Physical (PHY) layer of LAN/MAN interfaces. The Ethernet and the WLAN interface

• Application, Services, Topology, Data changing cables,
• Electrical characteristics of the wired interface,
• Frame format of the wired interface
• Wireless (WLAN) interface

Application:

LAN/MAN interface is used widespread for connecting various LAN/MAN network elements (terminals, servers, bridges, etc.).
This interface has numerous wired (CSMA/CD, Token ring, FDDI, ...) and wireless (WLAN, Wi-Fi) physical realizations. We will only review in details the CSMA/CD and according to the identifier of the relating standard, the 802.3 (Ethernet) in the tutorial.

Elements of the recommendation, standard describing the interface:

IEEE 802.3 wired physical layer
IEEE 802.11 wireless physical layer
IEEE 802.1 management
Services:

The interface provides the following services to the higher layers:

• line terminal functions:
  • transmission and reception of serial signals (PMD)
  • line coding/decoding (inversion, block coding) (PCS)
  • clock recovery
• generation and deposition of a synchronizing prefix (preamble) (64 bit 31x10 1x11)
• transmission and reception of serial signals
• channel observation (when can a transmitter transmit, beginning and end of a received frame) (CSMA/CD)
• collision detection (physical layer: comparison of the transmitted/reeived signal format)
• auto-negiotation - setting the optimal transmission mode based on the measurements of the parameters of the transmission medium (PCS)
• flow-control - (pause messages, 1000BASE only)

The notations of the two sides and internal sublayers of the interface in the IEEE 802.3 standard

           10BASE:
                ---+    +-+-----PHY-------+     ||
                   |    | |               |     ||transmission
               MAC |----|P|               |-----||medium
                   |    |L|      PMA      |     ||
                   |    |S|               |     ||
                ---+    +-+---------------+  |
                          |                  |
                         AUI                MDI


           100BASE, 1000BASE:

                ---+    +-+-----PHY-------+     ||
                   |    | |               |     ||transmission
               MAC |----|R|               |-----||medium
                   |    |S| PCS, PMA, PMD |     ||
                   |    | |               |     ||
                ---+    +-+---------------+  |
                          |                  |
                         MII                MDI

• PLS - Physical Layer Signalling
• AUI - Attachment Unit Interface
• MDI - Media Dependent Interface
• MII - Media Independent Interface
• RS - Reconciliation Sublayer
• PCS - Physical Coding Sublayer
• PMA - Physical Medium Attachment Sublayer (s/p clock recovery, p/s)
• PMD - Physical Medium Dependent Sublayer
  (tx_bit -> line (optical) signal line (optical) signal -> rx_bit

Data changing wires:

The interface was realized at several transmission media:

Medium	Notation according to standard	Remarks
thick coaxial cable	..BASE5	thick ethernet
thin 50 ohm coaxial cable	..BASE2	thin ethernet
twisted pair	..BASET	twisted-pair: UTP - unshielded twisted pair STP - shielded twisted pair TPE - twisted pair ethernet
optical fibre	..BASEF
wireless	802.11a/b/g/n

The word BASE relates to the base band signal transmission.

Pins of the jack: RJ-45  10BASE-T/100BASE-TX  (EIA/TIA 568-A/B: Commercial Building Wiring Standard)

                 -A     -B
             1   Tx+    Rx+
             2   Tx-    Rx-
             3   Rx+    Tx+
             4   nc     nc
             5   nc     nc
             6   Rx-    Tx-
             7   nc     nc
             8   nc     nc

topology	notation according to standard
bus	10BASE-5, 10BASE-2
star	10BASE-T, 100BASE-T
ring

Electrical characteristics:

Several bit rates:

bit rate	Notation according to standard
10 Mbit/s	10BASE
100 Mbit/s	100BASE
10/100 Mbit/s	10/100BASE
1000 Mbit/s	1000BASE

Signal levels, line coding:

