This chapter goes over the most important issues in data communications concerning protocol compatibility and standardization; it goes into detail about the actual procedures that data goes through; the devices data uses; and how data devices aren’t able to communicate unless their protocols match.
Concepts & Terminology ·
An example
of protocol is the standard protocols developed by international agencies, such
as ITU’s HDLC and X.25.
·
The simplest
protocols have been established by common usage; such as a standard EIA-232
serial interface uses asynchronous protocol.
·
This is universally
adapted and compatible, meaning it can communicate with any other device that
is identically set up.
·
Protocol Converters
(or Gateway): communicates with both connecting protocols converting their languages.
·
Incompatible
protocols can communicate through this.
·
They communicate
with DTE using that DTE’s won protocol, convert it to the network protocol,
and transport it to the distant DTE in its own language.
·
Incompatibility
of protocols is a huge problem for interconnectability of data networks.
·
Data protocols
can be implemented in firmware, such as a chip, software, or a combination of
both.
·
Layered protocols
allow developers to write software to a clearly defined interface.
·
Each layer
having its own function.
·
An example
of a layered protocol in action is in LAN standards, such as the OSI model.
·
LAN standards
further illustrate how the protocol is deployed.
·
Interoffice
Cards (NICs): Hardware vendors can build this network to connect to an of the
transmission media that LAN standards support, such as twisted-pair wire, fiber
optics, coaxial cable, and wireless.
·
Firmware:
The portion of the protocol that describes how the NIC communicates with the
transmission medium is implemented here.
·
Logical Link
Control (LLC): The card manufactures provide software drivers to enable the
functions in their cards to communicate with the network operating system across
this protocol.
Some other protocols include:
·
Address Resolution
Protocol (ARP)
·
Reverse Address
Resolution Protocol (RARP)
·
Routing Information
Protocol: used by Unix-based computers for exchanging routing information.
·
Open Shortest
Path First (OSPF) is now replacing this.
Every protocol is set up
by rules.
·
Data Protocols:
dictate some of the same types of relationships as a Diplomatic Protocol (which
suggest who is seated next to whom, how officials of different ranks are to
be addressed, what kind of response is appropriate to another’s statement, who
is introduced to whom, and other such niceties that govern diplomatic affairs).
·
In layered
Protocols, functions are assigned to one layer, but the rules regarding this
are rigid.
·
The internetworking
protocol is likely to be either Transport Control Protocol/ Internet Protocol
(TCP/IP).
·
IP is a higher-level
protocol that uses an addressing scheme entirely different from that used on
the LAN.
A station has two addresses:
·
A permanent
firmware address
·
An ad hoc
IP address
This is a major objective of interactions between protocols; it begins when devices establish communication, and ends hen the communication terminates.
Data networks handle sessions in two distinct ways:
Connectionless: Data is launched into the network
and delivered to the distant end based on its address.
·
Means that
each packet or frame must contain the address of the sending and receiving stations.
·
This is what
most LANs are
Connection-oriented: The devices have a physical or logical
connection across the network; the connection is set up at the start of the
session and remains for its duration.
·
Can be circuit-switched
or virtual connection.
·
Virtual connection:
Defined in a software path that shares bandwidth with other sessions.
·
The packets
or frames typically contain a path identifier, but do not need the address of
either the sender or the receiver after the session is set up.
Protocols can be classified
into two groups:
·
Peer-to-Peer:
Does not use a controller, so devices can communicate with one another at will.
·
Master-slave:
In the latter protocol the master controls the functioning of the data link
and controls data transfer between the host and its terminals.
All communication goes between slaves goes through the master.
Link Management
After the session is set up, the protocol controls the flow of data across the data link.
Synchronizing
Modems exchange signal to determine the highest speed at which they can exchange data, falling back to a lower speed if the circuit will not support the maximum.
Addressing
Every session requires an address to set up a connection if the protocol is connection-oriented or to route packets if it is connectionless.
Routing
In data networks having multiple routes to the destination, the protocol determines the appropriate route based on variables such as cost, congestion, distance, and type of facility.
