home *** CD-ROM | disk | FTP | other *** search
- -->[OO]::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
- -->]OO[::::[ Electronic Data Communication ]::[OO---------[ zomba ]----------
- -->]OO[:::::::::::::::::::::::::::::::::::::::[ z0mba@hotmail.com ]----------
- -->[OO]:::::::::::::::::::::::::::::::::::::::[ members.xoom.com/phuk ]::::::
-
-
- --oOo--> Covered in this Article: ]------------------------
- --oOo--> --------------------------------------------------
-
- --oOo--> Introduction ]---
- --oOo--> PRINCIPLE OF ELECTRONIC DATA COMMUNICATION ]---
- --OoO--> ========================================== ]---
- --oOo--> --> Communications Links ]---
- --oOo--> --> Communications Media ]---
- --oOo--> --> Modems ]---
- --oOo--> --> Multiplexors ]---
- --oOo--> COMMUNICATION METHODS ]---
- --oOo--> ===================== ]---
- --oOo--> --> Simplex/duplex Transmission ]---
- --oOo--> --> Synchronous and Asynchronous Transmission ]---
- --oOo--> --> Data transfer checks ]---
- --oOo--> --> Circuit Switching ]---
- --oOo--> --> Packet Switching ]---
- --oOo--> --> Advantages of Packet Switching ]---
- --oOo--> --> Data Compression ]---
- --oOo--> --> Data Encryption ]---
- --oOo--> --> The TCP/IP Protocol ]---
- --oOo--> --> The ISO OSI seven-layer Model ]---
- --oOo--> --> Bridges and Gateways/Routers ]---
-
-
-
-
- Introduction
- --oOo-------
-
- This article is meant to give you, the ereet public, a brief insight into how
- data communications werk. The parts on TCP/IP and the ISO OSI seven-layer
- model were origionally part of a file I was writing for ETG (now defunct) but
- I thought they were relevant to this article and so have included them. If
- you have been using the net for a while then the OSI model will be instantly
- recognisable even if you've never seen it before as it is basically just how
- the internet protocols werk and their ports etc (ie: Telnet, port 23). A lot
- of this article was taken from other sources as they explained better than I
- ever could :)
-
-
- PRINCIPLE OF ELECTRONIC DATA COMMUNICATION
- ==========================================
-
- Data communication involves sending and receiving data from one computer or
- data processing device to another. Applications using for example e-mail,
- supermarket EPOS (Electronic Point-Of-Sale) terminals, cash dispensers, fax
- machines and video conferencing are all examples of this.
-
- When the devices are close together, for example in the same building, they
- can be linked by means of cables. However, when devices are seperated by more
- than a few hundred yards, data has to be sent over a communications link (eg.
- tele- phone line) and extra equipment such as a modem is required.
-
-
- Communications Links
- --oOo---------------
-
- In the UK, BT, Mercury and other telcos provide services and data links.
- Telephone line may be either:
-
- --> public lines, on which the cost of sending data depends on the
- length of time taken;
- --> private or leased lines, for whiche there is a fixed annual fee
- and line can be used 24/7 with no extra cost.
-
-
- Communications Media
- --oOo---------------
-
- Communication may take place over a combination of different media.
-
- --> twisted pair (copper cable), used in much of the PSTN;
- --> coaxial cable - high quality, well-insulated cable that can transmit
- data at higher speeds;
- --> fibre optic cable through which pulses of light, rather than electricity,
- are sent in digital form;
- --> communications satallite, using one of the hundreds of satellites now
- in geosynchronous orbit about 22,000 miles above the Earth (for all you
- l4m3rs, geosynchronous means that they are rotating at the same speed as
- the Earth and are therefore stationary relative to it);
- --> microwave - similar to radio waves. Microwave stations cannot be much
- more than 30 miles apart because of the Earths curvature as microwaves
- travel in straight lines.
-
- The amount of data that can be sent over the line depends partly on the
- bandwidth, which is the range of frequencies that the line can carry. The
- greater the bandwidth, the greater the rate at which data can be sent, as
- several messages can be transmitted simultaneously.
-
- A network that is capable of sending voice, video and computer data is called
- an 'integrated services digital network' (ISDN), and this requires a high
- bandwidth.
