCII to EBCDIC.
  • Data conversion: bit order, CR-CR/LF, integer-floating point, and so on.
  • Data compression: reduces the number of bits that need to be transmitted on the network.
  • Data encryption: encrypt data for security purposes. For example, password encryption.
protocols used:

SESSION LAYER

The session layer allows session establishment between processes running on different stations. It provides:
  • Session establishment, maintenance and termination: allows two application processes on different machines to establish, use and terminate a connection, called a session.
  • Session support: performs the functions that allow these processes to communicate over the network, performing security, name recognition, logging, and so on.
protocols used:
 

TRANSPORT LAYER

The transport layer ensures that messages are delivered error-free, in sequence, and with no losses or duplications. It relieves the higher layer protocols from any concern with the transfer of data between them and their peers.
The size and complexity of a transport protocol depends on the type of service it can get from the network layer. For a reliable network layer with virtual circuit capability, a minimal transport layer is required. If the network layer is unreliable and/or only supports datagrams, the transport protocol should include extensive error detection and recovery.
The transport layer provides:
  • Message segmentation: accepts a message from the (session) layer above it, splits the message into smaller units (if not already small enough), and passes the smaller units down to the network layer. The transport layer at the destination station reassembles the message.
  • Message acknowledgment: provides reliable end-to-end message delivery with acknowledgments.
  • Message traffic control: tells the transmitting station to "back-off" when no message buffers are available.
  • Session multiplexing: multiplexes several message streams, or sessions onto one logical link and keeps track of which messages belong to which sessions (see session layer).
Typically, the transport layer can accept relatively large messages, but there are strict message size limits imposed by the network (or lower) layer. Consequently, the transport layer must break up the messages into smaller units, or frames, prepending a header to each frame.
The transport layer header information must then include control information, such as message start and message end flags, to enable the transport layer on the other end to recognize message boundaries. In addition, if the lower layers do not maintain sequence, the transport header must contain sequence information to enable the transport layer on the receiving end to get the pieces back together in the right order before handing the received message up to the layer above.
End-to-end layers
Unlike the lower "subnet" layers whose protocol is between immediately adjacent nodes, the transport layer and the layers above are true "source to destination" or end-to-end layers, and are not concerned with the details of the underlying communications facility. Transport layer software (and software above it) on the source station carries on a conversation with similar software on the destination station by using message headers and control messages.
Common TCP/IP Protocols and Ports
Protocol TCP/UDP Port Number Description
File Transfer Protocol (FTP)
(RFC 959)		 TCP 20/21 FTP is one of the most commonly used file transfer protocols on the Internet and within private networks. An FTP server can easily be set up with little networking knowledge and provides the ability to easily relocate files from one system to another. FTP control is handled on TCP port 21 and its data transfer can use TCP port 20 as well as dynamic ports depending on the specific configuration.
Secure Shell (SSH)
(RFC 4250-4256)		 TCP 22 SSH is the primary method used to manage network devices securely at the command level. It is typically used as a secure alternative to Telnet which does not support secure connections.
Telnet
(RFC 854)		 TCP 23 Telnet is the primary method used to manage network devices at the command level. Unlike SSH which provides a secure connection, Telnet does not, it simply provides a basic unsecured connection. Many lower level network devices support Telnet and not SSH as it required some additional processing. Caution should be used when connecting to a device using Telnet over a public network as the login credentials will be transmitted in the clear.
Simple Mail Transfer Protocol (SMTP)
(RFC 5321)		 TCP 25 SMTP is used for two primary functions, it is used to transfer mail (email) from source to destination between mail servers and it is used by end users to send email to a mail system.
Domain Name System (DNS)
