Real-time communications is any mode of communication that the user communicate instantly. Real implies 'live'. Real-time communications can include:
Telephony in the conventional sense
Mobile and cellular telephone
Two-way or multi-way amateur radio
IM (instant messaging)
VoIP (Voice over IP, also called Internet telephone)
IRC (Internet Relay Chat) or other chatting modes
Live videoconference communications
Live teleconference communications
Robotic telepresence.
Real Time Communication has two modes they are
Half duplex
Full duplex
Half Duplex is which data can be transmitted in both directions but not at the same time. Full Duplex is which data can be transmitted in both directions at the same time. To design this real time communication a protocol is needed which is the theme of this the real time communication protocol.
Factors to be considered while designing real time communication protocol:
Cost
Quality of service
Traffic load maintenance
Congestion control
Bandwidth
Speed
Protocols used for real time communication can be for
Wired communication
Wireless Communication
Wired Real time communication protocol:
Token ring network and broadcast network are two examples of Wired real time communication protocol.
Token ring network:
In the token ring network there are IEEE 802.5 and FDDI( Fiber distributed data interface). The token ring works as follows:
IEEE 802.5 is nothing but the token ring. Token Ring uses a ring topology whereby the data is sent from one machine to the next and so on around the ring until it ends up back where it started. It also uses a token passing protocol which means that a machine can only use the network when it has control of the Token; this ensures that there are no collisions because only one machine can use the network at any given time. Token ring uses a speed of 16Mbps.
Token ring in hub
A Token Ring hub simply changes the topology from a physical ring to a star wired ring. The Token still circulates around the network and is still controlled in the same manner, however, using a hub or a switch greatly improves reliability because the hub can automatically bypass any ports that are disconnected or have a cabling fault.
Further advancements have been made in recent years with regard to Token Ring technology, such as early Token release and Token Ring switching but as this site is primarily concerned with cabling issues.
FDDI:
The Fiber Distributed Data Interface (FDDI) specifies a 100-Mbps token-passing, dual-ring LAN using fiber-optic cable. FDDI is frequently used as high-speed backbone technology because of its support for high bandwidth and greater distances than copper.
FDDI uses dual-ring architecture with traffic on each ring flowing in opposite directions (called counter-rotating). The dual rings consist of a primary and a secondary ring. During normal operation, the primary ring is used for data transmission, and the secondary ring remains idle.
FDDI defines two types of optical fiber: single-mode and multimode. A mode is a ray of light that enters the fiber at a particular angle. Multimode fiber uses LED as the light-generating device, while single-mode fiber generally uses lasers.
Multimode fiber allows multiple modes of light to propagate through the fiber. Because these modes of light enter the fiber at different angles, they will arrive at the end of the fiber at different times. This characteristic is known as modal dispersion. Modal dispersion limits the bandwidth and distances that can be accomplished using multimode fibers. For this reason, multimode fiber is generally used for connectivity within a building or a relatively geographically contained environment.
Single-mode fiber allows only one mode of light to propagate through the fiber. Because only a single mode of light is used, modal dispersion is not present with single-mode fiber. Therefore, single-mode fiber is capable of delivering considerably higher performance connectivity over much larger distances, which is why it generally is used for connectivity between buildings and within environments that are more geographically dispersed.
FDDI's four specifications are the Media Access Control (MAC), Physical Layer
Protocol (PHY), Physical-Medium Dependent (PMD), and Station Management (SMT) specifications. The MAC specification defines how the medium is accessed, including frame format, token handling, addressing, algorithms for calculating cyclic redundancy check (CRC) value, and error-recovery mechanisms. The PHY specification defines data encoding/decoding procedures, clocking requirements, and framing, among other functions. The PMD specification defines the characteristics of the transmission medium, including fiber-optic links, power levels, bit-error rates, optical components, and connectors. The SMT specification defines FDDI station configuration, ring configuration, and ring control features, including station insertion and removal, initialization, fault isolation and recovery, scheduling, and statistics collection.
Broadcast Networks
Broadcast networks are typically set up as a shared-bus network, where any node
can talk directly to any other node or all other nodes. The best-known example of such a network is Ethernet, which is widely being used in the world today. Current Ethernet networks operate at a speed of 10 Mbit/s or 100 Mbit/s and can typically be found in home and office environments. Ethernet networks which operate at 1000 Mbit/s are beginning to be used in data and network centres.
Ethernet uses CSMA/CD to resolve concurrent medium accesses, which makes proper real-time guarantees for network messages impossible.
Switched real-time Ethernet networks
Ethernet networks nowadays uses switches instead of hubs. The difference between switches and hubs is the Hubs simply pass on incoming traffic on any port to all other ports, whereas switches learn the topology of the network, i.e. where all the nodes are
situated. When a packet, destined for a certain known Ethernet address, arrives, it is sent only to the port behind which the switch knows that Ethernet address to be residing.
Wireless networks:
Wireless networks have a number of protocols. The familiar one is the 802.11b. The 802.11b wireless communication standard operates in the unregulated 2.4 GHz frequency range. The maximum speed for 802.11b communications is 11 mbps.
The newer 802.11g standard improves on 802.11b. It still uses the same crowded 2.4 GHz shared by other common household wireless devices, but 802.11g is capable of transmission speeds up to 54 mbps. Equipment designed for 802.11g will still communicate with 802.11b equipment, however mixing the two standards is not generally recommended.
The 802.11a standard is in a whole different frequency range. By broadcasting in the 5 GHz range 802.11a devices run into a lot less competition and interference from household devices. 802.11a is also capable of transmission speeds up to 54 mbps like the 802.11g standard.
Another well-known wireless standard is Bluetooth. Bluetooth devices transmit at relatively low power and have a range of only 30 feet or so. Bluetooth networks also use the unregulated 2.4 GHz frequency range and are limited to a maximum of eight connected devices. The maximum transmission speed only goes to 1 mbps.
