Access Methods in Network Point to Point Protocol (PPP) Shared Link Access Protocol

Dedicated Link or Shared Link 2 devices in a network can be connected by : Dedicated link Shared link

Point to Point Access method for connection using dedicated link known as Point to Point access Most common protocol for this access is Point-to-Point Protocol (PPP) Exp: Internet user makes a connection from his/her home to ISP

Users have traditional modem (DSL or cable) Telephone line or TV cable provides physical connection, but to control and manage the transfer of data, there is a need for a PPP at data link layer.

What PPP provides: Defines the format of the frame to be exchanged between devices Defines how two devices can negotiate the establishment of the link and the exchange of data Defines how network layer data are encapsulated in the data link frame. Defines how two devices can authenticate each other

PPP Stack Although PPP is a data link layer protocol, it uses another set of other protocols to establish the link : Link Control Protocol Responsible for establishing, maintaining, configuring and terminating links Authentication Protocol Verification of user identity (exp: dial up session) PAP (password) and CHAP (challenge handshake) Network Control Protocol To encapsulate data (network layer protocol -> PPP frame)

PPP frame –HDLC version

Transition phase in PPP

Protocol stack

LCP packet encapsulated in a frame

Password Authentication Protocol

PAP Packet

Challenge Handshake Authentication Protocol

Example of CHAP

Shared link methods When multiple stations are connected and use a common link, called a multipoint or broadcast link, we need a multiple access protocol to coordinate access to the link

There are procedures to guarantee that : Shared link There are procedures to guarantee that : Each station can access the medium No 2 stations can access the medium at the same time No station interrupt the other station No station monopolize the access

Multiple access protocols

Random Access Protocol No station is superior to another station and none is assigned the control over another If > 1 station tries to access the link, there is an access conflict- collision- and the frames will be either destroyed or modified.

Random Access Protocol To avoid access conflict or to resolve it when it happens, each station follow the procedure that answer the following questions: When can the station access the medium ? What can the station determine the success or failure of the transmission ? How can the station determine the success or failure of the transmission ? What can the station do if there is an access conflict ?

Random access method evolusion

Multiple Access: ALOHA – The earliest random access protocol Developed at Univ. of Hawaii (1970-an) for radio wireless LAN with data rate 9600bps Base station– central controller To send data, needs to go through base station

Multiple Access : ALOHA ALOHA Protocol follows these regulations: Multiple access : Any stations sending frame when it has data to be sent Acknowledgement : After sending frame, station waiting for acknowledgement. If nothing received with certain allocated time, station waits (use backoff strategy) and resend it Station stop if has tried several times

ALOHA Protocol Procedure

Carrier Sense Multiple Access (CSMA) To minimize the chance of collision and to sense the medium before trying to use it Each station first listen to medium before sending However collision can still happen due to propagation delay – time spend by first bit to reach the destination

Collision in CSMA

Persistent strategy Dgn strategi gigih-p, bergtg kpd kebrgkalian tersebut. Jika p=0.2, stesen mempunyai 20 kemungkinan (drp 100) utk menggunakan talian. Jika stesen mengesan talian sedang idle, ia akan janakan suatu no. rawak (1-100), jika no. tersebut < 20, stesen akan menghantar data

Carrier Sense Multiple Access/Carrier Detect (CSMA/CD) CSMA tidak menakrifkan prosedur sekiranya berlaku perlanggaran Dlm CSMA/CD, mana2 stesen boleh menghantar data Stesen kemudian memantau talian utk mengetahui status penghantaran (berjaya/gagal) Jika berlaku perlanggaran, kerangka akan dihantar semula Bgimanapun utk elakkan perlanggaran drp berulang, ia menunggu (strategi backoff ) utk suatu jangkamasa Jangkamasa bertambah jika perlanggaran lebih kerap berlaku Diguna dlm rangkaian Ethernet tradisional

Prosedur CSMA/CD

Carrier Sense Multiple Access/ Collision Avoidance (CSMA/CA) Kaedah ini berbeza kerana ia mengelak berlakunya perlanggaran Stesen menggunakan strategi gigih. Jika talian didapati dlm keadaan melahu (idle), stesen menunggu selama IFG (interframe gap) Stesen menunggu lagi utk suatu jangkamasa rawak Kemudian ia menghantar data dan menunggu perakuan utk suatu jangkamasa tertentu Jika tidak diterima, nilai backoff ditingkatkan dan talian didengar sebelum penghantaran semula Diguna dlm rangkaian WLAN

CSMA/CA procedure

Controlled Access The stations consult one another to find which station has the right to send A station cannot send unless it has been authorized by other stations 3 popular methods: Reservation Polling Token Passing

A station needs to make a reservation before sending data Time is divided into intervals In each interval, a reservation frame precedes the data frames sent in that interval If there are N stations in the system, there are exactly N reservation minislots in the reservation frame. The stations that have made reservation can send their data frames after the reservation frame

Polling Works with topologies in which one device is designated as a primary station and others as secondary stations. All data exchanges must be made through the primary station. Primary station controls the link. If the primary wants to receive data, it asks the secondaries if they have anything to send; this is called poll function If the primary wants to send data, it tells the secondary to get ready to receive; this is called select function

Selecting process

Polling process

Token Passing A Station is allowed to send data when it receives a special packet called token In this method, the stations in a network are organized in a logical ring For each station, there is a predecessor and a successor Packet received from predecessor and sent to successor

Token Passing When data is sent, token circulates through the ring When a station has some data to send, it waits until it receives the token from its predecessor. It then holds the token and sends its data. When the station has no more data to send, it releases the token, passing it to the next logical station in the ring. The station cannot send data until it receives the token again in the next round. If a station receives the token and has no data to send, it just passes the data to the next station

Token Passing procedures

Channelization Multiple access method in which the available bandwidth of a link is shared in time, frequency or code between different stations. 3 popular channelization protocol: FDMA – the available bandwidth is divided into frequency bands TDMA – the stations share the bandwidth of the channel in time CDMA – differs from FDMA because only one channel occupies the entire bandwidth of the link; differs from TDMA because all stations can send data simultaneously, no time sharing

CDMA Still new in implementation A channel carry all data transmission simultaneously Based on coding theory . Each station is assigned a code, which is a sequence of numbers called chips

Data representation

CDMA multiplexing

CDMA demultiplexing

Sequence generation using Walsh Table : W1 and W2N

Sequence generation using Chips

Example 1 Check to see if the second property about orthogonal codes holds for our CDMA example. Solution The inner product of each code by itself is N. This is shown for code C; you can prove for yourself that it holds true for the other codes. C . C = [+1, +1, -1, -1] . [+1, +1, -1, -1] = 1 + 1 + 1 + 1 = 4 If two sequences are different, the inner product is 0. B . C = [+1, -1, +1, -1] . [+1, +1, -1, -1] = 1 - 1 - 1 + 1 = 0

Example 2 Check to see if the third property about orthogonal codes holds for our CDMA example. Solution The inner product of each code by its complement is -N. This is shown for code C; you can prove for yourself that it holds true for the other codes. C . (-C ) = [+1, +1, -1, -1] . [-1, -1, +1, +1] = - 1 - 1 - 1 - 1 = -4 The inner product of a code with the complement of another code is 0. B . (-C ) = [+1, -1, +1, -1] . [-1, -1, +1, +1] = -1 + 1 + 1 - 1 = 0