Sunday, December 28, 2014

What Is OSI

In the early 1980s, the International Standards Organization (ISO) saw the need ;to develop a network model to help vendors create interoperable network solutions. It developed what is now known as the open System Interconnection (OSI) reference model. Even though other networking models have been created, they often are related back to the OSI reference model when vendors want to provide education about their products.

What is the Open System

The concept of derived from a need standardization. Many people have encountered a situation in which they must choose between competing products. The major problem is that if you buy BigCorp’s XYZ product, you are tied to that product as your networking solution.

TCP/IP is good example of an o pen system for a protocol suite. Through the use of RFCs, all TCP/IP standards are fully documented. They have been designated as required or elective components to be included in a vendor’s implementation of TCP/IP. The goal of TCP/IP is to provide connectivity between heterogeneous systems. You might have to make some choices about how you implement the connectivity. By using TCP/IP, however, you know you have an underlying framework that is available on most platform.

Be careful with the term open systems. Many times it is bandied about as the end-all and be-all almost a religion. Competing products drive the market to come up with better solutions. If there is absolutely no difference between product A and Product B, why would you not always choose the cheaper of the two products?

Asynchronous Transfer Mode (ATM)

Asynchronous Transfer mode uses advanced technology to segment data into cells at high speeds. Each cell is fixed length, consisting of 5 byte of header information and 48 bytes of payload data. The use of a fixed-length packet results in higher transfer speeds because the network spends less time processing incoming data It also helps in planning application bandwidth. Cells cross the ATM network by the passing through devices known as ATM switches. These switches analyze header information to switch the cell to the next ATM switch that ultimately leads to the destination network. ATM enables more than one computer to transmit at the same time through the use of multiplexers.

The request includes the ATM address of the Target ATM device as well as quality of service (Qos) parameters. The QOS parameters essentially set minimum guidelines stat must be met for transmission. They include values for peak bandwidth, average sustained bandwidth, and burst size, if the actual traffic flow does not meet the QOS specifications, the cell can be marked as discard-eligible. This means any ATM switch that handles the cell can drop the cell in periods of congestion. At each switch, the signaling request is reassembled and examined. If the switch table has an entry of the destination ATM device and the ATM switch can accommodate the QOS requested for the connection, it forwards the cell to the next ATM switch. When the cell to the next ATM switch. When the cell signaling requested for the connection, it forwards the cell to the next ATM switch. When the cell signaling request reaches the destination endpoint, it responds with an accept message.

The wraps up the basics of the various network types that can be implemented for your network. The next section looks at a concept, Open Systems, that allows standardized protocols to be developed that provide network connectivity over the networks we deploy.

Frame Relay

Network communications have moved toward digital and fiber – optic environments. There is less need for the error checking found in the X.25 protocol. As the result, many large corporations use Frame Relay provide fast, variable-length packet-packet-switching over digital networks. Frame Relay includes a cyclic redundancy check (CRC) algorithm that can detect whether a packet is corrupted and can discard it. It does not; however ask for retransmission of the data. It leaves that up to higher levels that up to higher levels of the protocol.

Frame Relay uses permanent virtual circuits (PVCs) so the entire path between two hosts is known from end to end. This creates an optimal network environment in which the path between two hosts is predetermined. Instead of always having to calculate the best path to remote host, the PVC has predetermined that route. In addition, because the hosts are connected using a common frame relay network, packets do not have to be fragmented due to differing Maximum Transmission units (MTUs) The MTU is the largest packet size that canbe used on a network segment. Frame relay networks all have a same MTU, removing the issues with differing MTUs.

Frame Relay also includes the following local management interface (LMI) extensions:

· Virtual circuit status messages provide information about PVC integrity. They report the addition of any new PVCs and the deletion of existing PVCs. These status messages prevent hosts from sending messages to a PVC that has ceased to exist.

· Multicasting is an optional LMI extension that enables a host to send a single frame destined for multiple recipients. This reduces overall network traffic because a single frame can be sent to multiple hosts instead of one message per host.

