Overview Of The Application Layer

Computer networks are inherently insecure. To keep information secret, it must be encrypted. Encryption protocols fall into two general classes: secret key (e.g. DES, IDEA), and public key (e.g. RSA). Using these protocols is straight-forward; the hard part is key management.
In addition to providing secrecy, cryptographic protocols can also provide authentication. Finally, cryptography can also be used to allow messages to be signed in such a way that the sender cannot repudiate them after they have been sent. Naming in the Internet uses a distributed database system, DNS. Domain Name Server(DNS) holds records with IP addresses, mail exchanges, and other information. By querying a DNS server, a process can map an Internet domain name onto the IP address used to communicate with that domain.
As networks grow larger, they become harder to manage. For this reason, special network management systems and protocols have been devised, the most popular of which is SNMP. This protocol allows managers to communicate with agents inside devices to read out their status and issue commands to them.
Four major network applications are electronic mail, USENET news, the World Wide Web, and multimedia. Most email systems use the mail system defined in RFCs 821 and 822. Messages sent in this system use system ASCII headers to define message properties. These messages are sent using SMTP. Two systems for securing email exist, PGP and PEM.
USENET news consists of thousands of newsgroups on all manner of topics. People can join newsgroups locally, and can then post messages all over the world using the NNTP protocol, which has some resemblance to SMTP.
The World Wide Web is a system for linking up hypertext documents. Each document is a page written in HTML, possible with hyperlinks to other documents. A browser can display a document by establishing a TCP connection to its server, asking for the document, and then closing the connection. When a hyperlink is selected by the user, that document can also be fetched in the same way. In this manner, documents all over the world are linked together in a giant web.
Multimedia is the rising star in the networking firmament. It allows audio and video to be digitized and transported electronically for display. Most multimedia projects use the MPEG standards and transmit the data over ATM connections.

Overview Of The Transport Layer

The transport layer is the key to understanding layered protocols. It provides various services, the most important of which is an end-to-end, reliable, connection-oriented byte stream from sender to receiver. It is accessed through service primitives that permit the establishment, use and release of connection.
Transport protocols must be able to do connection management over unreliable networks. Connection establishment is complicated by the existence of delayed duplicate packets that can reappear at inopportune moments. To deal with them, three-way handshakes are needed to establish connections. Releasing a connection is easier than establishing one, but is still far from trivial due to the two-army problem.
Even when the network layer is completely reliable, the transport layer has plenty of work to do. It must handle all the service primitives, manage connections and timers, and allocate and utilize credits.
The main Internet transport protocol is TCP. It uses a 20-byte header on all segments. Segments can be fragmented by routers within the Internet, so hosts must be prepared to do reassembly. A great deal of work has gone into optimizing TCP performance, using algorithms from Nagle, Clark, Jacobson, Karn and others.
ATM has four protocols in the AAL layer. All of them break messages into cells at the source and reassemble the cells into messages at the destination. The CS and SAR sublayers add their own headers and trailers in various ways, leaving from 44 to 48 bytes of cell payload.
Network performance is typically dominated by protocol and TPDU processing overhead, and the situation gets worse at higher speeds. Protocols should be designed to minimize the number of TPDUs, context switches, and times each TPDU is copied. For gigabit networks, simple protocols using rate, rather than credit, flow control are called for.

Overview Of The Medium Access Sublayer

Some networks have a single channel that is used for all communication. In these networks, the key design issue is the allocation of this channel among the competing stations wishing to use it. Numerous channel allocation algorithms have been devised like :

- FDM : Dedicate a frequency band to each station.
- TDM : Dedicate a time slot to each station.
- Pure ALOHA : Unsynchronized transmission at any instant.
- Slotted ALOHA : Random transmission in well defined time slots.
- 1-persistent CSMA : Standard carrier sense multiple access.
- Nonpersistent CSMA : Random delay when channel is sensed busy.
- P-persistent CSMA : CSMA, but with probability of p of persisting.
- CSMA/CD : CSMA. but abort on detecting a collision.
- Bit Map : Round robin scheduling using a bit map.
- Binary countdown : Highest numbered ready station goes next.
- Tree walk : Reduced contention by selective enabling.
- Wavelength division : A dynamic FDM scheme for fiber.
- MACA, MACAW : Wireless LAN protocols.
- GSM : FDM plus TDM for cellular radio.
- CDPD : Packet radio within an AMPS channel.
- CDMA : Everybody speak at once but in different language.
- Ethernet : CSMA/CD with binary exponential backoff.
- Token bus : Logical ring on a physical bus.
- Token Ring : Capture the token to send a frame.
- DQDB : Distributed queuing on a two-bus MAN.
- FDDI : Fiber-optic token ring.
- HIPPI : Crossbar using 50-100 twisted pairs.
- Fibre channel : Crossbar using fiber optics.
- SPADE : FDM with dynamic channel allocation.
- ACTS : TDM with centralized slot allocation.
- Binder : TDM with ALOHA when slot owner is not interested.
- Crowther : ALOHA with slot owner getting to keep it.
- Roberts : Channel time reserved in advance by ALOHA.

FDM and TDM are efficient when the number of stations is small and the traffic is continous.
ALOHA protocol, with and without slotting and control, has been proposed when the number of stations is large and variable.
BINARY COUNTDOWN completely eliminates contention.
TREE WALK reduces contention by dynamically dividing the stations into two disjoint groups, one of which is permitted to transmit and one of which is not.
WIRELESS LANs have their own problems and solutions. The biggest problem is caused by hidden stations, so CSMA does not work. MACA attempts to stimulate transmissions around the destination, to make CSMA work better.
GSM, CDPD and CDMA are widely used for mobile computers and telephones.
The IEEE 802 LANs are : CSMA/CD, TOKEN BUS, and TOKEN RING. Each of these has its unique advantages and disadvantages, and each has found its own user community.
FDDI, FAST ETHERNET, HIPPI, and FIBER CHANNEL offer bandwidth in the 100 Mbps range and up.

Quick Tech Lesson: Overview Of The Physical Layer

The physical layer is the basis of all the networks. Nature imposes two fundamental limits on all channels, and these determine their bandwidth. These limits are the Nyquist limit, which deals with noiseless channels, and the Shammon limit, for noisy channels.
Transmission media can be guided or unguided. The principle guided media are twisted pair, coaxial cable, and fiber optics. Unguided media include radio, microwaves, infrared, and lasers through the air.
A key element in most wide area networks is the telephone system. Its main components are the local loops, trunks and switches. Local loops are analog-twisted pair circuits, which require modems for transmitting digital data. Trunks are digital, and can be multiplexed in several ways, including FDM, TDM and WDM. The switches include crossbars, space divison switches, and time divison switches. Both circuit switching and packet switching are important.
The telephone system are now becoming digital from end to end and carrying both voice and data traffic over the same lines. Two variants of this new system, known as ISDN, are being introduced. Narrowband ISDN is a circuit switched digital system that is an incremental improvement over the current system. In contrast, broadband ISDN represents a paradigm shift, since it is based on cell switching ATM technology. Various kinds of ATM switches exist, including the knockout switch and the Batcher-banyan switch.
For mobile applications, the hard-wired telephone system is not suitable. Alternatives to the telephone system include cellular radio and communication satellites. Cellular radio is now widely used for portable telephones but will soon be common for data traffic as well. The current generation of cellular systems (e.g. AMPS) are analog, but the next generation (e.g. PCS/PCN) will be fully digital. Transmission communication satellites are geosynchronous, and there was some interest in bigger systems, but the failure of Iridium slowed down this process.