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.

Quick Tech Tip: Overview Of The Network Layer

The network layer provides services to the transport layer through virtual circuits or datagrams. In both cases, its main job is routing packets from the source to the destination. In virtual circuit subnets, a routing decision is made when the virtual circuit is set up. In datagram subnets, it is made on every packet.
Many routing algorithms are used in computer networks. Static algorithms include shortest path routing, flooding, and flow-based routing. Dynamic algorithms include distance vector routing and link state routing. Most actual networks use one of these. Other important routing techniques are hierarchical routing, routing for mobile hosts, broadcast routing, and multicast routing.
Subnets can become congested, increasing the delay and lowering the throughput for packets. Techniques include traffic shaping, flow specifications, and bandwidth reservation. If congestion does occur, it must be dealt with. Choke packets can be sent back, load can be shed, and other methods applied.
Networks differ in various ways, so when multiple networks are connected together problems can occur. Sometimes problems can be finessed by tunneling a packet through a hostile network, but if the source and the destination networks are different, this approach fails. Fragmentation may be called for if different networks are having different maximum sizes.
The Internet has a rich variety of protocols related to the network layer. These include the data protocol, IP, but also the control protocols ICMP, ARP, and RARP, and the routing protocols OSPF and BGP. The Internet is rapidly running out of IP addresses, so a new version of IP, IPv6, has been developed.
Unlike the datagram-based Internet, ATM networks use virtual circuits inside. There must be a set up before data can be transferred and torn down after transmission is completed. Quality of service and congestion control are major issues with ATM networks.

Introduction to Packet Fragmentation

Each network imposes some maximum size on its packets. The network designers are not free to choose any maximum packet size they wish as there are various factors like hardware, operating system, protocols, compliance with some (inter)national standard, desire to reduce error induced transmissions to some level and desire to prevent one packet from occupying the channel too long.
Packets larger than the allowable MTU (Maximum Transmission Unit) must be divided into multiple smaller packets, or fragments, to enable them to traverse the network.
If a packet that is about to be sent (for eg : over an Ethernet link) is bigger than that, the router which is about to send the packet over that link will fragment the packet i.e. the router will split the packet up into smaller messages (known as fragments) that are each small enough to be transmitted over the link. When the fragments arrive at their destination (the computer to which they are being sent), that computer can reassemble the fragments to recover the original message - assuming none of the messages are lost in transit.

How can be fragmentation avoided ?
If the option of "don't fragment" is set ON in IP version 4, and the router wants to send the packet over a link for which the packet is too large, the router will not send the packet at all. Instead, the router will send a message back to the sender of the packet that was too large. The sending computer can then respond to this by sending out smaller packets. This is known as "path MTU discovery".

Strategies for recombining fragments :
- Transparent Fragmentation : When an oversized packet arrives at gateway, the gateway breaks it into smaller fragments, each fragment is addressed to same exit gateway, where pieces are recombined. In this way passage through the small packet network has been made transparent.
Benefits : It maximizes bandwidth on higher links and deterministic fragmentation unlikely.
Drawbacks : Packets may be reassembled/fragmented, gateways more complex, performance gains bounded because the max TU will be the MTU of the first hop. Plus, the IP layer at the destination may still have to perform reassembly if the last link had a smaller MTU than the first link. Only use on links with unusually small MTUs.



- Non transparent Fragmentation : This strategy includes refraining the recombining of fragmented packets at intermediate gateway. Once a packet is fragmented, each fragment is treated as an original packet. All fragments are passed through the exit gateway. Recombination exists only at destination host.
Benefits : Multiple exit gateways can now be used and higher performance can be achieved.
Drawbacks : Overhead increases. Also, it requires every host to be able to do reassembly.

Quick Technical Tip : Tunneling

Tunneling is a way in which data is transferred between two networks securely. All the data that is being transferred are fragmented into smaller packets or frames and then passed through the tunnel. This process is different from a normal data transfer between nodes. Every frame passing through the tunnel will be encrypted with an additional layer of tunneling encryption and encapsulation which is also used for routing the packets to the right direction. This encapsulation would then be reverted at the destination with decryption of data which is later sent to the desired destined node.
Example: People have written tunnels over ICMP, DNS, HTTP, e-mail messages, and TCP connections. Tunnels can either by of the "port redirector" style (which run on top of any TCP/IP stack) or of the network interface variety (below the TCP/IP stack requiring kernel mod).
VPN connection are of two type, PPTP (Point-to-Point tunneling protocol) and L2TP (Layer 2 tunneling protocol). Both PPTP and L2TP tunnels are nothing but local sessions between two different endpoints. Incase they have to communicate then the tunneling type must be negotiated between the endpoint, either PPTP or L2TP and then more configurable parameters like encryption, address assignment, compression etc must be configured in order to get the best possible security over the internet based private logical tunnel communication. This communication is created, maintained and terminated using a tunnel management protocol.
Data can be sent once the tunnel is in place and clients or server can use the same tunnel to send and receive data across the internetwork. The data transfer depends upon the tunneling protocols being used for the transfer.