OSI Reference Model

The Open Systems Interconnect Reference Model is a seven layer model defined by the ISO (International Standards Organization) to create an interoperable network. The layers describe all functions of a network from the physical medium up to the application.The idea of the ISO was to create separate layers independant of each other. This way it would be possible to use the best or most suitable layer from every protocol. This didn't succeed, but the OSI Reference Model is still the best way to describe a network.
OSI Reference Model
7 Application Layer
6 Presentation Layer
5 Session Layer
4 Transport Layer
3 Network Layer
2 Data Link Layer
1 Physical Layer7 Application Layer

The application layer contains all the management information for the distributed applications.The data units here is the real user data. Active devices for internetworking are called gateways.Note: This is not according to OSI which calls a gateway a device which is active on layer 3

6 Presentation Layer
The presentation layer is responsible for formating data in such way that it is ready for presentation to the application. This means translation of different character formats (ASCII/EBCDIC) is done here, but also text (de)compression, virtual terminal emulation and encryption/decryption. It is completely responsible for translation, formatting and the syntax selection.The data units on this layer are called messages

5 Session Layer
The session layer is responsible for establishing, maintaining and termination of session between applications. The session layer establishes and sets and synchonizes the parameters for dialogues between two devices.

4 Transport Layer
The transport layer is responsible for a guaranteed delivery of data. It provides a reliable service, it takes care of the setup, maintenance and shutdown of vitrual circuits and it is responsible for fault detection, error recovery and flow control.Data units on this layer are refered to as datagrams

3 Network Layer
The network layer is responsable for connectivity and path sellection between two end systems. By using the network layer it is possible to separate LANs into distinct networks. There is no guarantee of correct data delivery, since the network layer does not provide error correction.The devices active on this layer are called routers and the data units are refered to as packets.Note: There is a slight distinction with official OSI name for active devices on this layer. The OSI talks of gateways, but the OSI is probably the only one, we will refer to routers.

2 Data Link Layer
The data link layer is responsable for the physical addressing of the data. This means it's addressing is based on the true network interface card addresses, also known as MAC-addresses. The data link layer is also responsable for error notification, ordered delivery of frames and flow control.The IEEE has split this layer up into MAC and LLC, and even added SNAP.

The reason for this splitup is that the LLC, with or without SNAP, is identical for all networks and the MAC-layer is network specific. Ethernet and Tokenring have a different medium access control protocol (CSMA/CD and Tokenpassing) but the same LLC (IEEE 802.2)It's on the MAC part of the data link layer where bridges are active. The data units related to this layer are called frames.

1 Physical Layer
The physical layer is responsable for activating, maintaining and deactivating the physical link between to end stations. It defines electrical (voltage levels, timing and coding of voltage levels), mechanical (physical conntectors and cabling) and functional (data rates, max. transmission distances) specifications.Devices active on this layer are called repeaters. The units transmitted are bits (1 or 0).

Circuit Switching Vs Packet Switching

Circuit Switched Dial-Up Lines
Circuit switched, or digital dial-up (switched 56, ISDN) links form on demand. For the duration of a dial-up session, they behave as open pipes. Within the region of a service, the cloud that defines the boundary of that service in principle includes all end points. Establishing a link requires only a few seconds. Redundant paths exist among end points. Peer interaction among end points requires that the number of links grows beyond the number of end points.

Packet Switched Lines
Links from end points to the 'cloud' (boundary of the packet-switching service) (X.25, frame relay) are usually open pipes.Within the cloud, connections among end points are usually PVCs. Configuring a new PVC, which requires no equipment changes, can take as little as a few hours.Redundant paths can exist among end points. For every-to-every connectivity, the number of links into the cloud does exceed the number of end points (however, the number of PVCs must grow).

ARCnet

Introduction

To start off, ARCnet is not a standardized network. All information provided here is gathered through the years by different people and by just making mistakes, so if you have other experiences don't be surprised, but share them with us. This way we can all learn from it.
ARCnet (Attached Resource Computer NETwork) was originally developed by Datapoint Corporation in 1968, way before OSI and the like. The original purpose was a harddisk interface, later it became popular as a local area network.
According to the Datapoints ARCNET Designer's Handbook 61610 © 1988, ARCnet is: a high-performance, bus-structured, token-passing, base-band network linking two or more computers through high-speed connections. The network can support up to 255 nodes, transmitting data at a rate of 2.5 Mbps up to 4 miles in distance (when active hubs are used between nodes).
ARCnet uses a Token Passing bus structure much like IEEE802.4. It's most common speed is still 2.5 Mbps, but we know of versions runnig 20 Mbps (ARCnet plus by SMC, NCR and Datapoint) and the rumour is there is even a 100 Mbps (by Thomas Conrad) version. So it seems like it is still in progress. The reason for this is probably its easyness (ARCnet is very tolerable) in cabling and the predictable delivery of packets through the use of a token architecture.

