Data Terminals : Protocols

Protocols are a precise set of rules, timing, and a syntax that govern the accurate transfer of information between devices or software applications. Key protocols in data transmission networks include access protocols, handshaking, line discipline, and session protocols.

Access protocols are the set of rules that workstations use to avoid collisions when sending information over shared network media. Access protocols are also known as the media access control (MAC) protocols. Handshaking protocols involve the sequence of events that occur between communication devices that negotiate the data transmission rules and ensure reliable data transmission. When data devices begin to communicate, they discover the capabilities and agree on a common set of protocols to use during data communications session. Line discipline is the sequence of events that must occur to control the reception of data, perform error detection and correction, and multiplexing of control information, if necessary. Session protocols control the end-to-end connectivity of a data communication session. Session protocols ensure all the data is received and in the correct order.

Different protocols may be used in systems that provide similar functions. An example of this is token ring and Ethernet. Although these networks may actually use the same signaling system, they use incompatible protocols. To allow data to transfer between these networks, protocol converters are used. Protocol converts receive data and control messages, reformat data and convert control messages, and retransmit the data using the new protocol rules.

Network Management

Network management is set of procedures, equipment, and operations that keep a telecommunications network operating near maximum efficiency despite unusual loads or equipment failures. Network managers should be able to monitor, configure, and operate their network equipment from distant communication locations using a set of network management protocols.

A key network management protocol is simple network management protocol (SNMP). SNMP is an industry standard communication protocol that is used to manage multiple types of network equipment (most vendors comply at some level). By conforming to this protocol, equipment assemblies that are produced by different manufacturers can be managed by a single network management program. While many vendors supply proprietary configuration and administration software for their products, many support diagnostic and maintenance features through the use of SNMP.

Data networks can be characterized as premises distribution networks (PDNs), Local area networks (LANs), metropolitan area networks (MANs), wide area networks (WANs), and wireless data networks (WDNs).

Premises Distribution Network (PDN)
A premises distribution network (PDN) is a short-range network that is located at a customer’s facility or even within their personal area. A PDN is used to connect terminals (computers) to other networks and each other. The most common types of PDN are EtherNet, asynchronous transfer mode 25 (ATM 25), universal serial bus (USB), home packet data network (HomePDN), and FireWire (IEEE-1394).

Figure 1 shows several popular forms of PDN. This diagram shows that the data transfer rate varies with the length and type of interconnection cable. This diagram also shows that some PDN technologies are better suited for multimedia applications than others. For example, ATM25 can transfer (multiplex) multiple communication channels with different levels of quality of service (QoS). Other PDN systems are capable of very high-speed data transfer rates (up to 400 Mbps) for very short distances.


Figure 2: Premises Distribution Networks (PDNs)

Local Area Networks (LANs)
Local area networks (LANs) are private data communication networks that used high-speed digital communications channels for the interconnection of computers and related equipment in a limited geographic area. LANs can use fiber optic, coaxial, twisted-pair cables, or radio transceivers to transmit and receive data signals. LAN’s are networks of computers, normally personal computers, connected together in close proximity (office setting) to each other in order to share information and resources. The two predominant LAN architectures are token ring and Ethernet. Other LAN technologies are ArcNet, AppleTalk, and fiber distributed data interface (FDDI).

Token ring traditionally operates at either 4 or 16 Mbps. Token ring operates by passing tokens from computer to computer in the LAN. Ethernet is a packet data network that allows computers to randomly transmit data and each computer in an Ethernet system resolves the potential for packet data collisions.

Figure 2 shows several of the most popular LAN topologies and their configurations. Some data networks are setup as bus networks (all computers share the same bus), as start networks (computers connect to a central data distribution node), or as a ring (data circles around the ring). This diagram shows for popular types of LAN networks: Thinnet, Thicknet, token ring networks, and Ethernet star network.


Figure 2: Local Area Networks (LANs)

Metropolitan Area Networks (MAN’s)
A MAN is a data communications network or interconnected groups of data networks that have geographic boundaries of a metropolitan area. The network is totally or partially segregated from other networks, and typically links local area networks (LANs) together.

