R&M Data Center Handbook

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Layer 6 – Presentation Layer
Different computer systems use different structures for data formatting. Layer 6 interprets the data, and ensures
the syntax is uniform and correct (character set, coding language). Here, the data format from the transmitter is
converted into a format that is not specific to any terminal device. Cryptography is used in this layer for data security
purposes.
Layer 7 – Application Layer
In this layer, the data that was transmitted is made available to the user, or rather his/her programs. Standardization
is hardest for this layer because of the multitude of applications that are available. These include wellknown
application protocols like http, ftp, smtp, dns, dhcp, etc.
Order of events in a transmission
If a piece of information is being transmitted from one device to another, the transmitter begins with the information
flow in layer 7 and forwards it down, layer by layer, right down to layer 1. In the process, each layer places a socalled
header containing header information in front of the actual information to be transmitted. So, for example,
the IP source and destination address as well as other information are added in layer 3, and routers use this information
for path selection. An “FCS” field (checksum field) is added in layer 2 for a simple security check.
By adding all this extra information, the total amount of information which is transmitted on the cable becomes far
greater than the actual net information. However, the length of an Ethernet frame is limited: Its minimum length is
64 bytes, and its maximum 1,518 bytes. The frame length was expanded by 4 bytes to 1,522 bytes for the purpose
of tagging VLANs, as a result of the Frame Extension established in IEEE 802.3as.
VLAN
Virtual networks or virtual LANs (VLANs) are a technological concept for implementing logical work groups within
a network. A network of this type is implemented via LAN switching or virtual routing on the link layer or on the
network layer. Virtual networks are extended by a great number of switches, which for their part are connected to
one another through a backbone.
Half- / Full-Duplex
The terms full-duplex and half-duplex are understood to mean, in a telecommunications context, a transmission of
information that is based on direction and time. For half-duplex, this generally means that only one transmission
channel is available. Data can be either sent or received but never both at the same time.
In full-duplex operation, two stations connected with one another can send and receive data simultaneously. Data
streams are transmitted in both directions at the same speed. In a gigabit Ethernet (1 Gbit/s) full-duplex process,
for example, this translates into a transmission speed of 2 Gbit/s.
OSI & TCP/IP
Toward the end of the 1960s, as the “cold war” reached its high point, the United
States Department of Defense (DOD) demanded a network technology that would be
very secure against attacks. It should be possible for the network to continue to
operate, even in case of a nuclear war. Data transmission over telephone lines was not
suitable for this purpose, since these were too vulnerable to attacks. For this reason,
the United States Department of Defense commissioned the Advanced Research
Projects Agency (ARPA) to develop a reliable network technology. ARPA was founded
in 1957 as a reaction to the launch of Sputnik by the USSR, and had the task of
developing technologies that were of use to the military.
In the meantime, ARPA was renamed the Defense Advanced Research Projects Agency (DARPA), since its
interests primarily served military purposes. ARPA was not an organization that employed scientists and researchers,
but one which handed out orders to universities and research institutes.
Over time and as ARPANET grew, it became clear that the protocols selected up to that point were no longer
suitable for operating a larger network that also connected many (sub)networks to one another. For this reason,
other research projects were finally initiated, which led to the development of TCP/IP protocols, or the TCP/IP
model, in 1974. TCP/IP (Transmission Control Protocol / Internet Protocol) was developed with the goal of
connecting various networks of different types with one another for purposes of data transmission. In order to
force the integration of TCP/IP protocols into ARPANET, (D)ARPA commissioned the company of Bolt, Beranek &
Newman (BBN) and the University of California at Berkeley to integrate TCP/IP into Berkeley UNIX. This also laid
the foundation for the success of TCP/IP in the UNIX world.
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