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.

Quick Tech Tip: CDMA - Code Division Multiple Access

CDMA (Code-Division Multiple Access) refers to any of several protocols used in so-called second-generation (2G) and third-generation (3G) wireless communications. As the term implies, CDMA is a form of multiplexing, which allows numerous signals to occupy a single transmission channel, optimizing the use of available bandwidth. CDMA, a proprietary standard designed by Qualcomm in the United States, has been the dominant network standard for North America and parts of Asia.
Coverage: The most important factor is getting service in the areas you will be using your phone. Upon viewing competitors' coverage maps you may discover that CDMA carriers offer cellular service in your area. If so, there is no decision to be made, but most people will find that they do have a choice.
For radio systems there are two resources, frequency and time. Division by frequency, so that each pair of communicators is allocated part of the spectrum for all of the time, results in Frequency Division Multiple Access (FDMA). Division by time, so that each pair of communicators is allocated all (or at least a large part) of the spectrum for part of the time results in Time Division Multiple Access (TDMA). In Code Division Multiple Access (CDMA), every communicator will be allocated the entire spectrum all of the time. CDMA uses codes to identify connections.


CDMA is a form of spread-spectrum, a family of digital communication techniques that
have been used in military applications for many years. The core principle of spread
spectrum is the use of noise-like carrier waves, and, as the name implies, bandwidths
much wider than that required for simple point-to-point communication at the same data rate. CDMA is a Direct Sequence Spread Spectrum system. The CDMA system works directly on 64 kbit/sec digital signals. These signals can be digitized voice, ISDN channels, modem data, etc.
CDMA is altering the face of cellular and PCS communication by:
· Dramatically improving the telephone traffic capacity.
· Dramatically improving the voice quality and eliminating the audible effects of
multipath fading.
· Reducing the incidence of dropped calls due to handoff failures.
· Providing reliable transport mechanism for data communications, such as
facsimile and internet traffic.
· Reducing the number of sites needed to support any given amount of traffic.
· Simplifying site selection.
· Reducing deployment and operating costs because fewer cell sites are needed.
· Reducing average transmitted power.
· Reducing interference to other electronic devices.
· Reducing potential health risks.

Quick Tech Tip: Point-to-point tunneling protocol - PPTP

Overview of Point-to-point Protocol:

The Point-to-Point Protocol (PPP) provides a standard method for transporting multi-protocol datagrams over point-to-point links. PPP was originally emerged as an encapsulation protocol for transporting IP traffic between two peers.PPP is comprised of the following main components:
* Encapsulation: A method for encapsulating multi-protocol datagrams.
* Link Control Protocol: The LCP is used to automatically agree upon the encapsulation format options, handle varying limits on sizes of packets, detect a looped-back link and other common misconfiguration errors, and terminate the link.
* Network Control Protocol: An extensible Link Control Protocol (LCP) for establishing, configuring, and testing and managing the data-link connections.
* Configuration: Easy and self configuration mechanisms using Link Control Protocol. This mechanism is also used by other control protocols such as Network Control Protocols (NCPs).

Introduction TO PPTP :

PPTP packages data within PPP packets, then encapsulates the PPP packets within IP packets (datagrams) for transmission through an Internet-based VPN tunnel. PPTP supports data encryption and compression of these packets.
The PPTP protocol is designed to perform the following tasks:
* Query the status of Comm Servers
* Provide In-Band management
* Allocate channels and place outgoing calls
* Notify NT Server on incoming calls
* Transmit and Receive User Data with flow control in both directions
* Notify NT Server on disconnected calls.

PPTP-based Internet remote access VPNs are by far the most common form of PPTP VPN. In this environment, VPN tunnels are created via the following two-step process:
1. The PPTP client connects to their ISP using PPP dial-up networking.
2. Via the broker device (described earlier), PPTP creates a TCP control connection between the VPN client and VPN server to establish a tunnel.

Once the VPN tunnel is established, PPTP supports two types of information flow:
* control messages for managing and eventually tearing down the VPN connection. Control messages pass directly between VPN client and server.
* data packets that pass through the tunnel, to or from the VPN client.

PPTP also supports VPN connectivity via a LAN.
PPTP supports authentication, encryption, and packet filtering.

Though PPTP remains a popular choice for VPNs, one drawback of PPTP is its failure to choose a single standard for authentication and encryption. Two products that both fully comply with the PPTP specification may be totally incompatible with each other if they encrypt data differently.

Quick Tech Tip - Connectionless Internetworking

Connectionless internetworking follows the datagram model. In this model, the network layer offers the transport layer the ability to inject datagrams into the subnet. There is no notion of a virtual circuit at all in the network layer. This model does not require all packets belonging to one connection to traverse the same sequence of gateways.
A routing decision is made separately for each packet, possibly depending on the traffic at the moment the packet is sent. This strategy can use multiple routes and thus achieve a higher bandwidth than the concatenated virtual circuit model.