MAN’s offer the ability to connect networks across a metropolitan area as if they were co-located in the same building or on the same campus. To create a MAN, businesses install or lease communications links between the LANs. The backbone interconnection for a MAN is routinely fiber-based. This provides a fairly high data transfer rate and provides a high degree of fault tolerance. Fiber networks often are self-healing in case the fiber line is cut or damaged.

Figure 3 shows a five node MAN connecting that connects several LAN systems via a FDDI system. This diagram shows that each LAN may be connected within the MAN using different technology such as T1/E1 copper access lines, coax, or fiber connections. In each case, a router provides a connection from each LAN to connect to the MAN.


Figure 3: Metropolitan Area Network (MAN)

Wide Area Networks (WAN’s)
WANs are communication networks that provide data transmission services through large geographically separate areas. A WAN can be established by linking together two or more metropolitan area networks, which enables data terminals in one city to access data resources in another city or country.

Figure 4 shows that a WAN is usually composed of several different data networks. Different types of communication lines such as leased lines, packet data systems, or fiber transmission lines can interconnect these networks.


Figure 4: Wide Area Networks (WAN’s)

Wireless Local Area Network (WLAN)
WLANs allow computers and workstations to communicate with each other using radio propagation as the transmission medium. The wireless LAN can be connected to an existing wired LAN as an extension, or can form the basis of a new network. While adaptable to both indoor and outdoor environments, wireless LANs are especially suited to indoor locations such as office buildings, manufacturing floors, hospitals and universities.

Wireless data networks exist in three types: LAN’s, campus interconnect, and wide area wireless (e.g., cellular or PCS). Wireless LAN’s generally use either infrared or radio frequency (RF) as their transmission media. Infrared is line-of-sight only, and poses problems in many office environments when viewed as a single solution. When coupled with twisted pair wire (the basic LAN media) and used to bring in isolated workstations across a factory floor, it has proven to be a sound technology. RF is not line-of-sight and thus is not subject to the problems of infrared. It does, however, encounter interference from many devices found in the office and factory.

Wireless LANs often used radio channels in an unlicensed frequency band. These wireless data systems can transmit data up to 50 Mbps (2-11 Mbps is more typical). Point-to-point wireless data systems may be used to interconnect data networks between buildings within a campus. Providing this wireless data link only requires the installation of 2 antennas with a clear line of site communication. Point-to-point microwave data transmission rates can exceed 45 Mbps. Wide area wireless systems, such as cellular and PCS, can provide wireless coverage over large geographic areas. However, WANs have data transmission rates that are usually below 28 kbps and the usage cost is relatively high.

Wireless LAN systems typically use the unlicensed radio frequency bands instrument, scientific and medial (ISM) frequency bands. These bands include 902-928 MHz, 2.4 - 2.485 GHz, and 5.7 GHz ranges. Each of these frequency bands has usage limitations in different parts of the world. The only unlicensed frequency band that has common authorization to use throughout the world is the 2.4 GHz frequency band. WLANs typically operate up to a distance of 300 feet (100 meters). WLAN systems provide much larger coverage by interconnected radio access nodes. Wireless LAN standards include multiple versions of IEEE 802.11 and Bluetooth.

Figure 5 shows the three key types of wireless data networks. This diagram shows a wireless LAN system that has multiple access nodes. These access nodes operate as gateways between the data communication devices (e.g., mobile computer) and the data network hub. Building 1 uses an older 801.11 wireless LAN system that operates from 902-928 MHz at 2 Mbps. Building 2 uses a newer 802.11 wireless LAN system that operates at 2.4 GHz providing up to 11 Mbps data transfer rate. This diagram also shows a microwave data link that provides a 45 Mbps interconnection between campus buildings. Finally, a user who is operating in a remote area outside the core campus is using the wide area mobile system to transfer data files (at a data transfer rate below 28 kbps).


Figure 5: Wireless Data Networks