Yaakov (Jonathan) Stein, RAD Data Communications and Brian Stroehlein, TranSwitch Corporation

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1 Using ynchronization over PN Does IEEE 1588 M Really Mae a Difference? Yaaov (Jonathan) tein, RAD Data Communications an Brian troehlein, ranwitch Corporation pecial thans to Alon Geva, DP Algorithm eam Leaer at RAD Abstract It has been suggeste that IEEE 1588, being base on a biirectional protocol, can etect constant elay changes (CDCs) better than the uniirectional mechanisms employe in pure aaptive timing recovery. We test this suggestion both theoretically an using the new G.861 test suite an fin to the contrary. I. INRODUCION IEEE 1588 was originally evelope as a stanar for precision cloc synchronization of LAN-networe measurement an control systems. Recently, wor has been unertaen by the telecommunications community to enhance IEEE 1588 for frequency an time istribution over metro an wie area pacet-switche networs (PNs), an specifically to enable it to be use for synchronization of circuit emulation services (CE) over PNs, such as DMoIP. Once 1588 is enhance it will impart certain avantages as compare to straightforwar cloc recovery base on the circuit emulation flow. he 1588 messages may be use to attain frequency loc before particular CE flows are neee, an thus enabling rapi setup of synchronize CE flows. Furthermore, as the 1588 messages are not line to a particular flow, they nee not be sent at a pacet transmission creation rate that may interfere with neighboring DMoIP flows, nor even at a constant pacet transmission rate. 1

2 On the other han, DMoIP flows have native mechanisms for aaptive cloc recovery, an IEEE 1588 as aitional traffic that may not be truly neee. In aition, the aforementione use of 1588 creates a single point of failure, with any fault in the 1588 or its algorithms potentially affecting the timing of a very large number of DMoIP flows. Is the extra banwith an ris worthwhile? IEEE 1588 functions essentially as a pure aaptive cloc recovery mechanism an thus suffers from the same performance limitations. It has been suggeste that 1588 s main strength as compare with pure aaptive cloc recovery is its ability to etect constant elay changes introuce by reroute or suen changes in the loaing of the networ. his increase ability supposely erives from its being able to measure the absolute elay that the protocol pacets unergo while traversing the path from the remote master to the slave s receiver. G.861 (formerly G.pactiming) specifies test scenarios for such constant elay changes, an can be use to test this suggestion. IEEE 1588 performs timing istribution base on a biirectional protocol. imestamps are exchange in both irections from a master noe (usually connecte to an accurate cloc, often a tratum 1 cloc) to a slave noe (which performs a cloc recovery function) an vice versa. his biirectional approach allows the slave noe to evaluate the absolute elay that the protocol pacets (i.e., timestamps) unergo when traversing a path from the remote master to the slave s receiver an therefore to eliver absolute time (reference to a specific starting point). he IEEE 1588 protocol is esigne so that every networ element (e.g., a Layer switch or Layer router) can incorporate an IEEE 1588 master/slave pair on every port (e.g., bounary cloc), thus allowing a much more controlle networ environment in terms of the PDV that an IEEE 1588 timing istribution pacet experiences. Achieving this goal, however, requires upgraing all networ elements along the timing istribution path to support IEEE Protocols for circuit emulation services (CE) lie DMoIP, AoP, an CEoPN may also be use for elivering sync over a PN. hese protocols are uniirectional: the pacet flow is carrie in one irection from the master to the slave

3 only. herefore, the common CE protocols are only suite for elivering timing (frequency) rather than absolute time. II. CONAN DELAY CHANGE EVEN here are several types of common isruptive events that may cause suen changes in the constant elay component of a PN s performance. hese changes may be permanent or temporary. hese isruptive events inclue routing changes, suen changes in networ loaing, temporary networ overloa, an temporary loss of service. At first glance, it woul seem that the biirectional approach taen by IEEE 1588 coul allow the slave to better etect constant elay changes (CDCs) associate with isruptive events as compare to a uniirectional approach. In section III we shall emonstrate that the theoretical performance of a uniirectional protocol is actually better than that of a biirectional protocol. ince carrier-proven uniirectional systems are alreay available toay, there is thus no nee to push towar biirectional systems solely for the sae of CDC etection. he recently consente IU- G.861 specification ( iming an ynchronization Aspects in Pacet Networs, formerly G.pactiming) has specific test cases to test the ability of a slave noe to aequately etect CDCs in orer to eliver synchronization of acceptable quality in the real worl. In section IV we will present test results of a pure aaptive mechanism, an conclue that it is capable of passing the G.861 tests without 1588 assistance. III. ANALYI OF UNIDIRECIONAL AND BIDIRECIONAL PROOCOL Figure 1 shows the basic system iagram where master an slave noes are connecte via a pacet switche networ.