Data Segmenting and Reassembly
A continuous data stream from the source is segmented into frames, cells, or packets as appropriate and equipped with header and trailer records for transmission over the network.
Data Formatting
The bit stream may require conditioning before transmission and restoration after reception.
Supervision
The protocol establishes a connection, determines how the session will be started and ended, which end will control termination of the session, how charging will be handled, and so on.
Flow Control
Protocols protect networks from congestion by sending signals to the source to halt or limit traffic flow.
Error Detection and Correction
Protocols check for errors,
acknowledge correctly received data blocks, and send repeat requests when blocks
contain an error.
·
Most sophisticated
protocols can acknowledge multiple packets using one or two types of acknowledgment.
·
Selective
Repeat Acknowledgement: Enables the receiving device to request specific packets
to be repeated.
·
Go-Back-N
Method: The receiver instructs the sender to resend an errored packet and all
subsequent packets.
Failure Recovery
If the session terminates unexpectedly, the protocol determines how to prevent the application from being corrupted.
Sequencing
If data blocks are received out of their original sequence, the protocol delivers them to the receiving device in the correct order.
Setting Session Variables
The protocol determines such variables as whether the session will be half or full duplex, network login and authentication, file transfer protocols that will be used, and so on.
The Open Systems Interconnect Model (OSI)
The standardization efforts in protocols have resulted in layered protocols; they are easier to administer than single-layer protocols and provide greater opportunity for standardization.
Layer: A discrete set of functions the protocol is designed to accomplish.
The International Standards Organization (ISO): Published a seven-layer protocol model, the Open Systems Interconnect (OSI) model. The objective is to establish a framework that will allow any conforming system or network to connect and exchange signals, messages, packets, and addresses.
Layered control offers an opportunity for standardization and interconnection between the proprietary architectures of different manufacturers.
Layer 1-Physical
Describes the method of
physical interconnection over a circuit.
·
Contains the
rules for the transmission of bits between machines and standardizes pin connections
between DCE and DTE.
·
The standards
discuss modulation methods and multiplexing over the physical medium, which
is wire, fiber optics, coaxial cable, or wireless.
·
Null Modem:
An adapter that can send data to each other; connects the transmitting data
and signaling leads of each computer to the corresponding receiving leads of
the other.
Layer 2-Data Link
Protocols are concerned
with the transmission of frames of data between devices
·
Detects and
corrects errors so the user gets and error-free circuit.
·
Takes raw
data characters, creates frames of data from then, and processes acknowledgment
messages from the receiver.
·
High-Level
Data-Link Control (HDLC): the principal international standard that has numerous
subsets, of which Balanced Link Access Procedure (LAPB) and Designated Link
Access Procedure (LAPD) are common.
Layer 3-Network
The network layer forms
a logical connection between source and destination nodes on a network; it accepts
messages from the higher layers, breaks them into packets, routes them to the
distant end through the link and physical layers, and reassembles them in them
in the same form in which the sending end delivered them to the network.
·
The network
layer controls the flow of packets, controls congestion in the network, and
routes between nodes to the destination.
·
IP is one
of the most widely used protocols in the world which, together transport control
protocol, forms a common language used in most Internets.
·
It is a connectionless
layer 3 protocol.
Layer 4-Transport
Controls end-to-end integrity between
DTE devices, establishing and termination the connection. It segments data
into manageable Protocol Data Units (PDUs), and reassembles them at the receiving.
·
TCP is most
widely used transport layer protocol.
·
User Datagram
Protocol (UDP): a connectionless protocol that is used by Simple Network Management
Protocol (SNMP) and other functions that support connectionless sessions.
·
An abbreviated
version of TCP.
·
Used by simple
network management protocol (SNMP), Trivial File Transfer protocol (TFTP), and
Versatile Message Transfer Protocol (VMTP).
·
Voice and
video over IP use this sine they no requirement for the error-correction capabilities
of TCP.
Layer 5-Session
The user communicates directly
with the session layer, furnishing an address that the session layer converts
to the address the transport layer requires.
·
The session
layer determines whether machines can interrupt one another.
