Communication Systems/Packet Data Systems

Carrier Grade Packet Networks
In most data applications, user data is decomposed into a string of packets. This requires that additional information be added to the user's data to ensure its safe arrival at the correct destination.

If the packets are rather long or of variable length, the system may be referred to as frame relay. If the packets are small and of fixed length, the system may be referred to as cell relay.

The user data may be organized into fixed or variable length packets depending on the type of system. In any case, additional information is appended to the customers data in the form of a header.

Some of the header contents may include:
 * • Source address
 * • Destination address
 * • Time stamp or packet sequence number
 * • Maximum allowable delay (priority)
 * • ACK/NACK
 * • Network control/billing
 * • Error checking etc.

However, simplex connections with transmission delays are not suitable for voice connections, but are not a concern in data transmission. Each input data packet is stored in an input buffer or queue and sent to the appropriate output queue when resources become available. If the packets and queues are sufficiently short, a virtual full duplex link may be established, and interactive transactions may be possible.

In order to understand the operation of these systems, it is often necessary to resort to a standard reference model. The OSI model outlines the various functions and attributes, which are inherent in any communications system.

A further necessity is the standardization of the implementation of the OSI model. This work has largely been done by the IEEE and ITU (formerly CCITT).

The OSI model was started in 1978 and has evolved to provide a framework for interoperability. Standardization is required in order that various types of equipment or systems can be joined in some useful way. Some of the activities, which are regulated, include: • Interprocess communications • Data representation • Data storage • Resource and process management • Security • Program support

The majority of the OSI model is implemented in microprocessor based hardware. This means that new applications, network architectures, and protocols can generally be accommodated by making software changes. It also means that the traditional gap between the telecommunications and computer industry is being reduced.

Significant advances have been made in the physical layer interfaces, as networks migrate from twisted pair cabling to fiber optics. These developments have introduced new applications, architectures and protocols.

Some applications for public data packet networks include:
 * • Electronic funds transfer: Banks, Automated tellers, Clearinghouses, Stock exchanges
 * • Point-of-sale terminals
 * • Credit card verification
 * • Electronic mail
 * • Electronic purchasing
 * • Inventory management
 * • Database interworking

LANs
LANs are used to connect a group of terminals or workstations together. These may be found in a relatively confined space such as within a building or campus setting. The attachments may be dumb or intelligent terminals, file servers, routers, and repeaters.

The majority of LANs are privately owned. They can be connected to the PSTN or some other carrier to form MANs or WANs.

LANs can be categorized in a number of ways. One method examines how information is placed on the interconnecting transmission medium. Baseband LANs directly inject logic levels of on the medium and share access by some form of TDM. Broadband LANs however, use high frequency carriers and share access by FDM.

Some authors prefer to segregate a special category, the PBX:
 * LAN - local area network (baseband)
 * HSLN - high-speed local network (may be baseband or broadband)
 * PBX - private branch exchange (baseband)

The most widely used LANs today are ethernet, token ring, token bus, and Appletalk. They can be interconnected by gateways, routers, bridges, or repeaters to form MANs, which can in turn be used to create WANs.  Topology



LANs can be configured in three basic ways, namely: as a ring, star, or bus. Another arrangement known as a tree topology is really a variation of the bus. Cable TV is an example of a tree network employing frequency division multiplexing.


 * Star

The end office of the PSTN is configured as a star where all lines radiate from a single location. This type of wiring is also used in all modern office buildings. This center of the star or hub may contain a network manager of some other piece of hardware that exercises control over the LAN. This type of arrangement is used in a PBX, and the most common connection media is a twisted pair of wires.

Some networks may be configured as a physical star, but may have a different logical topology, such as a ring.


 * Ring

A ring consists of a closed loop, where each station is connected by an active or passive tap. An active tap is one has electronic components inserted into the loop to both extract and inject signals, thus all stations are effectively connected in series. A passive tap simply comes in contact with the loop., thus all stations are effectively connected in parallel.

A station wanting to transmit waits its turn to inject a packet onto the ring. This implies some sort of distributed protocol. This is most often implemented by means of a token.


 * Bus

A distributed protocol arrangement or token is needed to resolve bus contention problems. Each terminal monitors the common bus for any data with their address appended, at which point it simply makes a copy. This arrangement is often found in computer systems where a number of peripheral devices may be connected on a high-speed bus.

Two important issues in this type of arrangement are access and signal level. In a baseband or digital environment, there is often a polling mechanism imposed to create order. Also, the signals degenerate as they propagate down the line, this means that some sort of compensation must be made.

RS-422 is a simple protocol that is implemented on a twisted wire bus. The system allows a couple of dozen devices to be networked as far as 1 km, with a data rate of up to 1 Mbit/s.

Ethernet is an example of a baseband system implemented on a bus.