Understanding the network.pdf - Back to Home

and network transport is a very important distinction. By separating these two
functions, the ULP has no involvement in (or dependency on) the actual data
transport. This makes it possible for data to be moved across various Layer 2
transmission types with little, if any, modifications to the ULP. This allows for a
greater level of flexibility in what transport technologies are used, which means
faster data delivery and low transmission delays.
• ITU-T V.35 is the standard used for synchronous communications between
routers and a public packet data network. The interface is usually a Data
Service Unit/Channel Service Unit (DSU/CSU), a device used to provide data
conversion so the data can be sent over a digital telephone loop.
As you can see from the previous examples, any means by which data is actually
transported is addressed at the Layer 2 level.
Layer 1: Physical
The physical layer deals with specifications of the medium used to move bit data
from point to point. All physical, electrical, and mechanical aspects of the
transmission media are addressed at Layer 1. Layer 1 and Layer 2 are also
commonly looked at together because the physical layer standards are usually
taken for granted. Do not fall into the trap of grouping them. The physical layer of
the network is one of the most complex and, next to configuration errors, the most
common cause of problems found in networks. All physical media have
corresponding standards. When working with any medium, you should review, or at
least be aware of, the minimum operating specifications, such as connector type(s),
maximum cable length, and any environmental installation requirements, that
might interfere with the performance of the transport or affect the operation of
other network/non-network equipment.
Common physical layer standards are the following:
• IEEE 10-BaseT—The cabling standard for using unshielded twisted-pair
copper wire to transmit 802.3 Ethernet.
• IEEE 100-BaseT—The cabling standard for using unshielded twisted-pair
copper wire to transmit 802.3 Fast Ethernet.
• EIA/TIA-232—The standard used for unbalanced (async) circuits at speeds
up to 64Kbps. This is commonly known as the RS232 serialport standard.
The actual RS232 standard was based on the ITU-T V.24 standard that is no
longer used.