Communications protocol
In the field of telecommunications, a communications protocol is the set of standard rules for data
representation, signaling, authentication, and error detection required to send information over a
communications channel. These rules also include guidelines that regulate the following
characteristics of a network: access method, allowed physical topologies, types of cabling, and speed
of data transfer. The communication protocols for digital computer network communication have many
features intended to ensure reliable interchange of data over an imperfect communication channel.
Communication protocol is basically following certain rules so that the system works properly.
Systems engineering principles have been applied to create a set of common network protocol design
principles. These principles include effectiveness, reliability, and resiliency.
Effectiveness
Needs to be specified in such a way, that engineers, designers, and in some cases software developers
can implement and/or use it. In human-machine systems, its design needs to facilitate routine usage
by humans. Protocol layering accomplishes these objectives by dividing the protocol design into a
number of smaller parts, each of which performs closely related sub-tasks, and interacts with other
layers of the protocol only in a small number of well-defined ways.
Protocol layering allows the parts of a protocol to be designed and tested without a combinatorial
explosion of cases, keeping each design relatively simple. The implementation of a sub-task on one
layer can make assumptions about the behavior and services offered by the layers beneath it. Thus,
layering enables a "mix-and-match" of protocols that permit familiar protocols to be adapted to
unusual circumstances.
For an example that involves computing, consider an email protocol like the Simple Mail Transfer
Protocol (SMTP). An SMTP client can send messages to any server that conforms to SMTP's
specification. Actual applications can be (for example) an aircraft with an SMTP server receiving
messages from a ground controller over a radio-based internet link. Any SMTP client can correctly
interact with any SMTP server, because they both conform to the same protocol specification,
RFC2821. Some examples of layers,
1. At the lowest level, bits are encoded in electrical, light or radio signals by the Physical layer.
Some examples include RS-232, SONET, and Wi-Fi.
2. A somewhat higher Data link layer such as the point-to-point protocol (PPP) may detect errors
and configure the transmission system.
3. An even higher protocol may perform network functions. One very common protocol is the
Internet protocol (IP), which implements addressing for large set of protocols. A common
associated protocol is the Transmission control protocol (TCP) which implements error
detection and correction (by retransmission). TCP and IP are often paired, giving rise to the
familiar acronym TCP/IP.
4. A layer in charge of presentation might describe how to encode text (i.e.: ASCII or Unicode).
5. An application protocol like SMTP, may (among other things) describe how to inquire about
electronic mail messages.
Reliability
Assuring reliability of data transmission involves error detection and correction, or some means of
requesting retransmission. It is a truism that communication media are always faulty. The
conventional measure of quality is the number of failed bits per bits transmitted. This has the
wonderful feature of being a dimensionless figure of merit that can be compared across any speed or
type of communication media.
In telephony, links with bit error rates (BER) of 10-4 or more are regarded as faulty (they interfere
with telephone conversations), while links with a BER of 10 -5 or more should be dealt with by routine
maintenance (they can be heard).
Data transmission often requires bit error rates below 10-12. Computer data transmissions are so
frequent that larger error rates would affect operations of customers like banks and stock exchanges.
Since most transmissions use networks with telephonic error rates, the errors caused by these
networks must be detected and then corrected.
Communications systems detect errors by transmitting a summary of the data with the data. In TCP
(the internet's Transmission Control Protocol), the sum of the data bytes of packet is sent in each
packet's header. Simple arithmetic sums do not detect out-of-order data, or cancelling errors. A bitwise binary polynomial, a cyclic redundancy check, can detect these errors and more, but is slightly
more expensive to calculate.
Communication systems correct errors by selectively resending bad parts of a message. For example,
in TCP when a checksum is bad, the packet is discarded. When a packet is lost, the receiver
acknowledges all of the packets up to, but not including the failed packet. Eventually, the sender sees
that too much time has elapsed without an acknowledgement, so it resends all of the packets that
have not been acknowledged. At the same time, the sender backs off its rate of sending, in case the
packet loss was caused by saturation of the path between sender and receiver.
In general, the performance of TCP is severely degraded in conditions of high packet loss (more than
0.1%), due to the need to resend packets repeatedly. For this reason, TCP/IP connections are typically
either run on highly reliable fiber networks, or over a lower-level protocol with added error-detection
and correction features (such as modem links with ARQ). These connections typically have
uncorrected bit error rates of 10-9 to 10-12, ensuring high TCP/IP performance.
Resiliency
Resiliency addresses a form of network failure known as topological failure in which a communications
link is cut, or degrades below usable quality. Most modern communication protocols periodically send
messages to test a link. In phones, a framing bit is sent every 24 bits on T1 lines. In phone systems,
when "sync is lost", fail-safe mechanisms reroute the signals around the failing equipment.
In packet switched networks, the equivalent functions are performed using router update messages to
detect loss of connectivity.
Protocol Summary
Protocol
Cable
Speed
Topology
Ethernet
Twisted Pair, Coaxial, Fiber
10 Mbps
Linear Bus, Star, Tree
Fast Ethernet
Twisted Pair, Fiber
100 Mbps
Star
LocalTalk
Twisted Pair
.23 Mbps
Linear Bus or Star
Token Ring
Twisted Pair
4 Mbps - 16 Mbps
Star-Wired Ring
FDDI
Fiber
100 Mbps
Dual ring
ATM
Twisted Pair, Fiber
155-2488 Mbps
Linear Bus, Star, Tree
Topology
The physical topology of a network refers to the configuration of cables, computers, and other
peripherals. Physical topology should not be confused with logical topology which is the method used
to pass information between workstations.
Main Types of Physical Topologies
The following sections discuss the physical topologies used in networks and other related topics.