4 ( t) DM E1 Master Networ lave DM E1 x ~ ( t) Recovere DM cloc DM iming ource Fig. 1: Basic system iagram A pacet traversing the networ from the master to slave will unergo a elay of () t, which is itself time-epenent, while pacets traversing from the slave to master unergo a ifferent elay () t, which is also time-epenent. Figure epicts a simplifie version of the timestamp exchange protocol use by IEEE LAVE 1 ( ) t ( t + ( t ) + + ( t + ( t ) + ) time MAER ( t ) ( t + ( t ) + ) M M ( t ( )) t + ( t ) + t ( + ) time time t t t t + ( t ) + t t + ( t ) t t + ( t ) + + t + ( t ) ( + ) Fig. : IEEE 1588 timestamp exchange

5 At time instant ( ) t t t the slave creates a timestamp of its local time representation, puts it in a pacet an sens it to the master. he pacet unergoes a time 1 varying elay (pacet elay variation, or PDV) enote by ( t ). he master receives the pacet exactly at time t t + ( t ) an issues its own timestamp (accoring to its M own local time representation) t ( ). he master waits for a time ( ( ) + t secons), then at time instant t t + ( t ) + M accoring to its local time representation) t + ( t ) it issues another timestamp (again ( + ) an sens both an bac to the slave. his last exchange pacet unergoes a time varying elay of ( t ( t ) + ) + an arrives at the slave at time instant ( t ) + + ( t + ( t ) + ) ( t + ( t ) + + ( t + ( t ) + ) t t +.. he slave then issues a fourth timestamp: In orer to simplify the calculation we can mae a few common assumptions at this point: 1) he slave s PLL has achieve loc so there is a (almost) zero frequency error between the master an the slave. Hence, the relationship between the local time representation of the slave an that of the master is M () t ( t + ε ), where ε is a constant uncompensate phase (time) error between the master an the slave. ) he master s cloc is loce to a primary reference cloc (PRC or tratum 1 cloc). Hence the time representation of the master can be simplifie to: M () t t. Given these assumptions we can rewrite the four timestamps of Figure in a simpler manner: 1 ( ) t + ε ( ) t + ( ) ( ) t + ( ) + ( ) t + ( ) + + ( ) + ε 5

6 Where is the pacet s sequence number. At this last step, we also set ( ) ( t ), t ( t ) + to be ( ) ( ) +, respectively. In orer to calculate the current master-slave en-to-en elay the slave performs the following calculation: ~ way ( ) In general ( ) ( ) ( ) ( ) + ( ) ( ) ( ) + ( ) 1, woul be characterize as uncorrelate stochastic processes having the following means an stanar eviations: µ σ µ σ E E [ ( ) ] E[ ( ( ) ) ] ( µ ) [ ( ) ] E[ ( ( ) ) ] ( µ ) Where E is the expectation value operator. Hence, the total amount of noise power in the slave s en-to-en estimate ( ) ~ way woul be ( σ + σ ) / an, as a consequence, the etection of CDCs woul have to overcome this noise level. On the other han, a uniirectional timing istribution protocol (e.g. that use by pure aaptive cloc recovery) woul use a uniirectional elay estimate for CDC etection: ~ +. 1way ( ) ( ) ( ) ( ) ε ince ε is nearly constant, the total amount of noise power in the slave s en-to-en ~ is only σ. estimate ( ) 1way Now suppose that () changes by 0 secons. he respective NRs for etecting this CDC event will be: NR1way 0 σ ( ) 6

7 NRway ( 0 0 / ) ( σ ) + σ ) / ( σ ) + σ hus, for CDC etection, the uniirectional protocol has roughly twice the NR of the biirectional protocol given the same noise level for ( ) ( ),! IV. PERFORMANCE EING OF A UNIDIRECIONAL PROOCOL In this section we present recently conucte laboratory tests of our DMoIPbase uniirectional sync transmission using aaptive cloc recovery at a slave noe equippe with an OCXO reference cloc. RP was not use. hese tests were performe accoring to test Cases 1, an of Appenix VI of the recently consente IU- G.861 (formerly G.pactiming). For all tests networ raffic Moel 1 (see VI...1.1/G.861) was use, an Qo enhancing mechanisms were not employe. Our networ was compose of five Gigabit Ethernet switches in series. For raffic Moel 1 the pacet size profile is: 80% of the loa is minimum size pacets (6 octets) 15% of the loa is maximum size pacets (1518 octets) 5% of the loa is meium size pacets (576 octets) Maximum size pacets occur in bursts lasting between 0.1s an s. G.861 further ifferentiates between three test cases. In all three cases a stabilization perio is allowe before performing the measurements, an the pacets use to loa the networ are generate accoring to a specific networ traffic moel specifie in the Recommenation. 7

8 est Case 1 (VI.../G.861) Here a static loa is moele. Networ isturbance loa is maintaine at 80% for one hour assuming that the cloc recovery is in a stable conition. est Case (VI.../G.861) his case moels suen large an persistent changes in networ loaing, i.e. CDCs. It tests stability uner suen changes in networ conitions, an waner performance in the presence of low frequency PDV. he networ isturbance loa alternates between 80% for an hour an 0% for an hour. est Case (VI.../G.861) his case moels the slow change in networ loa over an extremely long timescale. It tests stability uner very slow changes in networ conitions, an waner performance in the presence of extremely low frequency PDV. he networ isturbance loa varies smoothly from 0% to 80% an bac over a -hour perio. Our MIE test results are presente in Figure. We see that that timing elivere using aaptive cloc recovery falls well within the limits for the common Deployment Cases anticipate by G

9 Figure MIE tests results for test cases an / G.861 V. CONCLUION While biirectional protocols lie IEEE 1588 allow slave noes to evaluate the absolute elay the protocol pacets (i.e., timestamps) unergo when traversing the path from the remote master to the slave s receiver an therefore eliver absolute time, they are also inherently noisier than uniirectional protocols, thus less well-suite for etecting isruptive events which cause CDCs. he IEEE 1588 solution also requires that almost every networ element along the timing istribution path support IEEE 1588, which means a very unappealing wholesale upgrae for carriers. Uniirectional protocols for circuit emulation services (CE) lie DMoIP, AoP, an CEoPN are more than sufficient for elivering sync over a PN. 9