Datagrams are constructed and sent in the usual way. Routing decisions are made on a packet by packet basis so that we cannot even guarantee that all the packets in a message will be sent over the same set of networks. Since each of the networks can have different properties the processing that the packets receive will be different depending on the route they took.
Since each network will have its own network layer protocol we cannot simply transfer network layer packets across the routers. One possibility is to try and convert from one protocol to another but this is not very successful for much the same set of reasons that converting between the different frame types of the 802 Ethernets was difficult.
A major issue in transferring from one network to another is that of addressing. In general different networks use different addressing schemes. One possibility would be to assign every host an address for every sort of network but apart from being inefficient (lots of addresses would never be used) it would also require a huge translation table to be kept.
What IP (Internet Protocol) attempts to do is to define a universal packet which can be carried across all networks. Of course others have also had this idea so there are several ‘universal’ schemes and these have to be dealt with as well.
A major advantage of the datagram approach is that it can be used over subnets that do not use virtual circuits inside. Many LANs, mobile networks and even some WANs fall into this category. When an internet includes one of these, serious problem occurs if the networking strategy is based on virtual circuits.

Quick Tech Tip: InterNetworking

What are we talking about ? The interconnection of two or more networks, usually local area networks so that data can pass between hosts on the different networks as though they were one network. This requires some kind of router or gateway to facilitate the process of interconnecting two or more individual networks to facilitate communications among their respective nodes. Note: The interconnected networks may be different types. Each network is distinct, with its own addresses, internal protocols, access methods, and administration.
For different networks to interact, devices such as repeaters, bridges, routers, gateways are required. Repeaters are low level devices that just amplify or regenerate weak signals. They are needed to provide current to drive long cables. They copy individual bits between cable segments.
Unlike repeaters, bridges are store and forward devices. A bridge accepts the whole frame and passes it to the data link layer where the checksum is verified. Then the frame is sent down to the physical layer for forwarding on a different network.
Multiprotocol routers are conceptually similar to bridges, except that they are found in the network layer. They take incoming packets from one line to another, just as all routers do, but the lines may belong to different networks and use different protocols.
Transport gateways connect byte streams in the transport layer. Application gateways connects two parts of an application in the application layer.
When a gateway is between two WANs run by different organizations in different countries, the gateway is ripped apart in the middle and the two parts are connected with a wire.

Quick Tech Explanation: Local Area Networks - LANs

Local Area networks, generally called LANs, are privately owned networks within a single building or campus of up to a few kilometers in size. Most local area networks are built with relatively inexpensive hardware such as Ethernet cables, network adapters, and hubs. Wireless LAN and other more advanced LAN hardware options also exist as a part of modern LANs.
The most common type of local area network is an Ethernet LAN. If you look at size of LAN's, the smallest home LAN can have exactly two computers; a large LAN can accommodate many thousands of computers. Many of the larger LANs are divided into logical groups called subnets.

LANs are distinguished from other kinds of networks by three characteristics:
1. Size: LANs have a size limit and cannot expand beyond this size, essentially meaning that it is possible to plan and design for the worst case transmission time.
2. Topology: This is the geometric arrangement of devices on the network. For example, devices can be arranged in a ring or in a straight line. In a bus network, one machine is master and is allowed to transmit. An arbitration mechanism is needed to resolve conflicts when two or more machines want to transmit simultaneously.
In ring network, each bit propgates around on its own, not waiting for the rest of the packet to which it belongs.
3. Protocols: The rules and encoding specifications for sending data. The protocols also determine whether the network uses a peer-to-peer or client/server architecture.
4. Media: Devices can be connected by twisted-pair wire, coaxial cables, or fiber optic cables. Some networks do without connecting media altogether, communicating instead via radio waves.

Most LANs connect workstations and personal computers. Each node (individual computer) in a LAN has its own CPU with which it executes programs, but it also is able to access data and devices anywhere on the LAN. This means that many users can share expensive devices, such as laser printers, as well as data. Users can also use the LAN to communicate with each other, by sending e-mail or engaging in chat sessions.

LANs are capable of transmitting data at very fast rates, much faster than data can be transmitted over a telephone line; but the distances are limited, and there is also a limit on the number of computers that can be attached to a single LAN.

Quick Tech Tip - FDDI - Fiber distributed data interface

Fiber distributed data interface (FDDI) provides a standard for data transmission in a local area network that can extend in range up to 200 kilometers (124 miles). FDDI is a token-passing, fiber ring, network. The fiber optic media can be multimode fiber and can be as large as 100 kilometers - with no more than 2 kilometers between nodes.

Why FDDI?
- More Powerful Workstations and Servers.
- Resource Intensive Network Applications.
- Growing Distributed Client/Server Applications.
- Larger Spans of Distributed Networks.
- Increasing Numbers of Network Users.
- Bigger and More Powerful Software Applications.

FDDI Benefits:
- Higher Capacity and Performance than older LANs.
- More Simultaneous Transactions.
- Higher Availability (dual ring topology).
- Predetermined Performance (adding users have minimal impact on throughput).
- Longer Distance Loops (2 kilometers to 100 kilometer).

FDDI uses dual-ring architecture with traffic on each ring flowing in opposite directions (called counter-rotating). The dual rings consist of a primary and a secondary ring. During normal operation, the primary ring is used for data transmission, and the secondary ring remains idle.

FDDI specifies the physical and media-access portions of the OSI reference model. FDDI is not actually a single specification, but it is a collection of four separate specifications, each with a specific function. Combined, these specifications have the capability to provide high-speed connectivity between upper-layer protocols such as TCP/IP and IPX, and media such as fiber-optic cabling.