Digital Signal Processing Reference
In-Depth Information
Ethernet is familiar to most of us, as it's commonly used within
buildings or campuses and is known as a local area network (LAN).
Ethernet uses another address, appended to the packet, known as
the media access control (MAC) address
this is a permanent
address assigned by the manufacturer of that equipment. TheMAC
address is a 12-hexadecimal digit address xx:xx:xx:xx:xx:xx. MAC
address ranges are assigned by an international agency to manu-
facturers of products containing Ethernet ports. For transport
within an Ethernet LAN the MAC address, appended to the IP
packet, is used. LANs are built up of Ethernet equipment
(computers for example), Ethernet hubs, Ethernet switches and
Ethernet bridges. Hubs simply act as repeaters as any packet
coming in on one port will be sent out on all the other ports.
Switches examine MAC addresses, and only forward packets on
ports that connect to the MAC addresses for those packets. Bridges
connect different LANs together. These could be all Ethernet based
LANs, or could be wireless LANs using 802.11 based wireless tech-
nology. Common Ethernet speeds are 100 BT (100Mbps) or gigabit
Ethernet (1 Gbps). Faster Ethernet speeds, such as 10 Gbps, are
possible on specially designed backplanes or over fiber interfaces.
There is a wider range of technologies used for wide area
networking (WAN), which, as the name implies, is over distances
ranging from a few miles to thousands of miles. These are usually
owned by a carrier, and used as shared resources (the internet for
example), or connections can be leased and used privately
(connections between corporate offices in different locations, for
example). However, IP is still used as the protocol over the
different transport services and technologies.
Synchronous Optical Network (SONET) and Synchronous
Digital Hierarchy (SDH) are two popular standards used for
transmission over fibers, and form the basis of most long-
distance telecommunications. The speeds used can be above
10 Gbps. Asynchronous Transfer Mode (ATM) is a protocol used
in these networks. ATM operates by allocating a specific amount
of bandwidth to a given connection using virtual circuits. It allows
for much finer control over the data bandwidth allocated for
a given connection or user. However, due to the “guaranteed”
bandwidth allocation, ATM tends to be an expensive way to
communicate when bandwidth requirements are dynamic.
Fortunately, IP can be layered over ATM, and the user will not
even be aware of the ATM protocol running underneath. Cable
and digital subscriber lines (DSL) are used for intermediate length
connections, typically from IP service providers to homes and
small businesses. This is often referred to as “last mile”. Rather
than using fiber, the physical connections are made with coaxial
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