(RFC 1034-1035)		 TCP/UDP 53 The DNS is used widely on the public internet and on private networks to translate domain names into IP addresses, typically for network routing. DNS is hieratical with main root servers that contain databases that list the managers of high level Top Level Domains (TLD) (such as .com). These different TLD managers then contain information for the second level domains that are typically used by individual users (for example, cisco.com). A DNS server can also be set up within a private network to private naming services between the hosts of the internal network without being part of the global system.
Dynamic Host Configuration Protocol (DHCP)
(RFC 2131)		 UDP 67/68 DHCP is used on networks that do not use static IP address assignment (almost all of them). A DHCP server can be set up by an administrator or engineer with a poll of addresses that are available for assignment. When a client device is turned on it can request an IP address from the local DHCP server, if there is an available address in the pool it can be assigned to the device. This assignment is not permanent and expires at a configurable interval; if an address renewal is not requested and the lease expires the address will be put back into the poll for assignment.
Trivial File Transfer Protocol (TFTP)
(RFC 1350)		 UDP 69 TFTP offers a method of file transfer without the session establishment requirements that FTP uses. Because TFTP uses UDP instead of TCP it has no way of ensuring the file has been properly transferred, the end device must be able to check the file to ensure proper transfer. TFTP is typically used by devices to upgrade software and firmware; this includes Cisco and other network vendors’ equipment.
Hypertext Transfer Protocol (HTTP)
(RFC 2616)		 TCP 80 HTTP is one of the most commonly used protocols on most networks. HTTP is the main protocol that is used by web browsers and is thus used by any client that uses files located on these servers.
Post Office Protocol (POP) version 3
(RFC 1939)		 TCP 110 POP version 3 is one of the two main protocols used to retrieve mail from a server. POP was designed to be very simple by allowing a client to retrieve the complete contents of a server mailbox and then deleting the contents from the server.
Network Time Protocol (NTP)
(RFC 5905)		 UDP 123 One of the most overlooked protocols is NTP. NTP is used to synchronize the devices on the Internet. Even most modern operating systems support NTP as a basis for keeping an accurate clock. The use of NTP is vital on networking systems as it provides an ability to easily interrelate troubles from one device to another as the clocks are precisely accurate.
NetBIOS
(RFC 1001-1002)		 TCP/UDP 137/138/139 NetBIOS itself is not a protocol but is typically used in combination with IP with the NetBIOS over TCP/IP (NBT) protocol. NBT has long been the central protocol used to interconnect Microsoft Windows machines.
Internet Message Access Protocol (IMAP)
(RFC 3501)		 TCP 143 IMAP version3 is the second of the main protocols used to retrieve mail from a server. While POP has wider support, IMAP supports a wider array of remote mailbox operations which can be helpful to users.
Simple Network Management Protocol (SNMP)
(RFC 1901-1908, 3411-3418)		 TCP/UDP 161/162 SNMP is used by network administrators as a method of network management. SNMP has a number of different abilities including the ability to monitor, configure and control network devices. SNMP traps can also be configured on network devices to notify a central server when specific actions are occurring. Typically, these are configured to be used when an alerting condition is happening. In this situation, the device will send a trap to network management stating that an event has occurred and that the device should be looked at further for a source to the event.
Border Gateway Protocol (BGP)
(RFC 4271)		 TCP 179 BGP version 4 is widely used on the public internet and by Internet Service Providers (ISP) to maintain very large routing tables and traffic processing. BGP is one of the few protocols that have been designed to deal with the astronomically large routing tables that must exist on the public Internet.
Lightweight Directory Access Protocol (LDAP)
(RFC 4510)		 TCP/UDP 389 LDAP provides a mechanism of accessing and maintaining distributed directory information. LDAP is based on the ITU-T X.500 standard but has been simplified and altered to work over TCP/IP networks.
Hypertext Transfer Protocol over SSL/TLS (HTTPS)
(RFC 2818)		 TCP 443 HTTPS is used in conjunction with HTTP to provide the same services but doing it using a secure connection which is provided by either SSL or TLS.
Lightweight Directory Access Protocol over TLS/SSL (LDAPS)
(RFC 4513)		 TCP/UDP 636 Just like HTTPS, LDAPS provides the same function as LDAP but over a secure connection which is provided by either SSL or TLS.
FTP over TLS/SSL
(RFC 4217)		 TCP 989/990 Again, just like the previous two entries, FTP over TLS/SSL uses the FTP protocol which is then secured using either SSL or TLS.

