Layer 7: Application Layer

Application Layer is a term used in categorizing protocols and methods in architectural models of computer networking. Both the OSI model and the Internet Protocol Suite (TCP/IP) define application layers.

In TCP/IP, the Application Layer contains all protocols and methods that fall into the realm of process-to-process communications via an Internet Protocol (IP) network using the Transport Layer protocols to establish underlying host-to-host connections.

In the OSI model, the definition of its Application Layer is not narrower in scope, distinguishing explicitly additional functionality above the Transport Layer at two additional levels: Session Layer and Presentation Layer. OSI specifies strict modular separation of functionality at these layers and provides protocol implementations for each layer.

The common application layer services provide semantic conversion between associated application processes. Note: Examples of common application services of general interest include the virtual file, virtual terminal, and job transfer and manipulation protocols.


Protocol examples
* 9P, Plan 9 from Bell Labs distributed file system protocol
* AFP,
* APPC, Advanced Program-to-Program Communication
* AMQP, Advanced Message Queuing Protocol
* BitTorrent
* Atom Publishing Protocol
* BOOTP, Bootstrap Protocol
* CFDP, Coherent File Distribution Protocol
* DDS, Data Distribution Service
* DHCP, Dynamic Host Configuration Protocol
* DeviceNet
* DNS, Domain Name System (Service) Protocol
* eDonkey
* ENRP, Endpoint Handlespace Redundancy Protocol
* FastTrack (KaZaa, Grokster, iMesh)
* Finger, User Information Protocol
* Freenet
* FTAM, File Transfer Access and Management
* FTP, File Transfer Protocol
* Gopher, Gopher protocol
* HL7, Health Level Seven
* HTTP, HyperText Transfer Protocol
* H.323, Packet-Based Multimedia Communications System
* IMAP, IMAP4, Internet Message Access Protocol (version 4)
* IRCP, Internet Relay Chat Protocol
* Kademlia
* LDAP, Lightweight Directory Access Protocol
* LPD, Line Printer Daemon Protocol
* MIME (S-MIME), Multipurpose Internet Mail Extensions and Secure MIME
* Modbus
* Netconf
* NFS, Network File System
* NIS, Network Information Service
* NNTP, Network News Transfer Protocol
* NTCIP, National Transportation Communications for Intelligent Transportation System Protocol
* NTP, Network Time Protocol
* OSCAR, AOL Instant Messenger Protocol
* PNRP, Peer Name Resolution Protocol
* POP, POP3, Post Office Protocol (version 3)
* RDP, Remote Desktop Protocol
* Rlogin, Remote Login in UNIX Systems
* RPC, Remote Procedure Call
* RTMP Real Time Messaging Protocol
* RTP, Real-time Transport Protocol
* RTPS, Real Time Publish Subscribe
* RTSP, Real Time Streaming Protocol
* SAP, Session Announcement Protocol
* SDP, Session Description Protocol
* SIP, Session Initiation Protocol
* SLP, Service Location Protocol
* SMB, Server Message Block
* SMTP, Simple Mail Transfer Protocol
* SNMP, Simple Network Management Protocol
* SNTP, Simple Network Time Protocol
* SPTP, Secure Parallel Transfer Protocol
* SSH, Secure Shell
* SSMS, Secure SMS Messaging Protocol
* TCAP, Transaction Capabilities Application Part
* TDS, Tabular Data Stream
* TELNET, Terminal Emulation Protocol of TCP/IP
* TFTP, Trivial File Transfer Protocol
* TSP, Time Stamp Protocol
* VTP, Virtual Terminal Protocol
* Waka, an HTTP replacement protocol
* Whois (and RWhois), Remote Directory Access Protocol
* WebDAV
* X.400, Message Handling Service Protocol
* X.500, Directory Access Protocol (DAP)
* XMPP, Extensible Messaging and Presence Protocol

Layer 6: Presentasi Layer

The Presentation Layer is Layer 6 of the seven-layer OSI model of computer networking.

The Presentation Layer is responsible for the delivery and formatting of information to the application layer for further processing or display. It relieves the application layer of concern regarding syntactical differences in data representation within the end-user systems. Note: An example of a presentation service would be the conversion of an EBCDIC-coded text file to an ASCII-coded file.

The Presentation Layer is the lowest layer at which application programmers consider data structure and presentation, instead of simply sending data in form of datagrams or packets between hosts. This layer deals with issues of string representation - whether they use the Pascal method (an integer length field followed by the specified amount of bytes) or the C/C++ method (null-terminated strings, i.e. "thisisastring\0"). The idea is that the application layer should be able to point at the data to be moved, and the Presentation Layer will deal with the rest.