Linear Bus
Star
Star-Wired Ring
Tree
Considerations When Choosing a Topology
Summary Chart
Linear Bus
A linear bus topology consists of a main run of cable with a terminator at each end .All nodes (file
server, workstations, and peripherals) are connected to the linear cable. Ethernet and LocalTalk
networks use a linear bus topology.
Linear Bus topology
Advantages of a Linear Bus Topology


Easy to connect a computer or peripheral to a linear bus.
Requires less cable length than a star topology.
Disadvantages of a Linear Bus Topology




Entire network shuts down if there is a break in the main cable.
Terminators are required at both ends of the backbone cable.
Difficult to identify the problem if the entire network shuts down.
Not meant to be used as a stand-alone solution in a large building.
Star
A star topology is designed with each node (file server, workstations, and peripherals) connected
directly to a central network hub or concentrator .
Data on a star network passes through the hub or concentrator before continuing to its destination.
The hub or concentrator manages and controls all functions of the network. It also acts as a repeater
for the data flow. This configuration is common with twisted pair cable; however, it can also be used
with coaxial cable or fiber optic cable.
Star topology
Advantages of a Star Topology



Easy to install and wire.
No disruptions to the network then connecting or removing devices.
Easy to detect faults and to remove parts.
Disadvantages of a Star Topology



Requires more cable length than a linear topology.
If the hub or concentrator fails, nodes attached are disabled.
More expensive than linear bus topologies because of the cost of the concentrators.
The protocols used with star configurations are usually Ethernet or LocalTalk. Token Ring uses a
similar topology, called the star-wired ring.
Star-Wired Ring
A star-wired ring topology may appear (externally) to be the same as a star topology. Internally, the
MAU (multistation access unit) of a star-wired ring contains wiring that allows information to pass
from one device to another in a circle or ring .The Token Ring protocol uses a star-wired ring topology.
Tree
A tree topology combines characteristics of linear bus and star topologies. It consists of groups of
star-configured workstations connected to a linear bus backbone cable . Tree topologies allow for the
expansion of an existing network, and enable schools to configure a network to meet their needs.
Tree topology
Advantages of a Tree Topology


Point-to-point wiring for individual segments.
Supported by several hardware and software venders.
Disadvantages of a Tree Topology



