Lan Performance LAB Ethernet : CSMA/CD TOKEN RING: TOKEN

Transcription

1 Lan Performance LAB Ethernet : CSMA/CD TOKEN RING: TOKEN

2 Ethernet Frame Format 7 b y te s 1 b y te 2 o r 6 b y te s 2 o r 6 b y te s 2 b y te s b y te s 4 b y te s P r e a m b le S ta r t F r a m e D e lim ite r D e s t. M A C S o u r c e M A C L e n g th (D a ta / P a d ) D S A P S S A P C T R L N L I F C S P r e a m b le T h is is a s tr e a m o f b its u s e d to a llo w th e tr a n s m itte r a n d r e c ie v e r to s y n c h r o n iz e th e ir c o m m u n ic a tio n. T h e p r e a m b le is a n a lte r n a tin g p a tte r n o f 5 6 o n e s a n d z e r o es. It is im m ed iately fo llo w e d b y th e S ta r t F r a m e D e lim te r. S ta r t F r a m e D e lim te r T h is is a lw a y s a n d is u s e d to in d ic a te th e b e g in n in g o f th e fr a m e - in fo r m a tio n. D e s tin a tio n M A C T h is is th e M A C a d d r e s s o f th e m a c h in e r e c e iv in g d a ta. W h e n lis te n in g to th e w ir e, a n e th e r n e t d e v ic e is c h e c k in g th is fie ld fo r it's o w n a d d r e s s. S o u r c e M A C T h is is th e M A C a d d r e s s o f th e m a c h in e tr a n s m ittin g d a ta. D a ta / P a d d in g T h e p a y lo a d d a ta is p la c e d h e r e. T h is is w h e r e th e IP h e a d e r an d d ata is p laced if yo u are ru n n in g IP o v e r E th e r n e t. C o n ta in e d w ith in th e D S A P - D e s tin a tio n S e r v ic e A c c e s s P o in t S S A P - S o u r c e S e r v ic e A c c e s s P o io n t C T R L - C o n tr o l b its fo r e th e r n e t c o m m u n ic a tio n N L I - N e tw o r k L a y e r In te r fa c e F C S T h is fie ld c o n ta in s th e F r a m e C h e c k Seq u en ce (FC S) w h ich is calcu lated u sin g a C yclic R ed u n d an cy C h e c k (C R C ). T h e F C S a llo w s e th e r n e t to d e te c t e r r o r s in th e Eth ern et fram e an d reject th em if th e fr a m e a p p e a r s d a m a g e d.

3 Token Ring Frame Format Frame Format Token Ring and IEEE support two basic frame types: tokens and data/command frames. Tokens are 3 bytes in length and consist of a start delimiter, an access control byte, and an end delimiter. Data/command frames vary in size, depending on the size of the Information field. Data frames carry information for upper-layer protocols, while command frames contain control information and have no data for upper-layer protocols. Both formats are shown in Figure 9-3. Figure 9-3: IEEE and Token Ring Specify Tokens and Data/Command Frames Token

4 FCS in both frames Both Ethernet and TR are link level protocol, they doesn t have the functionality of retransmit the dropped frames Higher level protocol must recover bad and dropped frames

5 Ethernet represents a RANDOM access scheme where channels contend by listening for silent periods and transmitting. If a collision occurs then the stations involved backoff a RANDOM time---the average backoff time increases on successive collisions causing a LIFO effect. The backoff times after stations are involved in a collision are random. However, the width of the time interval from which a station chooses its backoff time gets wider the more times it collides (on the same frame transmission attempt). Note: only stations INVOLVED with a collision (actively transmitting when a collision occurs) back off. The backoff time is an integer multiple of the slot-time (max round trip propagation delay). It is the INTEGER multiplier that grows exponentially, namely, if the first attempt to send a frame involves a collision you backoff 1 or 2 slot times, if the retransmission attempt collides you backoff 1,2,3 or 4 slot times, if the second retransmission attempt collides you backoff 1,2,3,4,5,6,7 or 8 slot times... since the actual back off is random (uniform) the expected value of the backoff is INCREASING. Now, suppose a frame arrives at a different station and is involved in a collision. On its first retrans attempt it waits 1 or 2 slot times while your frame, which has been trying for quite a while must wait between (1,16) slot times (expected value = 8)... who has a better chance of going first when the next transmission attempts occur? The LAST guy there...ergo LIFO. Note that once a frame is successfully sent the window start again at 1,2 slot times so we are referring to frames not stations... Under high load, esp in a large network with many nodes and long cable runs there is NO BOUND on the time it will take to transmit a frame. In fact, there is no guarantee the frame will even be sent (it may be dropped by the ethernet controller)

6 Token Ring represents a CONTROLLED access scheme where there is a special FRAME called the token that a station must possess in order to transmit. The a station transmits for a maximum allowed time when it has the token and all other station REPEAT the bits of the frame as the pass by (adding a one bit delay). If the intended receiver recognized its address it will COPY the frame into its RCV BUFFER and set the frame received bit while still repeating the frame. The transmitter may either pass the token to the NEXT node PHYSICALLY attached to the ring as soon as it completes frame transmission, or, it may wait to receive the leading edge of its transmitted frame back (which IT removes from the ring) before passing the token. The scheme BOUNDS the time each station waits before having a chance to transmit and guarantees that the frame will be transmitted.

7 Ethernet and Token Ring both are using shared media. Transmit mode in both is Broadcast rather than Point to Point We are comparing to MULTIPLE ACCESS methods for a shared media; in other words the TRANSMISSION MODE is BROADCAST versus POINT TO POINT.

8 Ethernet is simple Token ring is complex... The benefit of ethernet is that it is extensible and has been adapted well to newer technologies. The result is that much of the hardware investment and ALL of the software investments, and at least after the intro of 10BaseT the physical infrastructure investment has all been preserved even as the 'ethernet' technology has changed and become faster and more efficient. Furthermore, ethernet is much easier to manage...there are only a few simple rules.

9 Token ring, on the other hand, has required changes in interface cards for each advance, different cabling and new controllers for the rings (called MAU=media access unit). Thus, the expense is greater even if the performance is a step ahead. TR also requires a more knowledgeable operator to troubleshoot problems because there is LAN-Layer control (meaning control frames) and special purpose nodes. On Ethernet there are NO CONTROL FRAMES---any control you see is from a HIGHER LAYER entity. There are also no special purpose nodes.

10 TOPOLOGY: Ethernet is a PHYSICAL STAR, but a LOGICAL BUS Token ring was originall a physical ring...if a link between two nodes was faulty the network failed. Today it is a PHYSICAL STAR and a LOGICAL RING. If a station is powered off or faulty it is removed from the ring via a relay in the MAU. Each station acts an active repeater on the ring which bounds ring size since each station has an independent clock and the timing drift is cumulative.

11 Two SPECIAL STATION in Token Ring: The Active Monitor The Backup Monitor No Special station in Ethernet

12 The Setup in our Lab Something to be remember 1) To make FTP file transfer performance result stable, each transfer should begin after the network return stable, (wait for 10 seconds). When Network utilization is very high, e.g., 90%, FTP transfer will drag down the traffic generator utilization. After the FTP transfer finished, network utilization need time to recover to its normal level. So if immediately do FTP transfer again may be make the results a little bit better than the expected results. 2) Two packet sizes were using on traffic generator (1518 and 512). Comparing the FTP file transfer results in two cases when the offer load are same, try to figure out a reason.