Networking

1
Networking

• Networking began its infancy in the mid -1960s
  by the US Department of Defence (DoD). The
  original intention of networking was being
  developed to withstand a nuclear war. Telephone
  networks were to vulnerable and would terminate
  all conversations should a nuclear war occur.
• ARPA (Advanced Research Projects Agency) was
  created in response with the launching of Russian
  Sputnik in 1957. ARPA decided that a DoD network
  should be packet switched networked consisting of
  a subnet and host computers. Experimental network
  research was awarded to UCLA, UCSB, SRI and Univ.
  of Utah in 1969. These areas were chosen because
  they all had a large number of ARPA contracts.

2
Networking

• These 4 universities also had different and
  completely incompatible host computers. ARPANET
  protocols were not suitable for running over
  multiple networks, so TCP/IP model and protocols
  were invented in 1974. ARPA awarded several other
  contracts and specifically University of
  California at Berkeley to integrate the protocols
  with the Berkeley UNIX operating system.
• Berkeley developed a convenient program interface
  to the network and wrote many applications,
  utility, and management programs to make
  networking easier. In it early infancy, the OSI
  protocols were crushed and the TCP/IP protocols
  were already in widespread use. The OSI Model had
  seven layers because at the time, IBM had a
  propriety seven-layer protocol called SNA
  (Systems Network Architecture).

3
OSI-ISO 7-layer reference model

• The OSI model divides the functions of a protocol
  into a series of layers. Each layer has the
  property that it only uses the functions of the
  layer below, and only exports functionality to
  the layer above.
• A system that implements protocol behavior
  consisting of a series of these layers is known
  as a protocol stack or simply a stack. Protocol
  stacks can be implemented either in hardware or
  software, or a mixture of both.
• Typically, only the lower layers are implemented
  in hardware, with the higher layers being
  implemented in software.

4
OSI-ISO 7-layer reference model

• This OSI model is roughly adhered to in the
  computing and networking industry. Its main
  feature is in the interface between layers which
  dictates the specifications on how one layer
  interacts with another.
• This means that a layer written by one
  manufacturer can operate with a layer from
  another (assuming that the specification is
  interpreted correctly). These specifications are
  typically known as Requests for Comments or
  (RFCs) in the TCP/IP community. They are ISO
  standards in the OSI community.

5
OSI-ISO 7-layer reference model
6
OSI-ISO 7-layer reference model
Messages
Messages
Segments
Transport layer
Transport layer
Network service
Network service
Network layer
Network layer
Network layer
Network layer
End system a
End system b
Data link layer
Data link layer
Data link layer
Data link layer
Physical layer
Physical layer
Physical layer
Physical layer
7
OSI-ISO 7-layer reference model

• Usually, the implementation of a protocol is
  layered in a similar way to the protocol design,
  with the possible exception of a fast path where
  the most common transaction allowed by the system
  may be implemented as a single component
  encompassing aspects of several layers.
• This logical separation of layers makes reasoning
  about the behavior of protocol stacks much
  easier, allowing the design of elaborate but
  highly reliable protocol stacks.
• Each layer performs services for the next higher
  layer and makes requests of the next lower layer.

8
OSI-ISO 7-layer reference model
9
OSI-ISO 7-layer reference model - data
encapsulation
10
TCP/IP vs. OSI 7-layer reference model
Only a subset of the whole OSI model is used
today. It is widely believed that much of the
specification is too complicated and that its
full functionality has taken too long to
implement, although there are many people who
strongly support the OSI model.
11
Physical layer

• This physical layer refers to network hardware,
  physical cabling or a wireless electromagnetic
  connection. It also deals with electrical
  specifications, collision control and other
  low-level functions.
• The physical layer is the most basic network
  layer, providing only the means of representing
  and transmitting raw bits. The shapes of the
  electrical connectors, which frequencies to
  broadcast on, and similar low-level things are
  specified here.

12
Data-link layer

• The data link layer is the layer of the model
  which transfers data between adjacent network
  nodes in a wide area network or between nodes on
  the same local area network segment. The data
  link layer provides the functional and procedural
  means to transfer data between network entities
  and might provide the means to detect and
  possibly correct errors that may occur in the
  Physical layer.
• This layer is sometimes split into two
  sub-layers.
• The first sub-layer is Logical Link Control
  (LLC). This sub-layer multiplexes protocols
  running atop the data link layer, and optionally
  provides flow control, acknowledgment, and error
  recovery.
• The second sub-layer is Media Access Control
  (MAC). This sub-layer determines who is allowed
  to access the media at any one time. There are
  generally two forms of media access control
  distributed and centralized.