protocols used
 

NETWORK LAYER

The network layer controls the operation of the subnet, deciding which physical path the data should take based on network conditions, priority of service, and other factors. It provides:
  • Routing: routes frames among networks.
  • Subnet traffic control: routers (network layer intermediate systems) can instruct a sending station to "throttle back" its frame transmission when the router's buffer fills up.
  • Frame fragmentation: if it determines that a downstream router's maximum transmission unit (MTU) size is less than the frame size, a router can fragment a frame for transmission and re-assembly at the destination station.
  • Logical-physical address mapping: translates logical addresses, or names, into physical addresses.
  • Subnet usage accounting: has accounting functions to keep track of frames forwarded by subnet intermediate systems, to produce billing information.
Communications Subnet
The network layer software must build headers so that the network layer software residing in the subnet intermediate systems can recognize them and use them to route data to the destination address.

This layer relieves the upper layers of the need to know anything about the data transmission and intermediate switching technologies used to connect systems. It establishes, maintains and terminates connections across the intervening communications facility (one or several intermediate systems in the communication subnet).
In the network layer and the layers below, peer protocols exist between a node and its immediate neighbor, but the neighbor may be a node through which data is routed, not the destination station. The source and destination stations may be separated by many intermediate systems.
protocols used

DATA LINK LAYER

The data link layer provides error-free transfer of data frames from one node to another over the physical layer, allowing layers above it to assume virtually error-free transmission over the link. To do this, the data link layer provides:
  • Link establishment and termination: establishes and terminates the logical link between two nodes.
  • Frame traffic control: tells the transmitting node to "back-off" when no frame buffers are available.
  • Frame sequencing: transmits/receives frames sequentially.
  • Frame acknowledgment: provides/expects frame acknowledgments. Detects and recovers from errors that occur in the physical layer by retransmitting non-acknowledged frames and handling duplicate frame receipt.
  • Frame delimiting: creates and recognizes frame boundaries.
  • Frame error checking: checks received frames for integrity.
  • Media access management: determines when the node "has the right" to use the physical medium.
protocols used

PHYSICAL LAYER

The physical layer, the lowest layer of the OSI model, is concerned with the transmission and reception of the unstructured raw bit stream over a physical medium. It describes the electrical/optical, mechanical, and functional interfaces to the physical medium, and carries the signals for all of the higher layers. It provides:
  • Data encoding: modifies the simple digital signal pattern (1s and 0s) used by the PC to better accommodate the characteristics of the physical medium, and to aid in bit and frame synchronization. It determines:

    • What signal state represents a binary 1
    • How the receiving station knows when a "bit-time" starts
    • How the receiving station delimits a frame
  • Physical medium attachment, accommodating various possibilities in the medium:
    • Will an external transceiver (MAU) be used to connect to the medium?
    • How many pins do the connectors have and what is each pin used for?
  • Transmission technique: determines whether the encoded bits will be transmitted by baseband (digital) or broadband (analog) signaling.
  • Physical medium transmission: transmits bits as electrical or optical signals appropriate for the physical medium, and determines:
    • What physical medium options can be used
    • How many volts/db should be used to represent a given signal state, using a given physical medium
Technologies used
The following technologies provide physical layer services:

.
.
.
for simpler definitions

OSI MODEL
The open connection model describes about the networking process and their flow
It consists of some seven layers describes about working functions of networking.
 
APPLICATION
PRESENTATION
SESSION
TRANSPORT
NETWORKING
DATALINK
PHYSICAL LAYER
 
 The top three layers are the operating system related layers and the bottom four are lower layers
In normally the end to end networking depends on the three address [Physical address(MAC  address)+logical address(IP address)+ Port address]
APPLICATION LAYER
The application layer serves as the window for users and application processes to access network services. This layer contains a variety of commonly needed functions:
  • Resource sharing and device redirection
  • Remote file access
  • Remote printer access
  • Inter-process communication
  • Network management
  • Directory services
  • Electronic messaging (such as mail)
  • Network virtual terminals
PRESENTATION LAYER
The presentation layer formats the data to be presented to the application layer. It can be viewed as the translator for the network. This layer may translate data from a format used by the application layer into a common format at the sending station, then translate the common format to a format known to the application layer at the receiving station.

The presentation layer provides:
  • Character code translation: for example, ASCII to EBCDIC.
  • Data conversion: bit order, CR-CR/LF, integer-floating point, and so on.
  • Data compression: reduces the number of bits that need to be transmitted on the network.
  • Data encryption: encrypt data for security purposes. For example, password encryption.
SESSION LAYER
The session layer allows session establishment between processes running on different stations. It provides:
  • Session establishment, maintenance and termination: allows two application processes on different machines to establish, use and terminate a connection, called a session.
  • Session support: performs the functions that allow these processes to communicate over the network, performing security, name recognition, logging, and so on.

TRANSPORT LAYER

The transport layer ensures that messages are delivered error-free, in sequence, and with no losses or duplications. It relieves the higher layer protocols from any concern with the transfer of data between them and their peers.