Encryption is typically done at this level too, although it can be done on the Application, Session, Transport, or Network Layers; each having its own advantages and disadvantages. Another example is representing structure, which is normally standardized at this level, often by using XML. As well as simple pieces of data, like strings, more complicated things are standardized in this layer. Two common examples are 'objects' in object-oriented programming, and the exact way that streaming video is transmitted.

In many widely used applications and protocols, no distinction is made between the presentation and application layers. For example, HTTP, generally regarded as an application layer protocol, has Presentation Layer aspects such as the ability to identify character encoding for proper conversion, which is then done in the Application Layer.

Within the service layering semantics of the OSI network architecture, the Presentation Layer responds to service requests from the Application Layer and issues service requests to the Session Layer.

Layer 5: Session Layer

The Session Layer controls the dialogues (connections) between computers. It establishes, manages and terminates the connections between the local and remote application. It provides for full-duplex, half-duplex, or simplex operation, and establishes checkpointing, adjournment, termination, and restart procedures. The OSI model made this layer responsible for graceful close of sessions, which is a property of the Transmission Control Protocol, and also for session checkpointing and recovery, which is not usually used in the Internet Protocol Suite. The Session Layer is commonly implemented explicitly in application environments that use remote procedure calls.

Layer 4: Transport Layer

The Transport Layer provides transparent transfer of data between end users, providing reliable data transfer services to the upper layers. The Transport Layer controls the reliability of a given link through flow control, segmentation/desegmentation, and error control. Some protocols are state and connection oriented. This means that the Transport Layer can keep track of the segments and retransmit those that fail.

Although not developed under the OSI Reference Model and not strictly conforming to the OSI definition of the Transport Layer, typical examples of Layer 4 are the Transmission Control Protocol (TCP) and User Datagram Protocol (UDP).

Of the actual OSI protocols, there are five classes of connection-mode transport protocols ranging from class 0 (which is also known as TP0 and provides the least error recovery) to class 4 (TP4, designed for less reliable networks, similar to the Internet). Class 0 contains no error recovery, and was designed for use on network layers that provide error-free connections. Class 4 is closest to TCP, although TCP contains functions, such as the graceful close, which OSI assigns to the Session Layer. Also, all OSI TP connection-mode protocol classes provide expedited data and preservation of record boundaries, both of which TCP is incapable.

Perhaps an easy way to visualize the Transport Layer is to compare it with a Post Office, which deals with the dispatch and classification of mail and parcels sent. Do remember, however, that a post office manages the outer envelope of mail. Higher layers may have the equivalent of double envelopes, such as cryptographic presentation services that can be read by the addressee only. Roughly speaking, tunneling protocols operate at the Transport Layer, such as carrying non-IP protocols such as IBM's SNA or Novell's IPX over an IP network, or end-to-end encryption with IPsec. While Generic Routing Encapsulation (GRE) might seem to be a Network Layer protocol, if the encapsulation of the payload takes place only at endpoint, GRE becomes closer to a transport protocol that uses IP headers but contains complete frames or packets to deliver to an endpoint. L2TP carries PPP frames inside transport packet.

Layer 3: Network Layer

The Network Layer provides the functional and procedural means of transferring variable length data sequences from a source to a destination via one or more networks, while maintaining the quality of service requested by the Transport Layer. The Network Layer performs network routing functions, and might also perform fragmentation and reassembly, and report delivery errors. Routers operate at this layer—sending data throughout the extended network and making the Internet possible. This is a logical addressing scheme – values are chosen by the network engineer. The addressing scheme is hierarchical.

The best-known example of a Layer 3 protocol is the Internet Protocol (IP). It manages the connectionless transfer of data one hop at a time, from end system to ingress router, router to router, and from egress router to destination end system. It is not responsible for reliable delivery to a next hop, but only for the detection of errored packets so they may be discarded. When the medium of the next hop cannot accept a packet in its current length, IP is responsible for fragmenting the packet into sufficiently small packets that the medium can accept.

A number of layer management protocols, a function defined in the Management Annex, ISO 7498/4, belong to the Network Layer. These include routing protocols, multicast group management, Network Layer information and error, and Network Layer address assignment. It is the function of the payload that makes these belong to the Network Layer, not the protocol that carries them.

Layer 2: Data Link Layer

The Data Link Layer provides the functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the Physical Layer. Originally, this layer was intended for point-to-point and point-to-multipoint media, characteristic of wide area media in the telephone system. Local area network architecture, which included broadcast-capable multiaccess media, was developed independently of the ISO work in IEEE Project 802. IEEE work assumed sublayering and management functions not required for WAN use. In modern practice, only error detection, not flow control using sliding window, is present in data link protocols such as Point-to-Point Protocol (PPP), and, on local area networks, the IEEE 802.2 LLC layer is not used for most protocols on the Ethernet, and on other local area networks, its flow control and acknowledgment mechanisms are rarely used. Sliding window flow control and acknowledgment is used at the Transport Layer by protocols such as TCP, but is still used in niches where X.25 offers performance advantages.

