802.11b Vs 802.11g

Both the WLAN standards use the unregulated radio frequency band of 2.4GHz which means they can interfere with other appliances using the same frequency.

802.11b supports a maximum bandwidth of up to 11Mbps whereas the 802.11g supports a bandwidth of up to 54Mbps. 802.11g is the best of the two for office use since 802.11b is slower than 802.11g cards.

802.11g costs more than 802.11b network adapters or the wireless cards.

So, if you want to buy a wireless card for your notebook or your desktop, if your wireless access point supports 802.11g, then you should choose a 802.11g wireless card over a 802.11b card. There are many companies that manufacture wireless cards such as Realtek, Broadcom, Intel, Linksys and D-Link.

Some of the routing optimizations are based on the Diffusing Update Algorithm (DUAL) work from SRI, which guarantees loop-free operation. In particular, DUAL avoids the “count to infinity” behaviour of RIP when a destination becomes completely unreachable. The maximum hop count of EIGRP-routed packets is 224.

Basic Operation

The data EIGRP collects is stored in three tables:

1. Neighbor Table: Stores data about the neighbouring routers, i.e. those directly accessible through directly connected interfaces.
2. Topology Table: Confusingly named, this table does not store an overview of the complete network topology; rather, it effectively contains the aggregation of the routing tables gathered from all the neighbours. This table actually contains only those routes which are considered not to potentially be part of routing loops (part of the DUAL algorithm); in EIGRP terminology, they are considered “Feasible Successors”.
3. Routing table: Stores the actual routes to all destinations; routes may be marked either as “Passive”, which is the normal state when the routing has stabilized, or “Active” when the topology has changed, and the router is in the process of updating its route to that destination.

Multiple metrics

EIGRP associates five different metrics with each route:

1. Delay
2. Bandwidth
3. Reliability
4. MTU
5. Load

For the purposes of comparing routes, these are combined together in a weighted formula to produce a single metric:

[(K1*Bandwidth) + ((K2*Bandwidth)/(256-Load)) + (K3*Delay)] * [(K5/(Reliability + K4))]

where the various constants (K1 through K5) can be set by the user to produce varying behaviours. If K5 is set to zero, the K4/K5 term is not used (i.e. taken as 1). The default is for K1 and K3 to be set to 1, and the rest to zero, effectively reducing the above formula to:

Bandwidth + Delay

Obviously, these constants must be set to the same value on all routers in an EIGRP system, or permanent routing loops will probably result.

IGRP was created in part to overcome the limitations of RIP (maximum hop count, and a single routing metric) when used within large networks. IGRP supports multiple metrics for each route, including bandwidth, load, delay, and relability; to compare two routes these metrics are combined together into a single metric, using a formula which can be adjusted through the use of pre-set constants. The maximum hop count of IGRP-routed packets is 255.

Its successor is EIGRP, an advanced distance-vector routing protocol, that uses many features of link-state protocols, and adds Diffusing Update Algorithm (DUAL) ideas to the basic distance-vector mechanism of IGRP.

BGP supports classless interdomain routing and uses route aggregation to decrease the size of routing tables. Since 1994, version four of the protocol has been in use on the Internet; all previous versions are considered obsolete. Very large private IP networks can also make use of BGP; an example would be the joining of a number of large OSPF networks where OSPF by itself would not scale to size. Another reason to use BGP would be multihoming a network for better redundancy. All Internet service providers must use BGP to establish routing between one another, it is one of the most important protocols of the Internet.

BGP Operation

BGP neighbors, or peers, are established by manual configuration between routers to create a TCP session on port 179, BGP speaker will periodically, every 60 seconds by default, send 19-byte keepalive messages to maintain the connection. Among routing protocols, BGP is unique in using TCP as its transport protocol.

When BGP is running inside an AS, it is referred to as Internal BGP (IBGP Interior Border Gateway Protocol). When BGP runs between autonomous systems, it is called External BGP (EBGP Exterior Border Gateway Protocol). If the role of a BGP router is to route IBGP traffic, it is called a Transit Router. Routers that sit on the boundary of an AS and that use EBGP to exchange information with the ISP are called Border or Edge Routers.