The size and complexity of a transport protocol depends on the type of service it can get from the network layer. For a reliable network layer with virtual circuit capability, a minimal transport layer is required. If the network layer is unreliable and/or only supports datagrams, the transport protocol should include extensive error detection and recovery.
The transport layer provides:
  • Message segmentation: accepts a message from the (session) layer above it, splits the message into smaller units (if not already small enough), and passes the smaller units down to the network layer. The transport layer at the destination station reassembles the message.
  • Message acknowledgment: provides reliable end-to-end message delivery with acknowledgments.
  • Message traffic control: tells the transmitting station to "back-off" when no message buffers are available.
  • Session multiplexing: multiplexes several message streams, or sessions onto one logical link and keeps track of which messages belong to which sessions (see session layer).
Typically, the transport layer can accept relatively large messages, but there are strict message size limits imposed by the network (or lower) layer. Consequently, the transport layer must break up the messages into smaller units, or frames, prepending a header to each frame.
The transport layer header information must then include control information, such as message start and message end flags, to enable the transport layer on the other end to recognize message boundaries. In addition, if the lower layers do not maintain sequence, the transport header must contain sequence information to enable the transport layer on the receiving end to get the pieces back together in the right order before handing the received message up to the layer above.

End-to-end layers

Unlike the lower "subnet" layers whose protocol is between immediately adjacent nodes, the transport layer and the layers above are true "source to destination" or end-to-end layers, and are not concerned with the details of the underlying communications facility. Transport layer software (and software above it) on the source station carries on a conversation with similar software on the destination station by using message headers and control messages.

NETWORK LAYER

The network layer controls the operation of the subnet, deciding which physical path the data should take based on network conditions, priority of service, and other factors. It provides:
  • Routing: routes frames among networks.
  • Subnet traffic control: routers (network layer intermediate systems) can instruct a sending station to "throttle back" its frame transmission when the router's buffer fills up.
  • Frame fragmentation: if it determines that a downstream router's maximum transmission unit (MTU) size is less than the frame size, a router can fragment a frame for transmission and re-assembly at the destination station.
  • Logical-physical address mapping: translates logical addresses, or names, into physical addresses.
  • Subnet usage accounting: has accounting functions to keep track of frames forwarded by subnet intermediate systems, to produce billing information.

Communications Subnet

The network layer software must build headers so that the network layer software residing in the subnet intermediate systems can recognize them and use them to route data to the destination address.

This layer relieves the upper layers of the need to know anything about the data transmission and intermediate switching technologies used to connect systems. It establishes, maintains and terminates connections across the intervening communications facility (one or several intermediate systems in the communication subnet).
In the network layer and the layers below, peer protocols exist between a node and its immediate neighbor, but the neighbor may be a node through which data is routed, not the destination station. The source and destination stations may be separated by many intermediate systems.
DATA LINK LAYER
The data link layer provides error-free transfer of data frames from one node to another over the physical layer, allowing layers above it to assume virtually error-free transmission over the link. To do this, the data link layer provides:
  • Link establishment and termination: establishes and terminates the logical link between two nodes.
  • Frame traffic control: tells the transmitting node to "back-off" when no frame buffers are available.
  • Frame sequencing: transmits/receives frames sequentially.
  • Frame acknowledgment: provides/expects frame acknowledgments. Detects and recovers from errors that occur in the physical layer by retransmitting non-acknowledged frames and handling duplicate frame receipt.
  • Frame delimiting: creates and recognizes frame boundaries.
  • Frame error checking: checks received frames for integrity.
  • Media access management: determines when the node "has the right" to use the physical medium.
PHYSICAL LAYER
The physical layer, the lowest layer of the OSI model, is concerned with the transmission and reception of the unstructured raw bit stream over a physical medium. It describes the electrical/optical, mechanical, and functional interfaces to the physical medium, and carries the signals for all of the higher layers. It provides:
  • Data encoding: modifies the simple digital signal pattern (1s and 0s) used by the PC to better accommodate the characteristics of the physical medium, and to aid in bit and frame synchronization. It determines:
  • What signal state represents a binary 1
  • How the receiving station knows when a "bit-time" starts
  • How the receiving station delimits a frame
  • Physical medium attachment, accommodating various possibilities in the medium:
  • Will an external transceiver (MAU) be used to connect to the medium?
  • How many pins do the connectors have and what is each pin used for?
  • Transmission technique: determines whether the encoded bits will be transmitted by baseband (digital) or broadband (analog) signaling.
  • Physical medium transmission: transmits bits as electrical or optical signals appropriate for the physical medium, and determines:
  • What physical medium options can be used
  • How many volts/db should be used to represent a given signal state, using a given physical medium
 
 
 



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