13
Network layer

• The network layer addresses messages and
  translates logical addresses and names into
  physical addresses.
• It also determines the route from the source to
  the destination computer and manages traffic
  problems, such as switching, routing, and
  controlling the congestion of data packets.
• The network layer provides the functional and
  procedural means of transferring variable length
  data sequences from a source to a destination via
  one or more networks while maintaining the
  quality of service requested by the Transport
  layer.
• The Network layer performs network routing, flow
  control, network segmentation/desegmentation, and
  error control functions.

14
Transport layer

• The transport layer provides transparent transfer
  of data between hosts. It is usually responsible
  for end-to-end error recovery and flow control,
  and ensuring complete data transfer.
• In other words, the purpose of the transport
  layer is to provide transparent transfer of data
  between end users, thus relieving the upper
  layers from any concern with providing reliable
  and cost-effective data transfer.
• The transport layer usually turns the unreliable
  and very basic service provided by the Network
  layer into a more powerful one.

15
Session layer

• The session layer provides the mechanism for
  managing the dialogue between end-user
  application processes. It provides for either
  duplex or half-duplex operations and establishes
  check-pointing, adjournment, termination, and
  restart procedures.
• The session layer provides mechanisms to allow
  information on different streams, perhaps
  originating from different sources, to be
  properly combined. In particular, it deals with
  synchronization issues, and ensuring nobody ever
  sees inconsistent versions of data, and similar
  concepts.

16
Presentation layer

• The presentation layer is responsible for the
  delivery and formatting of information to the
  application layer for further processing or
  display.
• It relieves the application layer of concern
  regarding syntactical differences in data
  representation within the end-user systems. An
  example of a presentation service would be the
  conversion of an EBCDIC-coded text file to an
  ASCII-coded file.
• The presentation layer is the first one where
  people start to care about what they are sending
  at a more advanced level than just a bunch of
  ones and zeros.

17
Application layer

• The application layer interfaces directly to and
  performs common application services for the
  application processes. It also issues requests to
  the presentation layer.
• The common application layer services provide
  semantic conversion between associated
  application processes.
• Examples of common application services of
  general interest include the virtual file,
  virtual terminal, and job transfer and
  manipulation protocols.

18
Transmission errors in networks

• Transmission errors are caused by
• thermal noise
• impulse noise
• signal distortion during transmission
• crosstalk
• voice amplitude signal compression
• quantization noise
• jitter
• receiver and transmitter out of synchronization

19
Error detection

• Error detection is achieved via adding small but
  enough number of extra bits to deduce that there
  is an error but not enough bits to correct the
  error
• If only error detection is employed in a network
  transmission the retransmission process is
  triggered to recover the frame (data link layer)
  or the packet (network layer).
• At the data link layer, this is referred to as
  ARQ (Automatic Repeat reQuest).

20
Error detection and correction

• Error correction requires some number of
  additional (redundant) bits to deduce what the
  correct bits must have been.
• Example
• Hamming Codes
• Superimposed codes (related to selectors)
• FEC Forward Error Correction found in MPEG-4
• and others

21
Error detection and correction

• Hamming codes
• codewords in Hamming (error detecting and error
  correcting) codes consist of m data bits and r
  redundant bits.
• Hamming distance between two strings represents
  the number of bit positions on which two bit
  patterns differ (similar to pattern matching k
  mismatches).
• Hamming distance of the code is determined by the
  two codewords whose Hamming distance is the
  smallest.
• error detection involves determining if codewords
  in the received message match closely enough
  legal codewords.

22
Error detection and correction
A code with good distance properties
(b)
(a)
A code with poor distance properties
code distance
x codewords o non-codewords
23
Error detection and correction

• To detect properly d single bit errors, one needs
  to apply a d1 code distance.
• To correct properly d single bit errors, one
  needs to apply a 2d1 code distance.
• In general, the price for redundant bits is too
  expensive (!!) to do error correction for all
  network messages
• Thus safety and integrity of network
  communication is based on error detecting codes
  and extra transmissions in case any errors were
  detected