· Global addressing provides globally significant connection identifiers. Frame Relay uses data link connection identifiers (DLCIs) to identify a circuit ID. When global addressing is implemented, each connection has a globally unique ID. This ID is known to all other connections.

If Winnipeg must send a frame to Minneapolis, Winnipeg places a value of 40 in the DLCI field and sends the frame into the Frame Relay network. When the frame arrives in Minneapolis, the network changes the DLCI field contents to 10. This shows that the frame came from the Winnipeg network. This addressing scheme enables the Wan to function using the same methods as a LAN.

· Simple flow control provided an XON/XOFF flow-control mechanism. Frame Relay includes simple congestion-notification messages that enable the network to inform user devices when network resources are approaching a congested state. The simple flow control LMI extension is provided for devices that cannot use these notification messages and that need some level of flow control.

Tuesday, December 23, 2014

Packet Switching Networks

Packet-Switching Networks-

Packet switching network enable you to transmit data over an any-to-any connection Sometimes a packet-switched network is described as a mesh network. When information is transmitted over the network, it is known what path the information will take between the sender and the recipient of t he data.

The original data is broken into smaller packets. Each packet is tagged with the destination address and sequence number. As the packet traverses the network between the source and destination hosts, it travels on the best current path. This way, if a network link goes down during the transmission of a stream of packets, not all the packets have to be re-sent. Some of the packets will have found an alternate route when the link went down.

The following are three common implementations of packet-switching networks:

· X.25

· Frame relay

· Asynchronous transfer mode(ATM)

X.25 Networks-

X.25 is developed in the 1970s to provide users with WAN capabilities over public data networks. Phone companies developed it, and it attributes are international in nature, It is administered by an agency of the United Nations called the International Telecommunications Union (ITU).

In an X.25 network, a host calls another host tyo request a communications session. If the call is accepted, the two systems. Can begin a full-duplex information transfer. Either host can terminate the session.

A point –to-point connection takes place between data terminal equipment (DTE) at the client site and data circuit-terminating equipment (DCE) at the carries’ facilities. The DTE is connected to the DCE through a translation device known as a packet assembler/disassemble (PAD). The DCE connects to packet switching exchanges (PSEs), more commonly known as switches. The switches interconnect with each other until they reach the DCE of the destination host. This DCE connects to the DTE connects to the DTE of the host complete the communications session.

An association known as a virtual circuit accomplishes the end-to-end communication between the two DTEs. Virtual circuits enable communication between two defined end points to take place through any number of intermediate nodes. These nodes do not have to be a dedicated portion of the network. The circuit is not a physical data link; it is bandwidth that can be allocated on demand. The following are the two types of virtual circuits:

· Permanent virtual circuits (PVCs)- PVCs are used for common data transfers known to occur on a regular basis. Although the route is permanent, the client pays only for the time the line is in use.

· Switched virtual circuits (SVCs)- SVCs are used for data transfers that are sporadic in nature. The connection uses a specified route across the network. The route is maintained until the connection ceases.

Sunday, December 21, 2014

Duel-Ring Networks

Duel-Ring Networks-

Duel-Ring Networks commonly run the Fiber Distributed Data Interface (FDDI). FDDI is limited to a maximum ring length of 62 miles and operates at 100Mbps.

There are difference between token ring and FDDI when it comes to token passing. A computer on an FDDI network can transmit as many; frames as it can produce in a preset time interval before letting the token go. In addition, several frames can circulate the ring at once. This gives an overall edge in speed to FDDI over token ring.

One ring called primary ring; the other ring is the secondary ring. Under normal operation, all data flows on the primary ring, and the secondary ring remains idle. The secondary ring is used only if a break occurs in the primary ring. The ring automatically reconfigures itself to use the secondary ring when necessary and continues to transmit.

Workstation generally is connected only to the primary ring. These single-attachment hosts connect to the ring using a dual-attached concentrator (DAC). These clients only have a connection to the primary ring. The DAC and dual-homed stations have connections to both the primary and secondary rings. When the primary ring is broken, only stations with dual connections are involved in calculating an alternative route.

