13-LAN, packets, frames and topologies

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26 Οκτ 2013 (πριν από 3 χρόνια και 11 μήνες)

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13
-
LAN, packets, frames and
topologies


Dr. John Abraham

Professor

UTPA

What happens in data transfer?


For example an email message


Information

Data (alpha numeric
characters)


Data

Segment


Segment

Packet


Packet

Frame


Frame

Bits

Headers added


Data (including application header,
presentation header, and session header)


Segment Header


Data


Network Header


Segment Header


Data


Frame Header
-
Network Header
-

Segment
Header


Data


Frame Trailer

Circuit switching


Connection mechanism that establishes a path between
a sender and receiver, with guaranteed separation from
other paths. Originally used electromechanical switching
devices to make a physical circuit.


Today use electronic devices to establish circuits, and
multiple circuits are multiplexed over shared media.
Three general properties to define a circuit switched
paradigm:


Point
-
to
-
point communication


Separate steps for circuit creation, use and termination


Performance equivalent to an isolated physical path.

Packet Switching


Basis of internet


Sender must divide the message into blocks of
data called packets.


The size of a packet varies depending on the packet
switching technology used.


Uses statistical multiplexing


Communication from multiple sources competes for
the use of a shared wire.


3 properties of a packet switched paradigm


Arbitrary, asynchronous communication


No set
-
up required before communication starts


Performance varies due to statistical multiplexing.

Standards for Packet Format and
Identification


LAN


IEEE project 802


There may be sublayers, eg. Datalink layer is
dived into LLC
-
logical link control (upper
portion) and MAC

media access control
(lower portion)


Some 802 projects:


802.3 Ethernet 802.4 Token bus, 802.5 Token
Ring. 802.11 wirless. 802.15 bluetooth

LAN topologies


Bus


Ring


Star


Mesh


Advantages and disadvantages

Packet Identification,
demultiplexing, mac addresses


Only intended recipient should process the
packet. Demultiplexing requires an
address. Each computer has a unique
address (MAC address) and each packet
contains the address of this computer.


48 bit address on NIC. 3 bytes (MSB) identify
the manufacturer (Unique ID, OUI).
Remaining 3 bytes is the number for each
card.


Types of MAC addresses


Unicast


uniquely identifies a single computer,
only that computer get the packet. A source
address is always unicast.The lease significant
bit of the first byte defines the type of address. If
the bit is 0, it is unicast, otherwise multicast.


Broadcast


each computer on that network
should receive a copy of the packet. All 48 bits
are 1s.


Multicast


identifies a subnet of computers on a
given network and all computers in the subnet
will receive the packet.

Examples:


4A:30:10:21:10:1A


lsb of first byte is 0
so unicast. 0100 (4) 1010 (A)


47:20:1B:2E:08:EE


lsb is 1 so multicast.
0111 (7)


FF:FF:FF:FF:FF:FF


muticast

Broadcast, Multicast and
efficient multi
-
point delivery


LAN transmits over a shared medium. All
can hear, each machine should examine
the address to determine if it belongs self,
or discard the packet. See p.233 for
algorithm.


In case of broadcast or multicast only a
single copy needs to be transmitted.

Frames and Framing


Framing refers to the structure added to the
sequence of bits or bytes that allows a send and
receiver to agree on the exact format of the
message.


A frame header contains information used to
process the frame.


In a ethernet packet switched network a frame
has the same data length (payload) as a packet.


An ethernet frame contains seven fields:
preamble, SFD, DA, SA, length or type of data
unit, upper
-
layer data and the CRC.

TCP/IP Protocol Suite

13

Figure 3.3

Ethernet layers

TCP/IP Protocol Suite

14

Figure 3.4

Ethernet frame

Frame


Preamble 7bytes alerts the receiving system to the
coming frame and enables it to synchronize.


SFD


start frame delimiter. 1byte. Signals the beginning
of the frame.


DA/SA


destination and source address. 6bytes each.


Length/type


2 bytes. number of bytes n the data field.


CRC


4 bytes.


Must have DA(6), SA(6), Length(2) Data.. And CRC(4),
18 bytes plus data.


Data


encapsulated data from upper layers. minimum
of 46 bites and maximum of 1500 bytes. So minumum
length of the frame is 64 byte and maximum is 1518
bytes.

802.3 Frame


A SNAP header (8bytes) is added (3bytes
LLC, 3byte OUI and 2byte type/length)


LLC = Logical Link control


OUI = Organization unique identifier


This reduced the maximum payload of
1500 to 1492.

Frame


In order to provide service to the network layer, the data
link layer must use the service provided to it by the
Physical layer. The physical layer only accept a raw bit
stream. For a long bit stream, if error occurred during
transmission, the entire stream has to be sent again.
Therefore, it is broken to smaller frames. Each frame will
have to be distinguished somehow. Three methods are
used


Character count


Starting and ending characters, with character
stuffing


Starting and ending flags, with bit stuffing

Character count


The first byte indicates how many characters are in the
frame including the first byte.


Trouble with this scheme is that if the first byte of any
frame gets corrupted, the destination will get out of
sequence, having no way to find where the beginning of
the next frame is.


This method is rarely used any more.

Start and end with control chars with
character stuffing


Resynchronize at the beginning of each frame


Start with ASCII chars DLE & STX


Data Link Escape, Start of Text


End with the same sequence


If these ASCII char is part of the text then double it


like Jim’’s book
-

in pascal


Problem
-

must use 8 bit ASCII which is not suitable for all communications

Start and end with flags with stuffing


Each frame begins with bit pattern 01111110


If senders data has five consecutive ones then stuff it with a zero which is deleted at
the receiving end


This bit pattern called flag byte is added at the end of each frame as well


With this method boundary between two frames are easily recognized.