The ITU-T G.hn standard, which provides high-speed local area networking over existing wires (power lines, phone lines and coaxial cables), includes a complete Data Link Layer which provides both error correction and flow control by means of a selective repeat Sliding Window Protocol.

Both WAN and LAN services arrange bits, from the Physical Layer, into logical sequences called frames. Not all Physical Layer bits necessarily go into frames, as some of these bits are purely intended for Physical Layer functions. For example, every fifth bit of the FDDI bit stream is not used by the Layer.

WAN Protocol architecture

Connection-oriented WAN data link protocols, in addition to framing, detect and may correct errors. They are also capable of controlling the rate of transmission. A WAN Data Link Layer might implement a sliding window flow control and acknowledgment mechanism to provide reliable delivery of frames; that is the case for SDLC and HDLC, and derivatives of HDLC such as LAPB and LAPD.

IEEE 802 LAN architecture

Practical, connectionless LANs began with the pre-IEEE Ethernet specification, which is the ancestor of IEEE 802.3. This layer manages the interaction of devices with a shared medium, which is the function of a Media Access Control sublayer. Above this MAC sublayer is the media-independent IEEE 802.2 Logical Link Control (LLC) sublayer, which deals with addressing and multiplexing on multiaccess media.

While IEEE 802.3 is the dominant wired LAN protocol and IEEE 802.11 the wireless LAN protocol, obsolescent MAC layers include Token Ring and FDDI. The MAC sublayer detects but does not correct errors.

Layer 1: Physical Layer

The Physical Layer defines the electrical and physical specifications for devices. In particular, it defines the relationship between a device and a physical medium. This includes the layout of pins, voltages, cable specifications, hubs, repeaters, network adapters, host bus adapters (HBAs used in storage area networks) and more.

To understand the function of the Physical Layer in contrast to the functions of the Data Link Layer, think of the Physical Layer as concerned primarily with the interaction of a single device with a medium, where the Data Link Layer is concerned more with the interactions of multiple devices (i.e., at least two) with a shared medium. The Physical Layer will tell one device how to transmit to the medium, and another device how to receive from it (in most cases it does not tell the device how to connect to the medium). Standards such as RS-232 do use physical wires to control access to the medium.

The major functions and services performed by the Physical Layer are:

* Establishment and termination of a connection to a communications medium.
* Participation in the process whereby the communication resources are effectively shared among multiple users. For example, contention resolution and flow control.
* Modulation, or conversion between the representation of digital data in user equipment and the corresponding signals transmitted over a communications channel. These are signals operating over the physical cabling (such as copper and optical fiber) or over a radio link.

Parallel SCSI buses operate in this layer, although it must be remembered that the logical SCSI protocol is a Transport Layer protocol that runs over this bus. Various Physical Layer Ethernet standards are also in this layer; Ethernet incorporates both this layer and the Data Link Layer. The same applies to other local-area networks, such as token ring, FDDI, ITU-T G.hn and IEEE 802.11, as well as personal area networks such as Bluetooth and IEEE 802.15.4.

OSI Layer

The Open System Interconnection Reference Model (OSI Reference Model or OSI Model) is an abstract description for layered communications and computer network protocol design. It was developed as part of the Open Systems Interconnection (OSI) initiative. In its most basic form, it divides network architecture into seven layers which, from top to bottom, are the Application, Presentation, Session, Transport, Network, Data-Link, and Physical Layers. It is therefore often referred to as the OSI Seven Layer Model.

A layer is a collection of conceptually similar functions that provide services to the layer above it and receives service from the layer below it. On each layer an instance provides services to the instances at the layer above and requests service from the layer below. For example, a layer that provides error-free communications across a network provides the path needed by applications above it, while it calls the next lower layer to send and receive packets that make up the contents of the path. Conceptually two instances at one layer are connected by a horizontal protocol connection on that layer.

What is Telecommunication

What is telecommunication ?
The process of transmitting or receiving information over a distance by any electrical or electromagnetic medium. Information may take the form of voice, video, or data.

Telecommunication media
– Analog (traditional telephony, radio, and TV broadcasts)
– Digital (computer-mediated comm., telegraphy, and computer networks)

