Multiple Access

Definition of Terms

• We can consider the data link layer as two sublayers. The upper sublayer is responsible for data link control, and the lower sublayer is responsible for resolving access to the shared media.
• Many formal protocols have been devised to handle access to a shared link. We categorize them into three groups: random access protocols, controlled access protocols, and channelization protocols.
• In random access or contention methods, no station is superior to another station and none is assigned the control over another.
• ALOHA allows multiple access (MA) to the shared medium. There are potential collisions in this arrangement. When a station sends data, another station may attempt to do so at the same time. The data from the two stations collide and become garbled.
• To minimize the chance of collision and, therefore, increase the performance, the CSMA method was developed. The chance of collision can be reduced if a station senses the medium before trying to use it. Carrier sense multiple access (CSMA) requires that each station first listen to the medium before sending. Three methods have been devised for carrier sensing: I-persistent, nonpersistent, and p-persistent.
• Carrier sense multiple access with collision detection (CSMA/CD) augments the CSMA algorithm to handle collision. In this method, a station monitors the medium after it sends a frame to see if the transmission was successful. If so, the station is finished. If, however, there is a collision, the frame is sent again.
• To avoid collisions on wireless networks, carrier sense multiple access with collision avoidance (CSMA/CA) was invented. Collisions are avoided through the use three strategies: the interframe space, the contention window, and acknowledgments. 
• In CSMA/CA, the IFS can also be used to define the priority of a station or a frame.
• In CSMA/CA, if the station finds the channel busy, it does not restart the timer of the contention window; it stops the timer and restarts it when the channel becomes idle.
• In 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. We discussed three popular controlled-access methods: reservation, polling, and token passing.
• In the reservation access method, 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.
• In the polling method, all data exchanges must be made through the primary device even when the ultimate destination is a secondary device. The primary device controls the link; the secondary devices follow its instructions.
• In the token-passing method, the stations in a network are organized in a logical ring. Each station has a predecessor and a successor. A special packet called a token circulates through the ring.
• Channelization is a multiple-access method in which the available bandwidth of a link is shared in time, frequency, or through code, between different stations. We discussed three channelization protocols: FDMA, TDMA, and CDMA.
• In frequency-division multiple access (FDMA), the available bandwidth is divided into frequency bands. Each station is allocated a band to send its data. In other words, each band is reserved for a specific station, and it belongs to the station all the time.
• In FDMA, the available bandwidth of the common channel is divided into bands that are separated by guard bands.
• In time-division multiple access (TDMA), the stations share the bandwidth of the channel in time. Each station is allocated a time slot during which it can send data. Each station transmits its data in its assigned time slot.
• In TDMA, the bandwidth is just one channel that is timeshared between different stations.
• In code-division multiple access (CDMA), the stations use different codes to achieve multiple access. CDMA is based on coding theory and uses sequences of numbers called chips. The sequences are generated using orthogonal codes such the Walsh tables.
• In CDMA, one channel carries all transmissions simultaneously.
• The Point-to-Point Protocol (PPP) was designed to provide a dedicated line for users who need Internet access via a telephone line or a cable TV connection.
• A PPP connection goes through these phases: idle, establishing, authenticating (optional), networking, and terminating. 
• At the data link layer, PPP employs a version of HDLC.
• The Link Control Protocol (LCP) is responsible for establishing, maintaining, configuring, and terminating links. 
• Password Authentication Protocol (PAP) and Challenge Handshake Authentication Protocol (CHAP) are two protocols used for authentication in PPP.
• PAP is a two-step process. The user sends authentication identification and a password. The system determines the validity of the information sent.
• CHAP is a three-step process. The system sends a value to the user. The user manipulates the value and sends its result. The system verifies the result.
• Network Control Protocol (NCP) is a set of protocols to allow the encapsulation of data coming from network layer protocols; each set is specific for a network layer protocol that requires the services of PPP.
• Internetwork Protocol Control Protocol (IPCP), an NCP protocol, establishes and terminates a network layer connection for IP packets.    

Data link layer divided into two functionality-oriented sublayers

Three Categories of Multiple Access Protocols

• Random access method- 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.
• Controlled access method - there is no access control (as there is in controlled access methods) and there is no predefined channels (as in channelization). Each station can transmit when it desires. This liberty may create collision. The whole available bandwidth belongs to the station that wins the contention; the other stations needs to wait.
• Channelization - the available bandwidth is divided between the stations. If a station does not have data to send, the allocated channel remains idle.

Taxonomy of multiple-access protocols discussed in this chapter

Formula: The throughput for pure ALOHA

Formula: The throughput for slotted ALOHA

Flow diagram for three persistence methods

Flow diagram for the CSMA/CD

The number of sequences in a Walsh table

Note: You can proceed to take the multiple choice exam regarding this topic. Multiple Access - Set 1 MCQs

---

credit: Behrouz A. Forouzan©2013 www.FroydWess.com

Share Note :