-
-
- Modems
- --oOo-
-
- Telephone lines were origionally designed for speech, which is transmitted in
- analogue or wave form. In order for digital data to be sent over a telephone
- line, it must first be converted to analogue form and then converted back to
- digital at the other end. This is achieved by means of a modem (MOdulator
- DEModulator) at either end of the line.
-
-
- Digital Signal Digital Signal
- \ /
- \ Analogue Signal /
- Computer------Modem--------------------------------Modem------Computer
-
-
-
- Multiplexors
- --oOo-------
-
- A multiplexor combines more than one input signal into a stream of data that
- can be transmitted over a single communications channel. This means, for
- example, that a local area network of 48 PC's could all communicate with a
- mainframe at some geographically remote head office via a single leased line
- attached to a multiplexor. At the mainframe end, there is likely to be a
- front-end processor which will handle the communications, leaving the main
- processor free for other tasks.
-
-
- Computer\
- \ ___ Mini
- \ / mainframe
- Computer---Multiplexor---Modem------------------Modem---Multiplexor---- -or-
- / \___ front-end
- / processor
- Computer/
-
-
-
- COMMUNICATION METHODS
- =====================
-
-
- Simplex, half-duplex and full-duplex transmission
- --oOo--------------------------------------------
-
- There are three possible modes of transmission:
-
- --> Simplex - transmission can take place only in one direction. This type
- of transmission could be used for example when the sending
- device such as a temperature sensor never requires a response
- from the computer.
-
- --> Half-duplex - transmission can take place in both directions but not
- simultaneously. This type of transmission is often used
- between a central computer and terminals.
-
- --> Full-duplex - transmission can take place in both directions
- simultaneously. It is suitable for interactive computer
- applications.
-
-
- Synchronous and Asynchronous transmission
- --oOo-----------------------------------
-
- With asynchronous transmission, one character at a time is sent, with each
- character being preceded by a start bit and followed by a stop bit. A parity
- bit is also usually included as a check against incorrect transmission. This
- type of transmission is usually used by PC's, and is fast and economical for
- relatively small amounts of data.
-
- In Synchronous transmission mode, timing signals (usually the computers
- internal clock) control the rate of transmission and there is no need for
- start and stop bits to accompany each character. Mainframe computers usually
- use synchronous transmission. It is less error-prone than asynchronous
- transmission.
-
-
- Data Transfer Checks
- --oOo---------------
-
- The following checks may be made during data transmission:
-
- --> parity checks - an extra bit is transmitted with each character to make
- the number of bits set to 1 even (for even parity) or
- odd for (odd parity).
-
- --> checksum - may be sent with each block of data transmitted. All the
- elements in the block (eg: words or bytes) are added together
- (ignoring overflow) to produce a single element known as the
- checksum, and this is stored and transmitted with the block,
- and checked on receipt.
-
-
- Circuit Switching
- --oOo------------
-
- An excellent example of circuit switching is the public telephone system
- which uses circuit- switched paths. When a caller dials a number, the path
- between the two telephones is set up by operating switches in all of the
- exchanges involved in the path, and the circuit is set up and held for the
- entire duration of the call (even through periods of silence). This allows
- the two people on the phone ('leeto phreaks!)to hold a conversation with no
- waiting at either end.
-
-
- ph0ne____________
- \ \ /
- \ \ /
- ph0ne____________Local Exchange--------Trunk Exchange /
- / | \ /
- / | \ /
- ph0ne____________/ | \__Trunk Exchange----
- | /
- / _________/
- / /
- Trunk Exchange ph0ne
- ph0ne_______ ___________/ \__________ /
- \ / \ /
- Local Exchange Local Exchange
- / | \ \
- / | \ \ph0ne
- ph0ne ph0ne \__ph0ne
-
-
-
- Packet Switching
- --oOo-----------
-
- In a packet switching system (PSS) data is divided into packets - fixed
- length blocks of data say 128 bytes. As well as the data, each packet also
- carries:
-
- --> the source and destination address;
- --> a packet sequence number so that the whole message can be correctly
- reassembled;
- --> a checksum (longitudinal parity check) for the purpose of error checking.