The ARCnet devices

ARCnet knows 5 different types of devices:
Active hub
They split and amplify the signal. The are a bit like the hubs in Ethernet. Unused ports on an Active Hub do not need termination, although it is advised to terminate them. The active hubs provide electrical isolation at each port. They feature problem detection and segment partitioning, so only the segment on that port will be affected by problems of one of the attached devices to that segment.
Active Link
Which actually is a repeater. It has two ports and acts as a two port active hub.
Passive hub
They just split the signal, which means if it is a four port hub, every port gets 1/3 of the signal (one port was incoming). Unused ports on an Passive Hub do need termination. A Passive Hub is a small box with 4 BNC connectors and an internal resistor network.
Bus NIC (RIM)
A network card with a high-impedance driver (SMC 9068), only suitable for bus-segments.
Star NIC (RIM)
A network card with a low-impedance driver (SMC 9058), only suitable for a star-segments.

Local Loop

IntroductionThe Local Loop is the line from the customer premisis to the local telco office. This line connects you to the nearest facility of the local PTT. The cable used is generally 26 AWG unshielded twisted pair cable (but it can be fiber also), with runs of about 2.5 miles or 4 km length. Most lines do carry only a single call, but more calles are possible (ISDN, xDSL). The Local Loop is terminated at two points. One is the connection at the customer premisis and the other is the connection to the channel bank or mux at the local telco office.

There are two types of connections:

Analog
Digital

Analog lines
Analog lines are always copper. The lines could be used for voice and data and can be dial-up or leased lines. The specificness about these lines is that the information is transported through the wires in an analog way, meaning sine waves are used and not ones and zeros, like in digital lines. The terminals on these lines are often telephone sets or modems. For a description of these devices see their corresponding pages.

Loading Coils
To assure electrical consistency over the entire analog copper line loading coils are placed in the line. Loading coils can not be used in digital links, because they absorb digital pulses.

Digital lines
Digital lines may be copper or fiber.

DTE and DCE

DTE and DCE
The terms DTE and DCE are very common in the datacommunications market. DTE is short for Data Terminal Equipment and DCE stands for Data Communications Equipment. But what do they really mean? As the full DTE name indicates this is a piece of device that ends a communication line, whereas the DCE provides a path for communication.
Let's say we have a computer on which wants to communicate with the Internet through a modem and a dial-up connection. To get to the Internet you tell your modem to dial the number of your provider. After your modems has dialed the number, the modem of the provider will answer your call and your will hear a lot of noise. Then it becomes quiet and you see your login prompt or your dialing program tells you the connection is established.Now you have a connection with the server from your provider and you can wander the Internet.
In this example you PC is a Data Terminal (DTE). The two modems (yours and that one of your provider) are DCEs, they make the communication between you and your provider possible. But now we have to look at the server of your provider. Is that a DTE or DCE?The answer is a DTE. It ends the communication line between you and the server. Although it gives you the possibility to surf around the glode. The reason why it is a DTE is that when you want to go from your provides server to another place it uses another interface. So DTE and DCE are interface dependend. It is e.g. possible that for your connection to the server, the server is a DTE, but that that same server is a DCE for the equipment that it is attached to on the rest of the Net.

Multiplexing

Multiplexing
GeneralA multiplexor makes it possible for several devices to share a single communication line. Every device has a point to point connection to a device on the remote multiplexor, while there is just a single link available.The way the main link is shared is three folded:
Frequency Division
Statistical
Time Division
Multiplexors do not have any intelligence about the data they are multiplexing. They are interface dependend. The input ports are usually called channels and the output port is called the composite or main port.

Frequency Division Multiplexing:
FDMs devide the available bandwidth (Hz) of a link into multiple sub-channels with a smaller bandwidth. A good example is the way cable TV (CATV) is broadcasted to every home. A single cable contains all the channels you can choose from on your TV-set.Guardbands are used to separate the sub-channels. This means there is some overhead. This kind of multiplexing isn't used often (meaning in a wide range of applications), because FDMs can be as easilly expanded as the other multiplexing technics.

Time Division Multiplexing:
TDMs use the full bandwidth for every channel, but not at the same time. In a round robin fashion every channel gets its time slice (time slot) to the shared link.Each channel is sampled for a certain amount of time (the sample time depends on the number of channels and the input speed, length of a bit). The state of the channel is then send to the remote TDM where it is demultiplexed from the incoming bitstream and send to the corresponding channel. Because the sampling rate is a multiple of the bit time every bit is sampled more than once to prevent data loss. The aggregate rate of the channels can not exceed the rate of the composite port.When channels are not used, their bandwidth is still reserved (an empty slot is send). A more efficient way of multiplexing is statistical multiplexing.

Statistical Multiplexing:
A STM tries to use the capacity of the line as optimal as possible. Every channel is buffered and only those channels that have something to send are multiplexed and send to the remote side. This requires some intelligence from the muxes and a way of indicating which data came from which port.A large amount of data or many 'used' ports will soon flood the mux or the shared link.