Overall length of each segment is limited by the type of cabling used.
If the backbone line breaks, the entire segment goes down.
More difficult to configure and wire than other topologies.
5-4-3 Rule
A consideration in setting up a tree topology using Ethernet protocol is the 5-4-3 rule. One aspect of
the Ethernet protocol requires that a signal sent out on the network cable reach every part of the
network within a specified length of time. Each concentrator or repeater that a signal goes through
adds a small amount of time. This leads to the rule that between any two nodes on the network there
can only be a maximum of 5 segments, connected through 4 repeaters/concentrators. In addition,
only 3 of the segments may be populated (trunk) segments if they are made of coaxial cable. A
populated segment is one which has one or more nodes attached to it. In the above figure the 5-4-3
rule is applied. The furthest two nodes on the network have 4 segments and 3
repeaters/concentrators between them.
This rule does not apply to other network protocols or Ethernet networks where all fiber optic cabling
or a combination of a fiber backbone with UTP cabling is used. If there is a combination of fiber optic
backbone and UTP cabling, the rule is simply translated to 7-6-5 rule.
Considerations When Choosing a Topology:




Money. A linear bus network may be the least expensive way to install a network; you do not
have to purchase concentrators.
Length of cable needed. The linear bus network uses shorter lengths of cable.
Future growth. With a star topology, expanding a network is easily done by adding another
concentrator.
Cable type. The most common cable in schools is unshielded twisted pair, which is most often
used with star topologies.
Summary Chart:
Physical Topology
Common Cable
Common Protocol
Linear Bus
Twisted Pair
Coaxial
Fiber
Ethernet
LocalTalk
Star
Twisted Pair
Fiber
Ethernet
LocalTalk
Star-Wired Ring
Twisted Pair
Token Ring
Tree
Twisted Pair
Coaxial
Fiber
Ethernet
Cabling
UTP Ethernet cables
Note that the TX (transmitter) pins are connected to corresponding RX (receiver) pins, plus to plus
and minus to minus. And that you must use a crossover cable to connect units with identical
interfaces. If you use a straight-through cable, one of the two units must, in effect, perform the
cross-over function.
Two wire color-code standards depicted with RJ-45 jacks as follows (the view is from the front of the
jacks):
If we apply the 568A color code and show all eight wires, our pin-out looks like this:
Note that pins 4, 5, 7, and 8 and the blue and brown pairs are not used in either standard. Quite
contrary to what you may read elsewhere, these pins and wires are not used or required to implement
100BASE-TX duplexing--they are just plain wasted. However, the actual cables are not physically that
simple. In the diagrams, the orange pair of wires are not adjacent. The blue pair is upside-down.
The right ends match RJ-45 jacks and the left ends do not. If, for example, we invert the left side of
the 568A "straight"-thru cable to match a 568A jack--put one 180° twist in the entire cable from endto-end--and twist together and rearrange the appropriate pairs, we get the following can-of-worms:
Coaxial Cabling
10BASE2 is a variant of Ethernet that uses thin coaxial cable (RG-58 or similar, as opposed to the
thicker RG-8 cable used in 10BASE5 networks), terminated with BNC connectors. For many years this
was the dominant 10 Mbit/s Ethernet standard, but due to the immense demand for high speed
networking, the low cost of Category 5 Ethernet cable, and the popularity of 802.11 wireless
networks, both 10BASE2 and 10BASE5 have become obsolete.
In a 10BASE2 network, each segment of cable is connected to the transceiver (which is usually built
into the network adaptor) using a BNC T-connector, with one segment connected to each arm of the
T. At the physical end of the network a 50 Ohm terminator is required.
When wiring a 10BASE2 network, special care has to be taken to ensure that cables are properly
connected to all T-connectors, and appropriate terminators are installed. One, and only one,
terminator must be connected to ground via a ground wire. Bad contacts or shorts are especially
difficult to diagnose, though a time-domain reflectometer will find most problems quickly. A failure at
any point of the network cabling tends to prevent all communications. For this reason, 10BASE2
networks could be difficult to maintain and were often replaced by 10BASE-T networks, which
(provided category 3 cable or better was used) also provided a good upgrade path to 100BASE-TX.
10BASE2 networks cannot generally be extended without breaking service temporarily for existing
users and the presence of many joints in the cable also makes them very vulnerable to accidental or
malicious disruption. The name 10BASE2 is derived from several characteristics of the physical
medium. The 10 comes from the maximum transmission speed of 10 Mbit/s (millions of bits per
second). The BASE stands for baseband signaling, and the 2 represents a rounded up shorthand for
the maximum segment length of 185 meters (607 feet).