-
- The PSS takes the form of a computer network in which each computer redirects
- packets it receives to the next computer along an appropriate route to its
- destination.
-
-
- Advantages of packet switching
- --oOo-------------------------
-
- --> More efficient use of lines is possible.
- --> Cost depends only on the number of packets sent not on distance, so all
- data is transmitted at local call rates.
- --> It is less likely to be affected by network failure because of the
- multiple routes available to transmit data packets.
- --> Security is better; data is less likely to be intercepted because the
- packets may be sent along different routes or be interleaved with the
- other unrelated packets.
-
-
- Data Compression
- --oOo-----------
-
- Data compression is frequently used when transmitting large quantities of
- data, thereby reducing the number of blocks transmitted and hence the cost.
- It basically works by replacing repeated blocks by one copy of the byte plus
- a count of the repetitions.
-
-
- Data Encryption
- --oOo----------
-
- Data encryption is used for security purposes when transmitting or storing
- confidential data. The data to be transmitted is encoded using a mathematical
- algorithm or substitution of letters, so that even if it is intercepted it
- cannot be read.
-
-
- w0rd to the OfKIZk\$5zG w0rd to the
- darkcyde ---> ENCRYPTION --> OPbNd5%6S --> DECRYPTION --> darkcyde
- collective WeDgNC$รบ1GG8 collective
-
- Plaintext Ciphertext Plaintext
-
-
-
- The TCP/IP Protocol
- --oOo--------------
-
- Basically, TCP/IP is a set of protocols developed around the ARPAnet (where
- the internet began - just in case you didn't know!) which allows co-operating
- computers to share resources across a network. The most accurate name for
- this set of protocols is the 'Internet Protocol Suite' - TCP and IP are just
- two of the protocols in this suite. Due to the fact that TCP and IP are the
- best known of all the protocols, they have been joined to create the most
- common term - TCP/IP.
-
- TCP/IP protocols map to a four layered conceptual model: Applications,
- Transport, Internet, and Network Interface. Each layer on the TCP/IP model
- corresponds to one or more layers on the International Standards Organisation
- (ISO) seven-layer Open Systems Interconnection (OSI) model which I will go
- into more detail on later in the file. Below I have attempted to draw a
- diagram to shows this.
-
-
- OSI Model TCP/IP Model
-
- |--------------| |-----------------|
- | Application | | |
- |--------------| | |
- | Presentation | | Application |
- |--------------| | |
- | Session | | |
- |--------------| |-----------------|
- | Transport | | Transport |
- |--------------| |-----------------|
- | Network | | Internet |
- |--------------| |-----------------|
- | Data-link | | |
- |--------------| |Network Interface|
- | Physical | | |
- |--------------| |-----------------|
-
-
- Defined within the four layers of TCP/IP are protocols that dictate how
- computers connect and communicate. The most common of these are Transmission
- Control Protocol (TCP), User Datagram Protocol (UDP), Internet Protocol (IP),
- Address Resolution Protocol (ARP), and Internet Control Message Protocol
- (ICMP).
-
-
- Transmission Control Protocol (TCP)
- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-
- This is the most common higher-level protocol in the suite. TCP guarantees
- the delivery of packets, ensures proper sequencing of data, and provides a
- checksum feature that validates both the packet header and its data for
- accuracy. If the network either corrupts or loses a TCP packet during
- transmission, TCP is responsible for re-transmitting the faulty packet. This
- level of reliability makes TCP the protocol of choice for session-based data
- transmission, client- server applications, and critical services such as
- email.
-
- This reliability however has its downfalls - TCP headers require additional
- bits to provide proper sequencing of information, as well as a mandatory
- checksum to ensure reliabilty of both the TCP packet header and the packet
- data. To guarantee successful data delivery, the protocol also requires that
- the recipient acknowledge successful receipt of data.
-
- Such acknowledgements (ACK's) generate additional network traffic, thus
- diminishing the rate at which data passes. To reduce the impact on
- performance, most hosts send an acknowledgement for every other segment or
- when a specified time interval has passed.
