capsulating other protocols so that they can be transported via the
Internet Protocol (IP) in the form of a tunnel (VPN). GRE uses pro-
tocol number 47.
GSM The Global System for Mobile Communications (GSM), also known
as 2G, is a mobile communications standard. It achieves, along with
GPRS, a specified max. data transmission rate of 171.2 kbit/s.
Half-duplex With half-duplex, data can only be sent and received back-to-back
over a line.
Hash To ensure data integrity, the information needs to be protected from
unauthorised manipulation while it is being transmitted. To ensure
that this happens, every item of communication received has to
match the information originally sent. Therefore erratic mathematical
value functions (hash functions) are used to calculate checksums
(hash values). These are encrypted and sent as a digital signature
with the message. The recipient, in turn, checks the signature before
opening the packet. If the signature and, thus, the content of the
data packet has changed, the packet is discarded. The hash al-
gorithms used most frequently are Message Digest Version 5 (MD5)
and Secure Hash Algorithm (SHA1).
HDSL High Data Rate Digital Subscriber Line. See DSL.
Heartbeat A network's subscribers use heartbeats to signal that they are ready
to receive.
Hop Hop is the term for the connection from one network node to the
next.
Host A host is a computer system that provides its services to the net-
work.
Host name The domain name of a host. See DNS.
Host route A host route is the name for the route to a single host.
Hotspot A hotspot is a public internet access point via WLAN or wired Ether-
net.
HSDPA High Speed Downlink Packet Access (HSDPA, 3.5G, 3G+ or UMTS
broadband) is a data transmission method in the UMTS mobile com-
munications standard.
HTTP The HyperText Transfer Protocol (HTTP) is a protocol for transmit-
ting HTML pages (web pages) between server and client. By default
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Glossary
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