The primary medium for an FDDI network is fiber-optic cable. This means

· An FDDI network is more secure because it does not emit electromagnetic-field signals that can be tapped.

· An FDDI network can transmit over longer distances without the use of repeaters to strengthen the signal.

· An FDDI network is immune to electromagnetic noise.

Ring Network

Ring Network-

In the network, the entire computer joined in logical circle. Data travels around the circle and passes through each computer. In a physical layout, a ring network appears to be the same layout as a star network. The key difference is the connection unit known as a Multi-Station Access Unit (MAU). Within the MAU, data signals are passed in a ring a Multi-Station Access Unit (MAU). Within the MAU, data signals are passed in a ring from one host to the next.

Data is transmitted around the ring is using a method called token passing. When a host needs to transmit data, it modifies the token with the MAC address of the destination host. The data passes by each computer until it reaches the destination host. The destination host modifies the token of indicate that data was received successfully. After the sending host verifies that the data was received, the frame is removed from the network. The token is released so that another host on the network can transmit data.

Only a single token exists in a ring topology network. If a client wants to transmit data and the token is in use, he must wait. Although this sounds inefficient, the token travels at a very fast rate. If the total cable length for network is 400 meters, a token can circle this ring around 5,000 times per second.

Wireless Star Topology

Wireless Star Topology-

Wireless networking using the 802.11 standard use Wireless Access Point (WAP) as the central connection unit. Devices connect to the WAP using wireless network cards and the 802.11 standard. 802.11 communications can reach speeds of 11Mbps. For more information on Wireless networking,

New Categories for Twisted Pair Wiring

New Categories for Twisted-Pair Wiring-

Three new categories of referred to when talking about twisted pair wiring categories: Cat5e, Cat-6, and Cat7. Cat5e uses more stringent specifications than Cat5 and offers better performance. The Cat5e uses more stringent specifications than Cat5 and offer better performance. The Cat5E standard is approved to “Electronics Industries Association/Telecommunications Industry Association (EIA/TIA)” and the” Institute of Electrical and Electronics Engineers (IEEE)”. Cat6 not an official standard, supports frequencies up to 350MHz, about two and a half times the specification for Cat5. The Telecommunications Industry Association is currently working on Draft 6 for Cat6. Finally Cat7 uses a copper core wiring that supports frequencies up to 600MHz. Because of the use of Copper, Cat7 will require shielded wiring rather than unshielded wiring. At this time, Cat7 is a working item and even earlier in the standardization process.

Depending on the type of wiring you implement, different cable connectors are used interface the wiring segments with the network cards. RJ45 connectors commonly are used with UTP wiring. RJ45 connectors commonly used with UTP wiring. RJ45 connectors look much like phone connectors, but they are about twice as big. In some cases, especially with older token-ring network cards, DB9 connectors look like AUI connectors, expect there are 9 pins rather than 15 pins on the connector

Star Topology

Star Topology-

The star topology is the most prevalent network topology; implemented in networks today. The chief advantage of the star topology over the bus topology is that, if a cable segment is broken, only the host connected to the hub on that cable segment is affected. The following other benefits of using a star topology:

· It is easy to stake hubs to increase the number of ports that a host can link into the hub stake. This helps star based networks to grow in size.

· Different cable types can be used to connect to the hubs.

When implementing a star network, you use different cabling types than in a bus network. The most common wiring standard used with star-based networks is 10BASED-T wiring, which carries Ethernet signals on inexpensive twisted-pair wiring. The following 5 categories of unshielded twisted pair (UTP) cable can be used:

· Category 1 (Cat 1)- Used in traditional UTP telephone cable. It can carry only voice traffic, not data.

· Category2 (Cat2) - Certified for data transmissions of up to 4 Megabits per second (Mbps)(early token ring).

· Category3 (Cat3) - Certified for the data transmissions of up to 10Mbps (Ethernet).

· Category4 (Cat4) - Certified for the data transmissions of up to 16Mbps (token rind).