-
-
- User Datagram Protocol (UDP)
- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-
- If reliability is not totally essential then UDP, a TCP compliment, offers a
- connectionless datagram service that guarantees neither delivery nor correct
- sequencing of delivered packets (much like IP). Higher-level protocols or
- applications might provide reliability mechanisms in addition to UDP/IP. UDP
- data checksums are optional, providing a way to exchange data over highly
- reliable networks without unnecessarily consuming network resources or
- processing time. When UDP checksums are used, they validate both the
- integrity of the header and the data. ACKs are not enforced by the UDP
- protocol, this is left to higher-level protocols. UDP also supports sending
- data from a single sender to multiple receivers.
-
-
- Internet Protocol (IP)
- ~~~~~~~~~~~~~~~~~~~~~~
-
- IP provides packet delivery for all other protocols within the suite. It
- provides a best-effort, connectionless delivery system for computer data.
- They are not guaranteed to be delivered nor received in the order they are
- sent as the protocols checksum feature only confirms the headers integrity.
- The responsibitly of the data contained in the IP packets are only insured by
- using higher-level protocols
-
-
- Address Resolution Protocol (ARP)
- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-
- ARP is not directly related to data transport but is very important
- nonetheless. ARP is one of the maintenance protocols that supports the TCP/IP
- suite and is usually invisible to users and applications.
-
- If two systems are to communicate over a TCP/IP network, the system sending
- tha packet must map the IP address of the final destination to the physical
- address of the final destination. IP acquires this physical address by
- broadcasting a special inquiry packet (an ARP request packet) containing the
- IP address of the destination system. All ARP-enabled systems on the local IP
- network detect these broadcast messages, and the system that owns the IP
- address in question replies by sending its physical address to the requester
- (in an ARP reply packet). The physical/IP address is then stored in the ARP
- cache of the requesting system for subsequent use.
-
- Because the ARP reply can also be broadcast to the network, other systems on
- the network can use this information to update their own ARP caches. (you can
- use the 'arp' utility to view the ARP tables)
-
-
- Internet Control Message Protocol (ICMP)
- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-
- ICMP is another of the maintenance protocols. It allows two systems on an IP
- network to share status and error info. This information is often used by
- network admins to detect network trouble or recover from transmission
- problems. ICMP packets are stored within IP packets and are not really
- considered to be a higher-level protocol.
-
- The 'ping' utility uses the ICMP echo request and echo reply packets to
- determine whether a particular IP system on a network is functional. Because
- of this, the ping utility is useful for diagnosing IP networks or router
- failures.
-
-
- The ISO OSI seven-layer Model
- --oOo------------------------
-
- The seven layers of the Open System Interconnection (OSI) model are shown in
- my diagram below. The reason for the model was to try and introduce some
- standardisation into the protocols of network communication.
-
- |-----(7)-----|-----(6)----|---(5)----|----(4)-----|---(3)----|----(2)----|----(1)----|
- | Application |Presentation| Session | Transport | Network | Data Link | Physical |
- |-------------|------------|----------|------------|----------|-----------|-----------|
- |-------------|------------|----------|------------|----------|-----------|-----------|
- | Email | POP/SMTP | POP/25 | | | |RS-X, CAT 1|
- |-------------|------------|----------| | | |-----------|
- | Newsgroups | Usenet | 532 | | | | ISDN |
- |-------------|------------|----------| | | SLIP, PPP |-----------|
- | Web | HTTP | 80 | | | | ADSL |
- |Applications | | | | | | |
- |-------------|------------|----------|Transmission| Internet | |-----------|
- |File Transfer| FTP | 20/21 | Control | Protocol | | ATM |
- |-------------|------------|----------| Protocol | Version 6|-----------|-----------|
- |Host Sessions| Telnet | 23 | | | |
- |-------------|------------|----------| | |-----------|-----------|
- | Directory | DNS | 53 |------------|----------| | FDDI |
- | Services | | | | |802.2 SNAP |-----------|
- |-------------|------------|----------| | | | CAT 1-5 |
- | Network Mgt.| SNMP | 161/162 | User | Internet |-----------|-----------|
- |-------------|------------|----------| Datagram | Protocol | | Coaxial |
- |File Services| NFS | RPC | Protocol | Version 4|Ethernet II| Cable |
- | | |Portmapper| | | | |
- |-------------|------------|----------|------------|----------|-----------|-----------|
-
-
- By looking at the model in this way you will probably find that you are
- familier with the concept even if you have never seen it before as most pople
- know at the very least things like port 80 is for HTTP and 23 for Telnet etc.