X.25

Overview
In 1976 the CCITT adopted the X.25 standard labeled "Interface Between Data Terminal Equipment (DTE) and Data Circuit Terminating Equipment (DCE) for Terminals Operating in the Packet Mode on Public Data networks". X.25 is a peer-to-peer network and is only an interface protocol to packet-switched public networks. Packet-switching techniques are a flexible alternative that provides statistical multiplexing (the ability to have more than one logical channel over one physical channel) and any-to-any connectivity. The DTE interfaces with an X.25 service provider (DCE). Packet switches within the X.25 network are refered to as Data Switching Equipment (DSE).
X.25 specifies standards for the bottom three layers of the OSI-model.
1- The physical layer requires the use of interface recommendations X.20 (asynchronous) and X.21 (synchronous).
2-The link layer is split in two separate procedures based on ISO 3309 High Level Data Link Control (HDLC):
-Link Acces Procedure (LAP)
This is an original X.25 provision consisting of the HDLC "asynchronous response mode".
-Link Acces Procedure Balanced (LAPB)
This one was added later and is similar to the HDLC "asynchronous balanced mode". The LAPB makes it possible for communicating parties to operate in an autonomous, balanced mode in which neither is considered a master or slave.
-The network layer is called the packet level in X.25. This packet level accomplishes the flow of control information and data across the network.


Layer2 Layer 3 Layer 2
Flag Address Control Information Frame Check Sequence Flag


The frame format of X.25 is according to the ISO 3309 standard. The FLAG field is used to identify the beginning and ending of a frame. The ADDRESS field designates the binary address of the receiving station. The CONTROL field indicates the type of frame, such as:
-Unnumbered frames, consist of command information to start or stop the communication link. They do not contain data.
-Supervisory frames, contain routine operational commands, including the response to previously received frames. They do not contain data.
-Information frames, contain data packets, and may also contain response information

The INFORMATION fields contain the actual data and a data header, or is empty
The data header is 24 bits and split up in 4 parts:
-4 bits General Format Identifier (GFI)
-4 bits Logical Channel Group Number (LGN)
-8 bits Logical Channel Number (LCN)
-8 bits Packet Type ID (PTI) or P(R)MP(S)0 (Packet sequence numbers used when a packet contains user data)

The FRAME CHECK SEQUENCE is the CRC (Cyclic Redundancy Check) code.

The Transmission
The transmission is started in a virtual circuit operation with a call request. The initiating DTE asks the network for a virtual circuit with the destination DTE. The packet used for this is called a Call Request packet. The Call Request packet contains the destination address.The address (along with the originating DTE's identification) is used by the network for the duration of the call.The network responds with a "connect" message or with a message saying that the circuit can't be established and usually why. Now the actual data transfer can begin, on a routine or interrupt basis, and according to a set of "flow control" constraints imposed by the network. All these activities are handled by specific packet types. X.25 defines more than 30 packet types for various functions.

The Layer Discriptions
The Physical Layer:This one is not specified in the CCITT recommendation, but X.25 is traditionally used on leased lines, although there is a dial-up function which uses X.121 as an addressing scheme. This addressing scheme allows up to 14 digits arranged in a hierarchy of zones and domains.The connection is synchronous with speeds of 2.4, 4.8, 9.6, 56 and 64 kbps. There is also a 2 Mbps version.Common RS-232/EIA interfaces can be used, but more usually you'll find X.21. For asynchronous connections you'll need a Packet Assembler/Disassembler (PAD).

The Data Link Layer:
The data link layer is responsible for the packet format, which is described above. And will be discussed here in more detail.
-The first and last field of the packet is the FLAG (X'7E'). -The ADDRESS field has two possible values X'01' and X'03' which are also known as A and B. Which are used to indicate 'originate' and 'answer'.
-The CONTROL field is initialized with a Set Asynchronous Balanced Mode (SABM). There is also an extended operation (SABME) that provides a modulo-128 frame window. LAPB is a full-duplex mode of operation.

The Network Layer:
Often referred to as Packet Layer Protocol (PLP). The layer protocol provides a statistical multiplexing technique through the use of multiple logical connections across a physical link.The three possible logical (virtual) connections are:
-Logical Channel
-Switched Virtual Circuit (SVC)
-Permanent Virtual Circuit (PVC)

The PVCs and SVCs are both end-to-end connections between two users, but they are different in the fact that PVCs are predefined by the X.25 service provider and SVCs are not. PVCs also have the advantage that they do not require the connection setup and takedown time which are needed with SVCs.

The network layer is also responsible for the information field, which consists of two parts: the actual data and the data header.The first part of the data header is the 4-bit General Format Identifier (GFI) which indicates (among other things) the presence or absence of data.The 4-bit Logical Channel Group Number (LGN) is the group number.The 8-bit Logical Channel Number (LCN) is the channel number.The LGN and LCN combined can be used as one big channel number.LCN 0 is normally used for emergency network commands.And the last part is the 8-bit Packet Type Identifier (PTI) or Level 3 flow control sequence numbers. The PTI is only present at level 3 protocol packets.