All routers within a single AS and participating in BGP routing must be configured in a full mesh: each router must be configured as peer to every other router. This causes obvious scaling problems, since the number of required connections grows quadratically with the number of routers involved. To get around this, two solutions are built into BGP: route reflectors (RFC 2796) and confederations (RFC 3065).

Route reflectors reduce the number of connections required in an AS. A single router (or two for redundancy) can be made a route reflector: other routers in the AS need only be configured as peer to them. Confederations are used in very large networks where a large AS can be configured to encompass smaller more manageable internal autonomous systems. Confederations can be used in conjunction with route reflectors.

Router Flapping

A feature known as “dampening” is built into BGP to mitigate the effects of route flapping. Flapping of routes can be caused by WAN links or physical interfaces mending and breaking or by misconfigured or mismanaged routers. Without dampening, routes can be injected and withdrawn rapidly from routing tables, possibly causing a heavy processing load on routers thus affecting overall routing stability.

With dampening, a route’s flapping is exponentially decayed. At first instance when a route becomes unavailable but quickly reappears for whatever reason, then the dampening does not take effect, so as to maintain the normal fail-over times of BGP. At the second occurrence, BGP shuns that prefix for a certain length of time; subsequent occurrences are timed out exponentially. After the abnormalities have ceased and a suitable length of time has passed for the offending route, prefixes can be reinstated and its slate wiped clean. Dampening can also mitigate malicious denial of service attacks; dampening timings are highly customisable.

As backbone links and router processors have become faster, some network architects have suggested that flap dampening may not be as important as it used to be, since changes to the routing table can be absorbed much faster by routers. Some have even suggested that dampening may make things worse, not better, in such an environment. This topic is controversial, and the subject of much research.

Routing table growth

One of the largest problems faced by BGP, and indeed the Internet infrastructure as a whole, comes from the growth of the Internet routing table. If the global routing table grows to the point where some older, less capable, routers cannot cope with the memory requirements or the CPU load of maintaining the table, these routers will cease to be effective gateways between the parts of the Internet they connect. In addition, and perhaps even more importantly, larger routing tables take longer to stabilize (see above) after a major connectivity change, leaving network service unreliable, or even unavailable, in the interim.

Until 2001, the global routing table was growing exponentially, threatening an eventual widespread breakdown of connectivity. In an attempt to prevent this from happening, there is now a cooperative effort by ISPs to keep the global routing table as small as possible, by using CIDR and route aggregation. This has slowed the growth of the routing table to a linear process, greatly extending the time available before older routers need to be replaced.

Some of the features of OSPF are:-

• Support for MD5 based authentication
• Support for VLSM – Variable Length Subnet Mask (classless routing)
• Tagging of routes

OSPF based network can contain multiple smaller networks. OSPF uses both unicast and multicast for link state updates. Multicast address of 224.0.0.5 and 224.0.0.6 are used to send updates to other routers.

RIP is a distance-vector routing protocol employing the hop count as a routing metric. The maximum number of hops allowed with RIP is 15. The default update interval of RIP configured router is 30 seconds which results in large amount of network traffic. RIP runs above the network layer and uses UDP port 520 to transmit its updates. To avoid routing loops a technique called Split Horizon with Poison Reverse is used.

There are two version of RIP, RIPv1 and RIPv2.

RIPv1 uses classful routing and does not support VLSM (Variable Length Subnet Mask). This means that the internal subnets within the same network must be of same size and lack of authentication does not make it the first choice for networks.

RIPv2 was developed to overcome the limitations imposed by RIPv1 and as a result of new development, the RIPv2 supported classless Inter-Domain routing. But the limitation of 15 hop count still remained.

Some of the most popular and commonly used routing protocols are:-

• Open Shortest Path First (OSPF)
• Routing Information Protocol (RIP)
• Interior Gateway Routing Protocol (IGRP)
• Enhanced Interior Gateway Routing Protocol (EIGRP)
• Border Gateway Protocol (BGP)

The information about hostname and domains are stored on distributed database systems on public network such as Internet. Many types of information about the domain names such as IP address of each domain name, MX records (mail server), nameserver are stored in the DNS database. The DNS system provides with a friendly interface for data transmission. While computers use IP addresses to communicate with each other, humans find it easy to use domain names to access information on the Internet. It is easy for us to remember a domain name, email address instead remebering IP addresses of every website.