Notation according to standard	Signal level	Cable	Line coding	Remarks
10BASE-2	0V ... -2V	coaxial	Manchester	"thick" ethernet
10BASE-5	0V ... -2V	coaxial	Manchester	"thin" ethernet Baud rate: 20 MBaud
10BASE-T	+/-2.5V (2.2V...2.8V)	2 twisted pairs	Manchester
100BASE-TX	0V...+/-1V	2 twisted pairs (Cat 5)	4B/5B, MLT-3	"fast" ethernet, Input: MII bus Baud rate: 125 MBaud
100BASE-FX		optical fibre	4B/5B, NRZI	"fast" ethernet, Input: MII bus
100BASE-T4	3.5V +/-10%	4 twisted pairs (Cat 3,4,5)	8B/6T
100BASE-T2	1.81V +/-0.5dB	2 twisted pairs (Cat 3)		not used
1000BASE-SX		Two optical fibres (Short wave length, multimode)	block, 8B/10B	GigaBit Ethernet (GBE)
1000BASE-LX		Two optical fibres (Long wave length, mono/multimode)	block, 8B/10B	GigaBit Ethernet (GBE)
1000BASE-CX	ECL (3...1.1V)	4 twisted pairs (150 Ohm) length: 0.1...25m baud rate: 1250 MBaud	block, 8B/10B	GigaBit Ethernet

We illustrate the signal at the data cables with oscillograms.

10BASE-5 line signal oscillogram:

Lower part of the diagram shows the beginning of an Ethernet frame and its upper part shows its reached detail (SFD). If there are no any packets to be forwarded, then there is no signal on the interface. Line coding is Manchester (-2V -> 0V signal transmission corresponds to the binary 0, and the 0V -> -2V signal transmission corresponds to the binary 1.

10BASE-T oscillogram:

We can measure a signal among the strings of the pairs, which is similar to the line signal of 10BASE-5

100BASE-TX line signal oscillograms:

Time diagram details:

On the following figure we can see a typical 100BASE-TX signal detail. This signal has three levels, the signal amplitude is 1V. on the signal detail oscillations caused by the reflexion between the cable and the transceivers. However it cannot be seen on the figure, there is a signal between the two packages. It also cannot be seen, but the signal rate is 125 MBauds, which means that there are 10 line symbols located horizontally between two raster lines.

Eye diagram:

Illustration of MLT-3 coding (besides 1 line symbol/raster horizontal diversion):

  source series: 1 1 1 1 1 1 1 1 1 1

  line series: 0 1 0-1 0 1 0-1 0 1

  

Illustration of MLT-3 coding (besides 2 line symbol/raster horizontal diversion):

Spectrum of the signal:

We find the first minimum at the baud rate (125 MHz). Most of the energy of signal falls to the 0...125/4=31.25 MHz band. It is the result of the MLT-3 coding.

Wired interface frame formats (Layer1)

IEEE.802.3 (Ethernet) frame

          bytes
              +--------------------------------+
            7 | Preamble                       |   7 x  10101010  Syncronizing receiver's clock to the sender
              +--------------------------------+
            1 | SFD Start of Frame Delimiter   |        10101011  begining of the frame
              +================================+
              | D  basic MAC Data field        |
      60-1514 |    tagged MAC Data field       |
              |                                |
              +================================+
            4 | Frame Check Sequence           |
              +--------------------------------+

Interface processes, remote feeding

Test and maintenance functions

Most important elements of the 802.11 standard

IEEE 802.11 standard family has an important role in establishment of wireless local data transmission networks. Several members of this family are used currently in the establishment of WLAN networks.

Name	Year of acceptance	Frequency - GHz	Modulation	Bit rate - Mbps	Remarks
802.11	1997	2.4 or IR	PSK (FHSS or DSSS)	2, 1	-
802.11a	1999	5	OFDM	54 (24, 12, 6)	wireless ATM
802.11b	1999	2.4	CCK (DSSS)	11 (5.5, 2, 1)	wireless Ethernet
802.11g	2003	2.4	OFDM	20-54	-

Resolving the abbreviations:

• FHSS - Frequency-hopping spread spectrum
• DSSS - Direct-sequence spread spectrum
• OFDM - Orthogonal frequency-division multiplexing
• CCK - Complementary Code Keying
• CSMA/CA - Carrier Sense Multiple Access With Collision Avoidance -
  it is one of the CSMA-typed processes. It is used in cases, when we cannot use CSMA/CD. For example, in case of wireless networks (see the so called hidden terminal problem). That is why the transmitter and the receiver in case of IEEE 802.11 make a short packet changing: transmitter informs its own environment with sending the Request to Send signal, that it wants to make traffic, while the receiver sends the Clear to Send signal, informing with it all stations within its own range (IEEE 802.11 RTS/CTS exchange).