Basic GSM Configuration

A basic GSM configuration is shown.
It consists of the basic system blocks of the GSM network namely the MSC (Mobile Switching Center), TRAU (TRANSCODER), BSC (Base System Controller, BTS ( Base Transceiver Station) and MS (Mobile Station).
In Figure above, the mobile station talks with the BTS through the use of full duplex radio transmissions using a separate transmit and receive frequency to communicate with each other.
The BTS transmits on the frequency the mobile is tuned to and the mobile transmits to the frequency the BTS receiver is tuned to. The BTS in a system connect to the BSC through a transmission system. It can be one of these: Terrestrial transmission, Fiber Optic Transmission, Satellite Transmission, Leased line Transmission, etc. From BSC , the connection is going to TRAU (Trans-coder), and converting or adapting the bit rates from 16 kbps to 64 kbps (PCM-Standard) , and the end of connection is MSC.
The MS is a radiotelephone that may be used whenever "cell" coverage is provided. The term "mobile phone" has been used generically to include several forms of wireless communication. This term represents fully portable cellular and digital phones in addition to hand-held and hands-free car phones.
The BTS is a transceiver facility that provides the cell coverage. The cell provided by a cell site can be from one mile to twenty miles in diameter, depending on terrain and transmission power. Several coordinated cell sites are called a cellular network.
The BSC is the controller of BTS and manage the group of BTS for doing something like handover, radio resources management.
The TRAU is a unit which has function to convert the bit rate from 16 kbps (GSM bit rates Full rate) to 64 kbps (PCM Standard).
The MSC is the switching parts. These systems is doing the function like allocate the traffic channel, arranges handoffs, call routing, keeps track of billing information, etc.

Types of Telecommunication Networks

In its most basic form a network is an interconnected system of things or people. From a technical standpoint a network is a data communication system that interconnects computer systems at different sites, or the connection of two or more computers using a communications system.

Most networks can be classified into one of five different types. These include wide area networks (WAN), local area networks, (LAN), virtual private networks (VPN), client/server networks, network computing, and peer-to-peer networks.

Wide Area Network (WAN)

Any network that encompasses a large geographic area is referred to as a WAN. Many large businesses and government agencies use WANs to keep their employees and citizens connected as well as provide a quick and effective way to send and receive information.

Metropolitan Area Network (MAN)

A MAN is a network that covers a region, often a metropolitan area that is bigger than a Local Area Network and smaller than a Wide Area Network and consists of several interconnected LANs. This network often serves regional businesses that have several locations throughout the region or entire cities. With this configuration, a MAN often is then connected to larger WAN networks.

There are three features that differentiate MANs from LANs or WANs:

1. The area of the network size is between LANs and WANs. The MAN will have a physical area between 5 and 50 km in diameter.

2. MANs do not generally belong to a single organization. The equipment that interconnects the network, the links, and the MAN itself are often owned by an association or a network provider that provides or leases the service to others.

3. A MAN is a means for sharing resources at high speeds within the network. It often provides connections to WAN networks for access to resources outside the scope of the MAN.

Campus Area Network (CAN)

A CAN is a network that is restricted to a small geographic area such as a building complex or a college campus. It is smaller than a Metropolitan Area Network but larger than a Local Area Network. The CAN incorporates several LANs and usually has connections to a MAN or WAN.

Local Area Network (LAN)

Similar in many ways to WANs; LANs are responsible for connecting computers in a much smaller limited physical area. A good example of a LAN would be a hotel's wireless Internet offering which is self-contained within their own facility.

There are multiple standards for Local Area Networks. Examples include IEEE 802.3 (Ethernet), IEEE 802.11 (Wi-Fi) or ITU-T G.hn (using existing home wires, such as power lines, phone lines and coaxial cable).

Personal Area Network (PAN)

A Personal Area Network (PAN) is a network that is restricted to the area of a person's body. It is much smaller than Local Area Network. It typically incorporates ad hoc connections to other PANs or directly to BlueTooth devices.

Virtual Private Network (VPN)

VPNs are a type of network that builds on the concept of a WAN however relies upon the internet and an encrypted connection mechanism to establish a secure environment for internal or external employees or customers.

Client/Server Network

The Client Server network architecture continues to be the main architectural choice for most enterprise network computing. In a client/server environment the client (i.e. PC) relies on a LAN to connect with a back office network server that is responsible for the connection, retrieval, and storage of data and other critical company or personal information.

Network Computing

Network Computing is a network architecture that has grown with the Internet and resulting connection speeds. In a network computing architecture a computer uses its web browser to connect to another network computer that actually is running the application. A good example of this architecture in use is Google Docs, or Microsoft Office online. Both services allow users the ability to login to Google or Microsoft servers respectively and work similarly to how it would be performed on their own computing environment.

Peer-to-Peer Network

Peer to peer networks are now beginning to be realized for the positive benefits they provide and not as only used for the sharing of copyrighted material. Peer-to-peer networks can be separated into two major types: Central Server and Pure.

In a central server environment one host server maintains all active connections and shared information. When information is requested the central server informs the user where they can receive the file and allows the connection directly to the other PC to download.

A pure peer-to-peer network type has no central server to maintain active users relies instead on the individual computers to seek out all other computers offering the same information being requested. A good example of this type would be BitTorrent software which allows small parts of information to be pulled from many sources which once completed compiles into the one file that is being downloaded.