· Category5 (Cat5)- Certified for data transmissions of up to 100Mbps(fast Ethernet).

Mixing Media

Mixing Media-

At times, you might encounter a situation in which the computers in your office are networked using one type of cable medium, but the network cards do not support that cable medium, In this situation, it sometimes is easier to purchase transceivers to convert the network cards instead of purchasing new network cards.

Transceivers have two interfaces. One interface is a BNC connector and the other is an AUI connector. This enables a network card with an AUI interface to be used on a bus network utilizing BNC connectors. The transceiver passes information between the two connectors so the AUI type network card can still participate in the network.

Hardware of Bus Topology Network

Hardware Utilized in a Bus Topology Network-

Both thin Ethernet and thick Ethernet require the following additional network hardware to link the hosts:

· BNC connectors

· Terminators

· AUI connectors

British Naval Connector (BNC) connectors enable the various thin net coaxial cab le segments to interconnect. Each host has a T-connector that is used to link the cable segment to a host computer.

Wire Standards in a Bus Network

Wire Standards in a Bus Network-

Networked computers commonly are linked using network cabling. The following are two wiring standards for communications on a bus network:

· 10BASE-2- Also known as thin Ethernet, it allows network segments up to 185 meters on coaxial cable.

· 10BASE-5- Also known as thick Ethernet, it allows network segments up to 500 meters on coaxial cable.

Another common wiring standard implemented in local area networks is the 10Base-T standard. This standard is discussed in the section about star networks.

Topology of LANs

Topology of Lan-

A bus network is the simplest method used to network computers. A bus network consists of a single cable that connects all computers, servers, and network peripherals in a single network segment. Hosts on a bus network communicate with each other by putting information on the cable addressed to the physical address of the network card used to connect the destination computer to the segment. This physical address is called the Media Access Control (MAC) address.

Network card and MAC Address

Network card and MAC Address-

Each network card is assigned a unique MAC address. MAC addresses are 12-digit hexadecimal addresses such as 00-01-0E-6F-6D-62. Each network card manufacturer is assigned a prefix to be used for its network cards. It is the responsibility of the network card manufacturer to be sure no two of its cards have the same MAC address.

Server based Network

Server Based Network-

In server-based networks, at least one host is dedicated to server functionality. Client computers do not share any information with other computers. All data is stored on the central server. Most corporate networks are based on this methodology. Within a server-based network, servers can play several roles. These roles include the following:

· Directory Servers:These provide a central directory service for management of users, groups, and host objects to allow for centralized authentication and authorization using the central directory.

· File and Print Servers: These provide a secure repository for all data. They also can manage printqueues that provide access to network-sharable print resources.

· Application Servers: These provide the server side of client/server applications. In a client/server environment, the client runs a small version of the program that allows connectivity to the server. The server side of the application is used to perform processor-intensive queries on behalf of the client. Example of application servers includes Web servers and database servers.

· Mail Servers: These provide electronic-message capabilities for the clients of the network. With the use of gateways, mail transport can take place between heterogeneous mail systems.

· Security Servers: These provide security to the local area network if it is connected to any larger networks such as the Internet. Security servers include firewalls and proxy servers.

· Remote Access Servers: These enable external data flow to occur between the network and remote clients. A remote client can use a modem to dial in to the local area network, or alternatively use a technology known as tunneling or Virtual Private Networks (VPNs) to connect to the remote network over a public network such as the Internet. The system that allows the remote client to connect to the corporate network is the remote access server. A remote access to the network, or one or more virtual ports to allow tunnel connections. After the client has connected to the remote network, the user can function as if he or she were directly connected to network using a network card.

Peer-to-Peer network

Peer-to-Peer networks-

Peer-to-Peer networks operate with no dedicated servers on the network. Each host functions as both a client and a server. The user at each host determines what information or peripherals he is willing to share with the other members of the network. Peer-to-Peer networks generally are relegated to smaller organizations; they do not scale well to larger ones. They have several security issues as a result of each host’s capability to control its own security, which decentralizes security control.

