March 12, 2003
Welcome to Lesson 8 - Network Architecture
Check out the following link to learn about the origins of Ethernet:
Charles Spurgeon's Ethernet Web
Ethernet relies on the CSMA/CD Protocol. The CSMA/CD protocol functions somewhat like a dinner party in a dark room. Everyone around the table must listen for a period of quiet before speaking (Carrier Sense). Once a space occurs everyone has an equal chance to say something (Multiple Access). If two people start talking at the same instant they detect that fact, and quit speaking (Collision Detection.)
To translate this into Ethernet terms, each interface must wait until there
is no signal on the channel, then it can begin transmitting. If some other interface
is transmitting there will be a signal on the channel, which is called carrier.
All other interfaces must wait until carrier ceases before trying to transmit,
and this process is called Carrier Sense.
All Ethernet interfaces are equal in their ability to send frames onto the network. No one gets a higher priority than anyone else, and democracy reigns. This is what is meant by Multiple Access. Since signals take a finite time to travel from one end of an Ethernet system to the other, the first bits of a transmitted frame do not reach all parts of the network simultaneously. Therefore, it's possible for two interfaces to sense that the network is idle and to start transmitting their frames simultaneously. When this happens, the Ethernet system has a way to sense the "collision" of signals and to stop the transmission and resend the frames. This is called Collision Detect.
The CSMA/CD protocol is designed to provide fair access to the shared channel so that all stations get a chance to use the network. After every packet transmission all stations use the CSMA/CD protocol to determine which station gets to use the Ethernet channel next.
This brings up an interesting point, which is that the Ethernet system, in
common with other LAN technologies, operates as a best effort" data delivery
system. To keep the complexity and cost of a LAN to a reasonable level, no guarantee
of reliable data delivery is made. While the bit error rate of a LAN channel
is carefully engineered to produce a system that normally delivers data extremely
well, errors can still occur.
A burst of electrical noise may occur somewhere in a cabling system, for example,
corrupting the data in a frame and causing it to be dropped. Or a LAN channel
may become overloaded for some period of time, which in the case of Ethernet
can cause 16 collisions to occur on a transmission attempt, leading to a dropped
frame. No matter what technology is used, no LAN system is perfect, which is
why higher protocol layers of network software are designed to recover from
errors.
It is up to the high-level protocol that is sending data over the network to make sure that the data is correctly received at the destination computer. High-level network protocols can do this by establishing a reliable data transport service using sequence numbers and acknowledgment mechanisms in the packets that they send over the LAN.
Define the four basic 10Mbps Ethernet standards:
• 10Base5
• 10Base2
• 10BaseT
• 10BaseF
Describe the naming convention used for IEEE standards. The name is split into
three parts:
• Transmission speed (generally 10 or 100)
• Transmission type (baseband or broadband)
• Distance (in hundreds of meters) or Media type (twisted-pair or fiber-optic)
For example: 10Base2 = 10Mbps, baseband transmission, 200 meters (actually 185,
but close enough for standards work). 10BaseF = 10Mbps, baseband transmission,
fiber-optic cable.
There are two 100Mbps standards: 100VG-AnyLAN, and 100BaseT. These high-speed
networks are very well suited for video, CAD/CAM, and imaging.
100VG-AnyLAN
Although discussed with the Ethernet specifications, 100VG-AnyLAN encompasses
both Ethernet and Token Ring technologies; hence the term AnyLAN. AnyLAN is
currently still in development and is the only architecture to use the demand
priority channel access method discussed in Chapter 6. AnyLAN has significant
performance advantages by having intelligent hubs control network communication,
rather than each computer handling the process for itself.
100BaseT (Fast Ethernet)
100BaseT as an extension of the 802.3 specification which raises transmission
speeds to 100Mbps. There are three implementations of Fast Ethernet and their
cabling specifications:
• 100BaseT4—Uses all four pairs of Category 3, 4, or 5 UTP
• 100BaseTX—Uses two pairs of Category 5 UTP
• 100BaseFX—Uses two-strand fiber-optic cables.
Gigabit Ethernet: 1 Gbps IEEE 802.3z Standards
Gigabit Ethernet is also called 1000BaseX, Note that the Gigabit Ethernet standards
are based on the ANSI X3.230-1994 standard for Fiber Channel connection to Storage
Area Networks (SANs). IEEE 802.3z-1998 covers 1000BaseX, while 802.3z-1999 deals
with 1000BaseT specifications. Most Gigabit Ethernet implementations operate
in full-duplex mode.
Your boss has decided to network his house. He wants everything, lights, television, etc., to be controlled by a server in the basement. He has asked you to wire his house and make sure everything works. Knowing your boss, you anticipate that he will want to keep costs down, but have a high-speed network. Describe the networking solution you will put in place.
The following students from ASU need to take this question:
Erica Gollet-Reid
You have been hired by a state agency to provide connectivity between their existing Token Ring network and a new database server cluster. The cluster will need a network with transmission speeds of 100Mbps. The cluster’s network must support prioritization and should be redundant. This particular agency has very deep pockets and there is no concern for cost. What network architecture, or combination of architectures best suits this installation?
The following students from ASU need to take this question:
Chris Willis
Remember to keep up with your on-line lessons and quizzes!
mdm and drw