Line hits are caused by physical disturbances, electronic influences
on the transmission medium. The question draws attention to the serial
connection between B1 and B2, and a possible difference between Ethernet
connections. Ethernet makes no provision for physical layer protocol
retransmission in the face of erred communications. An explanation follows.
There are different physical layer protocol entities for Ethernet,
notably MLT-3 for fast Ethernet, Manchester for Ethernet, etc. These are
actual protocols for transferring bit streams over a common medium and only
serve to perform line encoding. When an error presents itself, most often
these errors register as invalid code signals to the distant end, which
somehow gets translated into a data signal, forcibly in the case of
Ethernet. After this process is complete the bit streams are compacted and
provisioned into 8-bit boundaries and are passed up to the data link layer.
At this point, the communication enters the prevue of a central processing
unit. The OS controlling the CPU would naturally do a CRC function on the
received data stream and extract the CRC that was computed by the sending
node, and do a comparison of the two. Actual implementations could vary.
This in essence is an overview of Ethernet Technology. The important thing
to remember is that there is not a protocol function that occurs at the
point the bit streams are sent from the hardware to the main CPU (channel
access functions are handled in hardware on a NIC). All communication is
accepted carte blanche, and naturally so. Ethernet is primarily a LAN
technology were error prone communications (caused by EMI or other naturally
occurring phenomenon) is tightly controlled and minimized. In serial
communication technology there is such a protocol function because there is
a higher chance of their being electromagnetic influences, propagation
delay, etc.
In serial communications at the point that the bit streams are
decoded into logical binary words (that 8 bit provisioning scheme
aforementioned). There is a protocol function implemented to control the
actual reception and healthiness of the bit streams. HDLC is the default
protocol for Cisco Routers, but there are other notables. Such as Bi-Sync,
SDLC, LAPB, PPP, etc. In some of these protocols there are provisions for
the retransmission of frames when errors are detected, channel multiplexing,
stream windowing as well as frame sequencing and acknowledgements.
Why this long answer? Remember the question draws attention to the
physical layer when 'line hits' are mentioned. Further clues were given
when the only difference mentioned was a change in physical composition.
It's up to you to decide if the test maker in this instance is testing to
see if you know all of this, judged by the overall difficulty of the exam.
To answer your question if there is a line hit between B1 and B2, B1
will always retransmit. In most serial encapsulations method, the frame
never clears the buffers on B1 until B2 acknowledges reception to B1.
There was an effort to add this amount of reliability to Ethernet
and Token Ring environments, hence LLC which is a spin off of sorts of HDLC
for serial communications. With LLC there are actually 3 different modes of
communication. Type 1 is the normal mode that you would normally see in
modern networking environments (Type 2 is more usual for Token Ring). Type
2 is modeled after communication qualities that serial communications need
to overcome. Type 3 is not commonly used. To be succinct, it is like
slapping a serial protocol over Ethernet or Token Ring. When Ethernet is
behaving like a serial interface it will buffer, acknowledge and retransmit
erred frames just like a serial interface (In that case, each intermediate
device is responsible for retransmitting any frames with errors). Like
everything else in life, the true answer depends on what you are doing.
To be safe, let me point out that all of this nonsense usually is spoken of
in the books as residing at the Data Link layer. I believe the test
question may also be trying to confuse you with what are the
responsibilities of the Transport layer (TCP to be exact). But what if you
aren't even using TCP, What if you are using UDP over IP over Ethernet?
There is clearly no retransmission effort going on here. All confusing
isn't it? Don't worry in time you'll sort it all out.
Cheers and Good Luck,
WAYNE BAETY, MCSE, A1C, USAF
Network Systems Trainer
-----Original Message-----
From: Todd Carswell [mailto:[EMAIL PROTECTED]]
Sent: Monday, November 12, 2001 11:09 PM
To: [EMAIL PROTECTED]
Subject: 2 "Line Hit" Scenarios... [7:25928]
Here's the setup for my 2 questions...
PCA-------B1-----------B2--------PCB
Bridge 1 and Bridge 2 are running Transparent Bridging between them.
Question 1: There's a SERIAL connection between B1 and B2. B1 and B2 are
configured for transparent bridging. If PCA sends a packet to PCB and the
frame is errored somehow, who takes care of the retransmission? I assume
it's PCA because it's a serial connection. Am I right?
Question 2: There's an ETHERNET connection between B1 and B2. The bridges
are still using Transparent Bridging and PCA sends a packet to PCB. Again,
the frame has an error. Will B1 be the device to handle the retransmission?
Thanks, guys!
Todd
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=26064&t=25928
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