Wide Area Networks (WANs)

Wide Area Networks (WANs)-

LAN implementations have physical and geographic limitations. Wide area networks (WANs) meet a need for networking that requires connectivity over larger distances.

Most WAN are simply combinations of local area networks and additional communications links between the LANs. The following terms are used to dercribe the scope or size of a WAN:

· Metropoliton Area Networks (MANs)- MANs are WANs is a small geographic area. Generally, they are localized to single city or region.

· Campus area networks (CANs)- CANs is a common designation for WANs that link regions of a university campus.

For practical implementation, these are no different from a wide area network except for the area they physically cover.

Communications over a WAN use one of the following transmission technologies:

· Analog

· Digital

· Packet Switching

Analog and digital technologies commonly are implemented as point-to-point technologies. In other words, they are configured between two distinct hosts. Packet switching, on the other hand, links several hosts using a mesh or cloud technology. Any host participating in t;he cloud can establish a session to another host in the cloud.

Analog WAN Connectivity-

Analog phone lines can be used to connect networks despite the poor line quality and slower speeds. The public switched telephone network (PSTN) was primarily designed for voice traffic; it also can be used for data traffic. Remote users connecting to the home network from the road often use PSTN access. Although it is possible to purchase a dedicated analog line to connect networks, the cost of a conditioned line generally is prohibitive, and other networking solutions are investigated.

Digital Wide Area Network Connectivity-

A more common method of linking a WAN is to use digital data service (DDS) lines. DDS provides a point-to-point synchronous connection. A company can lease dedicated circuits that provide full-duplex bandwidth by setting up a permanent link from each endpoint of the network.

Digital lines are preferable to analog lines due to increased speed and lack of transmission errors. Digital traffic does not require a modern Instead, data is sent from a router on a network to a channel service unit/data service unit (CSU/DSU).

The following are common digital connectivity methods:

· T1/E1

· T3/E3

· ISDN

· Switched 56

T1/E1- T1 service (known as E1 in Europe) is the most widely used digital service at higher data speeds. T1 can transmit a full duplex signal at a rate of 1.544 Mbps. It can be used to transmit voice, data, and video signals.

Because of the high cost of a T1 line, many subscribers opt for fractional-T1 service. Instead of using a T1’s full bandwidth, the subscriber uses one or more T1 channels, Each T1 channel is a 64Kbps increment.

T3/E3- T3 service (known as E3 in Europe) can provide voice and data-grade service at speeds up to 45Mbps. This is the highest-capacity service available to the consumer today. As with T1 service, fractional-T3 service is available as an alternative to multiple T1 lines.

Integrated Services Digital Network(ISDN)- ISDN is an inter LAN connectivity method that can carry data, voice and imaging signals. Two flavors olf ISDN are available: basic rate and primary rate.
Basic rate ISDN provides two bearer channels (known as B Channels) that communicate at 56Kbps; an 8Kbps link-management channel; and one data channel (known as a D channel) that carry signal and link management data at a r ate of 16Kbps. A network using both B channels can provide a 128Kbps data stream.

Primary rate ISDN provide the entire bandwidth of a T1 link by provide the entire bandwidth of a T1 link by providing 23B channels and one D Channel. In Europe, 30B channels are used. The D channel under primary rate ISDN communicates at 64Kbps and still is used only for signal and link management data.

ISDN is a demand-dial interface, Instead of remaining active at all hours, it demand-dials whenever a connection is required.

What is Local Area Network(LANs)

Local Area Network(LANs)-

Local area Networks (LANs) are the most common networks. A LAN has the following characteristics:

· The network operates in a contained area. This could be a single floor in a building or simply within a single building.

· The host within the LAN is interconnected with high-bandwidth network connections such as Ethernet or token ring, or use newer technology such as wireless networks.

· All facets of the LAN often are privately managed. No third parties are required for connectivity solutions.

· LAN services are available on a 7-dayh, 24-hour basis.

Type of Network -

· Peer-to-peer networks

· Server-based networks