Lan Access Protocols 1161 Introduction

In our context here an access protocol is a means of permitting all users to access a LAN fairly and equitably. Access can be random or controlled. For the random access schemes

DSAP Address

SSAP Address

Control

Information

8 bits

8 bits

8 or 16 bits

M * 8 bits

DSAP Address = destination service access point address field

SSAP Address = source service access point address field

Control = control field (16 bits for formats that include sequence numbering, and 8 bits for formats that do not ) Information = information field = multiplication

M = an integer value equal to or greater than 0. (Upper bound of M is a function of the medium access control methodology used.)

DSAP Address = destination service access point address field

SSAP Address = source service access point address field

Control = control field (16 bits for formats that include sequence numbering, and 8 bits for formats that do not ) Information = information field = multiplication

M = an integer value equal to or greater than 0. (Upper bound of M is a function of the medium access control methodology used.)

Figure 11.4 LLC PDU frame format.

Figure 11.5 DSAP and SSAP address field formats (Ref. 2).

Information transfer command/response (l-format PDU)

Supervisory command/response (S-format PDUs)

Unnumbered command/response (U-format PDUs)

LLC PDU Control field bits 3 4 5 6 7 8 9

10-16

LLC PDU Control field bits 3 4 5 6 7 8 9

10-16

0

N(S)

P/F

N(R)

1 0

S S

X XXX

P/F

N(R)

1 1

M M

P/F

M MM

N(S) = Transmitter send sequence number (Bit 2 = low-order bit) N(R) = Transmitter receive sequence number (Bit 10 = low-order bit) S = Supervisory function bit M = Modifier function bit X = Reserved and set to zero P/F = Poll bit—command LLC PDU transmissions Final bit—response LLC PDU transmissions (1 = Poll/final)

Figure 11.6 LLC PDU control field formats (Ref. 2).

we will review CSMA (carrier sense multiple access) and CSMA/CD, where CD stands for collision detection. These schemes have given way to forms of controlled access. Economy and efficiency had more to do with this changeover than anything else. We will describe token ring, which is one of the protocols that uses a token to control access. Most new LANs and older CSMA/CD LANs have been upgraded using the electrical interface of CSMA and CSMA/CD and now incorporate hubs, switching hubs, switches, and/or routers. This eliminated any possibility of collisions and mitigated the very limited traffic capacity typical of Ethernet (CSMA/CD).

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