24
Error-Detection System using Check Bits
Received information bits
Information bits
Recalculate check bits
Channel
Calculate check bits
Compare
Information accepted if check bits match
Received check bits
Check bits
25
Cyclic Redundancy Checking (CRC)

cyclic redundancy check (CRC) is a popular
technique for detecting data transmission
errors. Transmitted messages are divided into
predetermined lengths that are divided by a
fixed divisor. According to the calculation, the
remainder number is appended onto and sent with
the message. When the message is received, the
computer recalculates the remainder and compares
it to the transmitted remainder. If the numbers
do not match, an error is detected.
26
Error detection --via parity of subsets of bits
Note Check bits occupy power of 2 slots
12345678 .
27
Detection via parity of subsets of bits
Consider 4 bit words.
Add 3 parity bits.
P2P1P0
D3D2D1D0
0
1
0
1
? ? ?
Each parity bit computed on a subset of bits
P2 D3 xor D2 xor D1 0 xor 1 xor 1 0
P1 D3 xor D2 xor D0 0 xor 1 xor 0 1
P0 D3 xor D1 xor D0 0 xor 1 xor 0 1
Use this word bit arrangement
D3D2D1P2D0P1P0
0
1
1
0
0
1
1
Check bits occupy power of 2 slots!
28
Detection via parity of subsets of bits -no
error occurred
D3D2D1P2D0P1P0
First, we send
No error occurred. But how do we know that?
1
1
0
1
1
0
0
Later, someone gets
D3D2D1P2D0P1P0
1
1
0
1
1
0
0
And computes
B2 P2 xor D3 xor D2 xor D1 0 xor 0 xor 1 xor 1
0
If all
B2,B1,B0 0
B1 P1 xor D3 xor D2 xor D0 1 xor 0 xor 1 xor
0 0
there are no errors!
B0 P0 xor D3 xor D1 xor D0 1 xor 0 xor 1 xor 0
0
These equations come from how we computed
P2 D3 xor D2 xor D1 0 xor 1 xor 1 0
P1 D3 xor D2 xor D0 0 xor 1 xor 0 1
P0 D3 xor D1 xor D0 0 xor 1 xor 0 1
29
Detection via parity of subsets of bits -single
bit is twisted
D3D2D1P2D0P1P0
What if a cosmic ray hit D1? How would we know
that?
First, we send
0
1
1
0
0
1
1
D3D2D1P2D0P1P0
Later, someone gets
1
1
0
0
1
0
0
And computes
B2B1B0 101 5
B2 P2 xor D3 xor D2 xor D1 0 xor 0 xor 1 xor 0
1
B1 P1 xor D3 xor D2 xor D0 1 xor 0 xor 1 xor
0 0
And what does 101 5 mean?
B0 P0 xor D3 xor D1 xor D0 1 xor 0 xor 0 xor 0
1
7
5
6
3
4
2
1
The position of the flipped bit! To repair, just
flip it back
We number the least significant bit with 1, not
0! 0 is reserved for no errors.
D3D2D1P2D0P1P0
0
0
1
0
0
1
1
30
Detection via parity of subsets of bits -magic
trick revealed
For any 4 bit word
we add 3 parity bits
P2P1P0
D3D2D1D0
? ? ?
0
1
0
1
Observation The parity bits need to encode no
error scenario, plus a number for each bit (both
data and parity bits)
For p parity bits and d data bits d p 1 2p
31
Detection via parity of subsets of bits -magic
trick revealed
For any 4 bit word
we add 3 parity bits
P2P1P0
D3D2D1D0
Question Why do we arrange bits?
Start by numbering, 1 to 7.
7
5
6
3
4
1
2
D3 D1 D0 P0
With this order, an odd parity means an error in
1,3,5, or 7. So, P0 is the right parity bit to
use
D3D2 D0P1P0
D3D2D1P21P0
etc ... each bit narrows down the suspect bits,
until it is certain.
D3D2D1P2D0P1P0
An odd parity means a mistake must be in 2, 3, 6,
or 7 -- the four numbers possible if P1 1!
32
Detection via parity of subsets of bits -magic
trick revealed
For any 4 bit word
we add 3 parity bits
P2P1P0
D3D2D1D0
P2 D3 xor D2 xor D1
P0 D3 xor D1 xor D0
P1 D3 xor D2 xor D0
D3D2D1P2D0P1P0
7 bits can code 128 numbers, but only 16 of these
numbers are legal.
It takes 3 bit flips to move from one legal
number to another (for all 16 numbers)
If only one bit flips, we can always figure out
the closest legal number, and correct the
number.