·
It establishes
how to begin and terminate a session and how to restore or terminate the connection
in case a failure interrupts the session.
Layer 6-Presentation
Interprets the character stream that flows between terminals during the session.
Layer 7-Application
The interface between the
network and the application running on the computer.
·
Message-Handling
Service (MHS): a protocol for enabling X.400 e-mail systems to communicate.
·
File Transfer,
Access, and Management (FTAM): a protocol for managing and manipulating files
across a network.
·
Virtual Terminal
(VT): provides a standard terminal interface.
·
Electronic
Document Interchange (EDI): uses the MHS platform for transferring electronic
documents across networks.
Transport Control Protocol/Internet Protocol (TCP/IP)
TCP/IP is a collection of protocols that were developed beginning in the 1970s by the Department of Defense as a way of providing interoperability among equipment manufacturers. This is designed for operation on the Internet, but it is equally adaptable to communication within a closed network.
This is a true international standard that is administered through the Internet Engineering Task Force (IETF), which is a voluntary body that distributes its recommendations through Internet Requests for Comments that are open to anyone.
The Internet has four primary
purposes:
·
To provide
electronic mail service to the users
·
To support
file transfer between hosts
·
To permit
users to log on to remote computers
·
To provide
users with access to information databases
The primary Application
layer protocols that supports the four functions of the internet (these are
all in the same structure as VT, OSI, FTAM, and X.400):
·
Simple Mail
Transfer Protocol (SMTP) for e-mail
·
File Transfer
Protocol (FTP) for transfers
·
TELNET allows
users to log on a remote computer over the network and operate as if they were
directly attached
·
Hypertext
Transport Protocol (HTTP) is used to support database access through web browsers.
It fits in a three-layer framework atop the physical and data-link layers.
Functions of TCP/IP
It has three-level hierarchy,
lowest being:
·
Subnetwork
or segment: they linked together through switches or routers to comprise domains.
·
Domains: Linked
by an enerprisewide internetwork.
An IP network has two types
of nodes:
·
Hosts: A source
or destination of information such as a computer, printer, server, router, or
other addressable unit.
·
Gateways:
Select routes to a host based on the address, which is unique for every device.
·
Today routers
handle the gateway function in most networks.
·
They have
two types of Gateways:
·
Core Gateways:
have information about the structure of the network.
·
Noncore Gateways:
have incomplete routing information; they know the route to a core gateway but
have no knowledge of routing beyond the core.
TCP’s role is to ensure
that any irregularities during the journey from source to destination are corrected.
·
All devices
on the network are given addresses that correspond to their physical position
in the hierarchy.
·
Open Shortest
Path First (OSPF): the routing algorithm currently used in most networks in
which nodes need only know the shortest route to the destination.
·
IP lacks end-to-end
error checking and acknowledgment.
·
TCP takes
care of those functions.
·
TCP has the
function of disciplining an otherwise chaotic path through the Internet
·
It sets up
a connection at the start of a session, and terminates it at the end.
·
It performs
flow control by using a sliding window.
Internet Protocol (IP)
This is a packet protocol
that segments data so it fits within the packet-size limitations, and launches
the packets into the network as datagrams; it routes information between devices.
·
Datagrams:
a connectionless and unacknowledged packet.
·
Internet Control
Message Protocol (ICMP): reports on delivery of IP datagrams.
·
It warns users
of when a destination is unreachable, and reports on how long it takes to reach
a host or when datagrams exceed their time-to-live parameters.
·
Internet Group
Management Protocol (IGMP): allows devices to be added and removed from address
groups that use class D addresses.
·
It is unreliable,
connectionless, best-effort, datagram protocol that delivers data across an
Internet.
·
It routes
packets and defines the rules under which host and gateways handle packets.
·
It defines
the basic PDU of traffic passing across an Internet, which is an IP datagram.
·
This is a
simple PDU that contains a 28-octet header plus a data area that can be up to
65,535 octets long.
·
Maximum Transfer
Unit (MTU): the maximum length permitted by a network.