History of the DNS

The practice of using a name as a more human-legible abstraction of a machine’s numerical address on the network predates even TCP/IP, all the way back to the ARPAnet era. Originally, each computer on the network retrieved a file called HOSTS.TXT from SRI (now SRI International) which mapped an address (ex. 145.97.39.135) to a name (ex. www.google.com) The Hosts file still exists on most modern operating systems either by default or through configuration and allows users to specify an IP Address to use for a hostname without checking the DNS. This file is now used primarily for troubleshooting DNS errors or mapping local addresses to more organic names. Such a system had inherent limitations, because of the obvious requirement that every time a given computer’s address changed, every computer that wanted to communicate with it would need an update to its Hosts file.

The growth of networking called for a more scalable system: one which recorded a change in a host’s address in one place only. Other hosts would learn about the change dynamically though a notification system, thus completing a globally accessible network of all hosts names and their associated IP Addresses.

How DNS works?

The practical operation of the DNS system consists of three players:

1. The DNS resolver, a DNS client program which runs on a user’s computer and generates DNS requests on behalf of software programs.
2. The recursive DNS server, which searches through the DNS in response to queries from resolvers and returns answers to those resolvers; and,
3. The authoritative DNS server which hands out answers to queries from recursors, either in the form of an answer, or in the form of a delegation (i.e. referral to another authoritative DNS server).

Understanding the parts of a domain name

A domain name usually consists of two or more parts, separated by dots.

The rightmost label conveys the top-level domain (also known as TLD) for example, the address networkguruz.com has the top-level domain com.

Each label to the left specifies a subdivision or subdomain of the domain above
it. Note that “subdomain” expresses relative dependence, not absolute dependence: for example, networkguruz.com comprises a subdomain of the com domain, and en.networkguruz.com could form a subdomain of the domain networkguruz.com (in practice, however, en.networkguruz.com actually represents a hostname – see below). In theory, this subdivision can go down to 127 levels deep, and each label can contain up to 63 characters, as long as the whole domain name does not exceed a total length of 255 characters. But in practice some domain registries have shorter limits than that.

Types of DNS records

Important categories of data stored in the DNS include the following:

• An A record or address record maps a hostname to its 32-bit IPv4 address.
• An AAAA record or IPv6 address record maps a hostname to its 128-bit IPv6 address.
• A CNAME record or canonical name record makes one domain name an alias of another. The aliased domain gets all the subdomains and DNS records of the original.
• An MX record or mail exchange record maps a domain name to a list of mail exchange servers for that domain.
• A PTR record or pointer record maps an IPv4 address to the canonical name for that host. Setting up a PTR record for a hostname in the in-addr.arpa domain that corresponds to an IP address implements reverse DNS lookup for that address. For example (at the time of writing), www.icann.net has the IP address 192.0.34.164, but a PTR record maps 164.34.0.192.in-addr.arpa to its canonical name, referrals.icann.org.
• An NS record or name server record maps a domain name to a list of DNS servers for that domain. Delegations depend on NS records.
• An SOA record or start of authority record specifies the DNS server providing authoritative information about an Internet domain.
• An SRV record is a generalized service location record.
• A TXT record allows an administrator to insert arbitrary text into a DNS record. For example, this record is used to implement the Sender Policy Framework specification.

Other types of records simply provide information (for example, a LOC record gives the physical location of a host), or experimental data (for example, a WKS record gives a list of servers offering some well-known service such as HTTP or POP3 for a domain).

ATM sought to resolve the conflict between circuit-switched networks and packet-switched networks by mapping both bitstreams and packet-streams onto a stream of small fixed-size ‘cells’ tagged with virtual circuit identifiers. The cells are typically sent on demand within a synchronous ‘time-slot’ pattern in a synchronous bit-stream: what is asynchronous here is the sending of the cells, not the low-level bitstream that carries them.

Why cells?

The motivation for the use of small data cells was the reduction of jitter (delay variance, in this case) in the multiplexing of data streams; reduction of this (and also end-to-end round-trip delays) is particularly important when carrying voice traffic.

This is because the conversion of digitized voice back into an analog audio signal is an inherently real-time process, and to do a good job, the codec that does this needs an evenly spaced (in time) stream of data items. If the next data item is not available when it is needed, the codec has no choice but to produce silence – and if the data does arrive, but late, it is useless, because the time period when it should have been converted to a signal has already passed.