Of course, range depends on the actual place, or on modulations used for the individual standards, in case of B and G standards compatible backwards together with it, it can be within a building 30-50 metres, and outside the building it can be 1 kilometer (if we can see the AP). In case of the A standard the range is a bit smaller. Note, that for wireless communication with small range, and small energy (but with fast rate, compared to that) the Bluetooth (IEEE 802.15), and for large ranged connections in urban environment the WiMax (IEEE 802.16) is used.

It could be interesting, so we will mention here further two members of the 802.11 family:

• IEEE 802.11n is a field under construction, which tries to reach 600 Mbps rate in the physical layer using among others the MIMO (multiple-input multiple-output) technology.
• IEEE 802.11y is a protocol issued in November 2008, which uses OFDM on bearers between 3650 and 3700 MHzs in the USA. Its maximum speed can be 54 Mbps lehet, so it is not bigger, than in case of A or G standards, however, its transmitting performance could be quite bigger, which allows to reach even a 5-kilometer range at an outdoor environment.

We can study on the figure below the channel division used in the most prevalent WLAN-standard, the 802.11g. We can see, that channels may overlap each other, and they have a broadness of 22 MHz. Bearers are to 5 MHz from each other, thus a channel located in the centre of the frequency domain can interfere with 8 other at the most. Arrangement of the 14th channel does not follow this regularity. We just note, that not all the channels are in use in all geographic regions. Besides, selection of aggregation of non-interfering channels is not the same in the individual regions. (Spectral details indicated with a continuous line on the figure demonstrate the selection valid in the USA and China.)

Protocol Structure - WLAN: Wireless LAN by IEEE 802.11, 802.11a, 802.11b,802.11g, 802.11n

801.11 protocol family MAC frame structure:

• Frame Control Structure:

• Protocol Version - indicates the version of IEEE 802.11 standard.
• Type - Frame type: Management, Control and Data.
• Subtype - Frame subtype: Authentication frame, Deauthentication frame; Association request frame; Association response frame; Reassociation request frame; Reassociation response frame; Disassociation frame; Beacon frame; Probe frame; Pro be request frame and Probe response frame.
• To DS - is set to 1 when the frame is sent to Distribution System (DS)
• From DS - is set to 1 when the frame is received from the Distribution System (DS)
• MF- More Fragment is set to 1 when there are more fragments belonging to the same frame following the current fragment
• Retry indicates that this fragment is a retransmission of a previously transmitted fragment. (For receiver to recognize duplicate transmissions of frames)
• Pwr - Power Management indicates the power management mode that the station will be in after the transmission of the frame.
• More - More Data indicates that there are more frames buffered to this station.
• W - WEP indicates that the frame body is encrypted according to the WEP (wired equivalent privacy) algorithm.
• O - Order indicates that the frame is being sent using the Strictly-Ordered service class.
• Duration/ID (ID) -
• Station ID is used for Power-Save poll message frame type.
• The duration value is used for the Network Allocation Vector (NAV) calculation.
• Address fields (1-4) - contain up to 4 addresses (source, destination, transmittion and receiver addresses) depending on the frame control field (the ToDS and FromDS bits).
• Sequence Control - consists of fragment number and sequence number. It is used to represent the order of different fragments belonging to the same frame and to recognize packet duplications.
• Data - is information that is transmitted or received.
• CRC - contains a 32-bit Cyclic Redundancy Check (CRC).

"6.1.4 MSDU format This standard is part of the IEEE 802 family of LAN standards, and as such all MSDUs are LLC PDUs as defined in ISO/IEC 8802-2: 1998. In order to achieve interoperability, implementers are recommended to apply the procedures described in ISO/IEC Technical Report 11802-5:1997(E) (previously known as IEEE Std 802.1H-1997 [B13]), along with a selective translation table (STT) that handles a few specific network protocols, with specific attention to the operations required when passing MSDUs to or from LANs o r operating system components that use the Ethernet frame format. Note that such translations may be required in a non-AP STA."