·
This is a
laissez-faire protocol, for the Internet lacks flow control and has no way of
detecting duplicate, out-of-sequence, or lost packets.
·
Therefore,
every packet has a time-to-live field that has a maximum value of 255.
·
Internet Control
Message Protocol (ICMP): enables gateways and host to send messages over the
Internet to other gateways and hosts to do functions that lower levels handle
in the OSI model.
IP Addressing
This is both the strength
and the weakness of the TCP/IP protocol. Addressing is composed of three parts:
·
Class
·
Network Portion
·
Host
Each IP address is made
up of four classes:
·
Class A Addresses:
has 24 bits for hosts and 8 bits for networks, 1 bit of which identifies the
network class, leaving 7 bits for network number.
·
Class B Addresses:
Allocates 14 bits for host addresses and 16 bits for networks.
·
Class C Addresses:
Has 8 bits for hosts and 21 bits for the network address.
·
Class D Addresses:
Are multicast addresses.
IPv6 (IP version 6): Is an expanded addressing method that has been approved.
Internet Network Information Center (InterNIC): This is how Internet addresses are assigned. Anyone using TCP/IP is advised to obtain an address from InterNIC.
Transport Control Protocol (TCP)
This is a connection-oriented
guaranteed delivery protocol.
·
It provides
reliable end-to-end data delivery, and is al so responsible for sequencing,
flow control, deleting duplicate packets, and arranging delivery of missing
packets.
·
It is analogous
to OSI’s transport layer.
·
It sets up
a connection at the start of a session, and terminates it at the end.
·
Provides positive
acknowledgment of the receipt of packets from the distant end.
·
It uses a
sliding window protocol to control the session between two hosts.
Address Resolution
When computers share a physical
network, they are assigned a network address.
·
Hosts and
gateways must have a method of mapping IP addresses to physical addresses in
order to send data across an Internet.
·
Address Resolution
Protocol (ARP): when a host needs to know the physical address of a station,
it sends a broadcast message request in the physical address of a station with
a given IP address.
·
Reverse Address
Resolution Protocol (RARP): At start up, stations broadcast their physical address,
asking hosts to respond with their IP address, which is contained in a table.
Once this initial transaction is completed, the workstation is able to respond
to ARP messages.
Dynamic Address Assignment
Bootp:
the TCP/IP model specifies this protocol that is intended to enable diskless
PCs to request an IP address assignment from a server.
·
This eliminates
the need to visit each node to enter an IP address.
·
Drawback:
Once it assigns an address, it has no way of unassigning it if it is not used.
Gateways
These are the key to a host’s
finding its way through the Internet.
·
They contain
routing tables that the programmer enters or that the gateway builds by querying
neighboring gateways.
·
It has detailed
routing information for all directly attached networks and knowledge of where
to send traffic for remote networks.
Point-To-Point Protocol (PPP)
PPP was designed by the
Internet Engineering Task Force to route multiple protocols over dial-up and
dedicated point-to-point links.
·
Most dial-up
Internet programs support PPP in addition to the les-effective synchronous-line
interface protocol (SLIP).
·
It permits
interoperability of hosts, routes, and bridges over serial links.
·
The data link
protocol is HDLC.
·
Operating
above HDLC are two higher-level protocols:
·
Link Control
Protocol (LCP): responsible for negotiating link options and authenticating
the link between devices.
·
This can include
frame compression, adjustment of frame size, and setup of link monitoring.
·
Network Control
Protocol (NCP): once the link is established, this provides a framework to enable
the network layer protocols to establish a connection.
·
A link quality
monitoring protocol provides for checking link quality during the session.
PPP
has two methods of authentication:
·
Password Authentication
Protocol (PAP): in this option the originator sends a password; the receiver
either accepts the password or closes down the link.
·
Challenge
Handshake Authentication Protocol (CHAP): The host transmits a challenge that
contains a random character string to the distant station; the distant station
responds with a calculated value using a private algorithm and the receiving
station’s identifier.
·
The most common
use is over networks that include dial-up connections as opposed to leased-line,
which is what the PPP operates with.