Now consider a speech signal reduced to packets, and forced to share a link with bursty data traffic (i.e. some of the data packets will be large). No matter how small the speech packets could be made, they would always encounter full-size data packets, and under normal queuing conditions, might experience maximum queuing delays.

ATM was designed to implement a low-jitter network interface. However, to be able to provide short queueing delays, but also be able to carry large datagrams, it had to have cells. ATM broke all packets, data, and voice streams up into 48-byte chunks, adding a 5-byte routing header to each one so that they could be reassembled later. The choice of 48 bytes was, as is all too often the case, political instead of technical. When the CCITT was standardizing ATM, parties from the United States wanted a 64-byte payload because having the size be a power of 2 made working with the data easier and this size was felt to be a good compromise between larger payloads optimized for data transmission and shorter payloads optimized for real-time applications like voice; parties from Europe wanted 32-byte payloads because the small size (and therefore short transmission times) simplify voice applications with respect to echo cancellation.

Most of the interested European parties eventually came around to the arguments made by the Americans, but France and a few allies held out until the bitter end. With 32 bytes, France would have been able to implement an ATM-based voice network with calls from one end of France to the other requiring no echo cancellation. 53 bytes was chosen as a compromise between the two sides, but it was ideal for neither and everybody has had to live with it ever since. 5-byte headers were chosen because it was thought that 10% of the payload was the maximum price to pay for routing information. ATM multiplexed these 53-byte cells instead of packets. Doing so reduced the worst-case queuing jitter by a factor of almost 30, removing the need for echo cancellers.

Why virtual circuits?

ATM is a channel-based transport layer. This is encompassed in the concept of the Virtual Path (VP) and Virtual Circuit (VC). Every ATM cell has an 8- or 12-bit Virtual Path Identifier (VPI) and 16-bit Virtual Circuit Identifier (VCI) pair defined in its header. The length of the VPI varies according to whether the cell is sent on the user-network interface (on the edge of the network), or if it is sent on the network-network interface (inside the network).

As these cells traverse an ATM network, switching is achieved by changing the VPI/VCI values. Although the VPI/VCI values are not necessarily consistent from one end of the connection to the other, the concept of a circuit is consistent (unlike IP, where any given packet could get to its destination by a different route than the others).

Another advantage of the use of virtual circuits is the ability to use them as a multiplexing layer, allowing different services (such as voice, Frame Relay, n*64 channels, IP, SNA, etc.) to share a common ATM connection without interfering.

The Transmission Control Protocol (TCP) is one of the core protocols of the Internet protocol suite. Using TCP, applications on networked hosts can create connections to one another, over which they can exchange data in packets. The protocol guarantees reliable and in-order delivery of data from sender to receiver. TCP also distinguishes data for multiple connections by concurrent applications (e.g. Web server and e-mail server) running on the same host.

TCP supports many of the Internet’s most popular application protocols and resulting applications, including the World Wide Web and e-mail.

In the Internet protocol suite, TCP is the intermediate layer between the Internet Protocol (IP) below it, and an application above it. Applications often need reliable pipe-like connections to each other, whereas the Internet Protocol does not provide such streams, but rather only unreliable packets. TCP does the task of the transport layer in the simplified OSI reference model of computer networks.

Protocol operation

Unlike TCP’s traditional counterpart – User Datagram Protocol – that can immediately start sending packets, TCP provides connections that need to be established before sending data. TCP connections have three phases:

1. Connection establishment
2. Data transfer
3. Connection termination

Connection establishment

To establish a connection, TCP uses a 3-way handshake. Before a client attempts to connect with a server, the server must first bind to a port to open it up for connections: this is called a passive open. Once the passive open is established, a client may initiate an active open. To establish a connection, the 3-way (or 3-step) handshake occurs:

1. The active open is performed by sending a SYN to the server
2. In response, the server replies with a SYN-ACK.
3. Finally the client sends an ACK back to the server.

At this point, both the client and server have received an acknowledgement of the connection.

Data transfer

There are a few key features that set TCP apart from User Datagram Protocol:

1. Error-free data transfer
2. Ordered-data transfer
3. Retransmission of lost packets
4. Discarding duplicate packets
5. Congestion throttling