-
- The OSI model was introduced to describe how messages should be transmitted
- between two computers on a network so that product implementors could produce
- products that would consistently work with each other. The idea is that
- messages are only transmitted in the physical layer, if the message is
- received by a host that is not the target then it will not proceed up the
- layers, it will just be passed on. The top four layers (4,5,6,7) are known as
- the 'upper layers' and the bottom three layers (1,2,3) are known as the
- 'lower layers'. The upper layers are used whenever a message passes from or
- to a user. The lower layers are used whenever a message passes through a host
- computer.
-
-
- Layer 7: Application Layer
- ~~~~~~~~~~~~~~~~~~~~~~~~~~
-
- This is the layer at which communication partners are identified, quality iof
- service is identified, user authenticity and privacy are considered, and any
- constraints on data syntax are identified. They are /not/ the actual
- applications themselves, but having said that, some applications perform
- application layer functions.
-
-
- Layer 6: Presentation Layer
- ~~~~~~~~~~~~~~~~~~~~~~~~~~~
-
- This layer is usually a part of the operating system. It converts incoming
- and outgoing data from one presentation format to another ie. ASCII to
- EBCDIC. It is sometimes called the syntax layer. It also handles encryption
- and compression of data.
-
-
- Layer 5: Session Layer
- ~~~~~~~~~~~~~~~~~~~~~~
-
- This layer basically sets up, co-ordinates and terminates conversations,
- exchanges and dialogs between the application at each end. It deals with
- session and connection co-ordination. It allows application processes to
- register unique addresses, such as NetBIOS names. It also has some other
- support functions inclusing user authentication and resource-access security.
-
-
- Layer 4: Transport Layer
- ~~~~~~~~~~~~~~~~~~~~~~~~
-
- This layer manages the end-to-end control ie: determining whether all packets
- have arrived. It also deals with error checking to ensure complete data
- transfer.
-
-
- Layer 3: Network Layer
- ~~~~~~~~~~~~~~~~~~~~~~
-
- This layer handles the routing of the data ie: sending it in the right
- direction to the right destination on outgoing transmissions and receiving
- incoming transmissions at the packet level. It basically deals with routing
- and forwarding. It control subnet traffic to allow intermediate systems to
- instruct a sending station not to transmit its frame when the router's buffer
- is full. If the router is busy, the network layer can instruct the sending
- station to use an alternate router.
-
-
- Layer 2: Data Link Layer
- ~~~~~~~~~~~~~~~~~~~~~~~~
-
- This layer provides error control and synchronisation for the physical level
- and does bit-stuffing for strings of 1's in excess of 5. It furnishes
- 'transmission protocol' knowledge and management. It establishes and
- terminates a logical link (virtual-circuit connection) between two computers
- identified by their unique network interface card (NIC) addresses.
-
-
- Layer 1: Physical Layer
- ~~~~~~~~~~~~~~~~~~~~~~~
-
- This layer conveys the bit-stream through the network at the electrical and
- mechanical level. It provides the hardware means of sending and receiving
- data on a carrier. Data-encoding modefies the digital-signal pattern (1s and
- 0s) used by the computer to better accommodate the characteristics of the
- physical medium and to assist in bit and frame synchronisation. Data-encoding
- resolves which signal pattern represents a binary 1, how the receiving
- station recognises when a 'bit-time' starts and how the receiving station
- delimits a frame.
-
-
- Bridges and Gateways/Routers
- --oOo-----------------------
-
- A bridge is a connection between two local area networks. Wide area networks
- may be connected throught a system of routers/gateways, a gateway being a
- computer which acts as a point of connection between different networks.
-
-
-
- Shouts and Greetz
- --oOo------------
-
- The usual peeps:
-
- Werd to the darkcyde collective, extra shouts to hybrid, bodie and force.
- Also greetz to [JaSuN], darkflame, xio, PUBLiC NUiSANCE, shadow, gossi, elf,
- downtime, kryptus. L8r.
-
-
-
-